A4490 Datasheet by Allegro MicroSystems

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Description
Designed to provide the power supply requirements of printers,
office automation, industrial, and portable equipment, the
A4490 provides three high current, high performance, switching
regulator outputs with independent soft start.
High frequency switching allows selection of inexpensive
inductors and small ceramic output capacitors. The turn-on
cycles of the regulators are interleaved to minimize stresses on
the input capacitors and to reduce EMI. A charge pump is used
to provide the supply for driving the power switches, ensuring
operation at very wide operating duty cycles and avoiding the
need for power-draining clamp circuits.
A power-on-reset circuit with user configurable delay indicates
when enabled regulators are in specification. The power-on-
reset flag also indicates when the input voltage drops below
specification, giving the system controller advance warning
while the switchers continue to operate down to the shutdown
level.
Internal diagnostics provide comprehensive protection against
overloads, input undervoltages, and overtemperatures.
4490-DS, Rev. 10
Features and Benefits
Three buck converters
4.5 to 34 V input voltage range
550 kHz fixed frequency
Multiphase switching
Independent control of each converter
Power-on-reset flag
Internal compensation
4 × 4 mm QFN Package, small PCB footprint
Triple Output Step-Down Switching Regulator
Package: 20-contact QFN (suffix ES)
A4490
Approximate size
Continued on the next page…
Microcontroller or
Controller Logic
VBB
GND
CP1 CP2 VCP
PGND
PORZ
ENB1
ENB2
ENB3
CPOR VREG1
VREG2
VREG3
VDD
A4490
VBB2
LX2
FB2
VBB3
LX3
FB3
VBB1
LX1
FB1
Typical Application
Triple Output Step-Down Switching Regulator
A4490
2
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
The A4490 is provided in a 20-contact, 4 mm × 4 mm, 0.75 mm
nominal overall height QFN, with exposed pad for enhanced thermal
dissipation. It is lead (Pb) free, with 100% matte tin leadframe
plating.
Applications include the following:
Photo, inkjet, and portable printers
Industrial
Hand-held devices
Portable applications
Description (continued)
Selection Guide
Part Number Packing Operating Temperature Range
(°C)
A4490EESTR-T 1500 pieces per 7-in. reel –40 to 85
Thermal Characteristics may require derating at maximum conditions, see application information
Characteristic Symbol Test Conditions* Value Units
Package Thermal Resistance RθJA On 4-layer PCB based on JEDEC standard 37 ºC/W
*Additional thermal information available on the Allegro website.
Absolute Maximum Ratings (reference to GND)
Characteristic Symbol Notes Rating Units
Load Supply Voltage VBB 36 V
LX1, LX2, and LX3 Pins VLXn–1 to 36 V
PORZ and VDD Pins VIN –0.3 to 7 V
ENBx Pin Input Current IENBx Driven by a current-limited voltage source 1 mA
Operating Ambient Temperature TARange E –40 to 85 ºC
Maximum Junction Temperature TJ(max) 150 ºC
Storage Temperature Tstg –55 to 150 ºC
Recommended Operating Conditions
Characteristic Symbol Conditions Min. Typ. Max. Units
Load Supply Voltage VBB
To operate at VBB < 6 V, connect VDD
supply to the VBB supply. See Powering
Configurations section.
4.5 – 34 V
LX1, LX2, and LX3 Pins VLXn–0.7 – 34 V
Operating Ambient Temperature TA–40 – 85 ºC
Junction Temperature TJ–40 125 ºC
EE____________54: g j T j :
Triple Output Step-Down Switching Regulator
A4490
3
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Functional Block Diagram
ENB2
FB2
GND
Switcher #2
PWM Control
LX2
5 V / 1.5 A
VBB3
FB3
LX3
3.3 V / 1.5 A
Switcher #3
PWM Control
ENB1
4.7 μH
10 μF
10 μH
ENB3
PORZ
VBB1
FB1
Switcher #1
PWM Control
LX1
VCP
V
BB
Bias Supply
PGND
VCP
VCP
SS
SS
SS
CP1
CP2
Charge Pump
VCP
Regulator
VDD
Switch
POR Block
CPOR
V
BB
C2
C3
C4
C5
C7
C10
C14
L1
L2
10 μF
10 μF
C11
10 μF
C12
10 μF
C13
10 μF
15 μH
R1
R2
R3
R4
R5
R6
D1
D2
D3
L3
C1
100 nF
47 nF
47 nF
470 nF
10 μFVREG1
VREG2
VREG3
1.0 V / 1.5 A
C9
10 μF
C8
10 μF
VBB2
10 μF
C6
V
BB
V
DD
100 kΩ
Note: All capacitors ceramic X5R.
‘a‘ ‘3: 52‘ :2: ‘2:
Triple Output Step-Down Switching Regulator
A4490
4
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Terminal List
Number Name Function
1 FB1 Feedback REG1
2 VDD Bias supply
3 GND1 Ground
4 FB2 Feedback REG2
5 ENB2 Enable REG2, logic input, active high
6 LX2 Switch node REG2
7 VBB22 Input supply for REG2
8 CPOR POR delay adjustment
9 PORZ Power on reset output, active low
10 VCP Charge pump reservoir
11 CP2 Charge pump capacitor terminal
12 CP1 Charge pump capacitor terminal
13 PGND1 Ground for charge pump circuitry
14 FB3 Feedback REG3
15 ENB3 Enable REG3, logic input, active high
16 LX3 Switch node REG3
17 VBB32 Input supply for REG3
18 VBB12 Input supply for REG1
19 LX1 Switch node REG1
20 ENB1 Enable REG1, logic input, active high
–PAD
3Exposed pad for enhanced thermal dissipation
1GND and PGND should be connected externally.
2The three VBBx pins should be connected together externally.
3Thermal pad should be connected to the ground (0 V) plane using thermal vias.
PAD
15
14
13
12
11
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
ENB1
LX1
VBB1
VBB3
LX3
LX2
VBB2
CPOR
PORZ
VCP
ENB3
FB3
PGND
CP1
CP2
FB1
VDD
GND
FB2
ENB2
Pin-out Diagram
Triple Output Step-Down Switching Regulator
A4490
5
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
ELECTRICAL CHARACTERISTICS1,2 at TA = 25°C, VBB = 6.0 to 34 V, VDD supplied externally, unless noted otherwise
Characteristics Symbol Test Conditions Min. Typ. Max. Unit
General
VBB Quiescent Current IBBON ENBx = high, ILOAD = 0 mA, VBB = 12 V,
current drawn by feedback resistors ignored – 1 2 mA
IBBOFF ENBx = 0 V 1 μA
VDD Supply Range VDD 3.3 5.5 V
VDD Quiescent Current IDD ENBx = high 6 mA
ENBx = 0 V 1 μA
REG1, REG2, and REG3
Feedback Input Bias Current IBIAS –400 –100 100 nA
Feedback Voltage VFB With respect to 0.8 V target voltage ±1.5 %
Output Voltage Regulation3VOUT VREGx = 5 V, IOUT = 0 to 1.5 A, TA = –20°C to 85°C –2.5 2.5 %
VREGx = 5 V, IOUT = 0 to 1.5 A, TA = –40°C to 85°C –3.5 3.5 %
PWM Frequency fSW 470 550 630 kHz
Maximum Duty Cycle Dmax 90 %
Minimum Duty Cycle Dmin 5 %
Buck Switch On-Resistance RDS(on)
TJ = 25°C, ILOAD = 1.5 A, VBB = 6.0 V 450 mΩ
TJ = 125°C, ILOAD = 1.5 A, VBB = 6.0 V 700 mΩ
TJ = 25°C, ILOAD = 1.5 A, VBB = 4.5 V 560 mΩ
TJ = 125°C, ILOAD = 1.5 A, VBB = 4.5 V 870 mΩ
Current Limit Threshold ILIM Peak current through switch with D = 0.9 2.0 A
Soft Start Duration tss 0.625 1.25 1.875 ms
Logic Inputs and Outputs
ENBx Input Voltage VIL 0.8 V
VIH 2.0 V
ENBx Input Hysteresis VI(hys) 300 500 mV
ENBx Input Current IIL V
IH 5 V –1 1 μA
PORZ Output (Open Drain) VPORZL I
PORZL = 1 mA, fault asserted 0.4 V
PORZ Output Leakage Current IPORZH V
PORZ = 5 V, fault not asserted –1 1 μA
Power-On Reset Duration tPOR C
POR = 470 nF 75 115 155 ms
Continued on the next page…
Triple Output Step-Down Switching Regulator
A4490
6
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Protection
VREGx Undervoltage Lockout Startup VREGUV(su) FB1, FB2, and FB3 rising 85 %VFB
VREGx Undervoltage Lockout Shutdown VREGUV(sd) FB1, FB2, and FB3 falling 80 %VFB
VREGx Undervoltage Lockout
Startup Hysteresis VREGUV(suhys) –5%
VBB Undervoltage Lockout Startup VBBUV(su) No external VDD supply, VBB rising 3.7 4.3 4.7 V
VBBCPUV(su) External VDD supply, VBB rising 3.8 4.2 4.6 V
VBB Undervoltage Lockout Shutdown VBBUV(sd) No external VDD supply, VBB falling 3.6 4.1 4.7 V
VBBCPUV(sd) External VDD supply, VBB falling 3.0 3.5 4.3 V
VBB Undervoltage Lockout
Shutdown Hysteresis
VBBUV(sdhys) No external VDD supply 500 mV
VBBCPUV(sdhys) External VDD supply 600 mV
VBB Undervoltage Warning Threshold VBBUV(por)
VBB falling (forces PORZ low); switchers continue
to operate 3.6 V
Junction Overtemperature Shutdown TJTSD Temperature rising 165 °C
Junction Overtemperature Shutdown
Hysteresis TJTSD(hys) Recovery = TJTSD –TJTSD(hys) 15 °C
1For input and output current specifications, negative current is defined as coming out of (sourcing) the specified pin.
2Specifications over the junction temperature range of –40°C to 125°C are assured by design and characterization.
3Average value of VOUT relative to target voltage. The effects of the feedback resistors are not taken into account.
ELECTRICAL CHARACTERISTICS1,2 (continued) at TA = 25°C, VBB = 6.0 to 34 V, VDD supplied externally, unless noted otherwise
Characteristics Symbol Test Conditions Min. Typ. Max. Unit
Triple Output Step-Down Switching Regulator
A4490
7
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Basic Operation
The A4490 contains three fixed frequency, buck switching con-
verters with peak current-mode control, including slope compen-
sation. Each converter can be independently turned on and off via
the enable inputs (EN1, EN2, and EN3), which are active high.
When enabled, the corresponding output is brought-up under the
control of a soft start routine, which avoids output voltage over-
shoot and minimizes input inrush current.
The output voltage is typically divided down by an external
potential divider, and is compared against an internal reference
voltage to produce an error signal, also known as the current
demand signal. The current signal through the buck switch is
converted into a voltage. This signal is then compared against the
current demand signal to create the required duty cycle.
At the beginning of each switching cycle, the buck switch is
turned on. When the current signal through the switch reaches the
level of the current demand signal, the on-time of the switch is
terminated. On the next switching cycle, the switch is turned on
again and the cycle is repeated.
One shared clock is used to define the switching frequency for
each regulator. Each of the three switching cycles (REG1, REG2,
and REG3) are phase shifted with respect to one another by 120°
in an attempt to minimize the pulsed current drawn from the
input filter capacitors. Under certain conditions, for example at
low VBB conditions and relatively high user-set output voltages,
switching overlap between channels is inevitable.
Under conditions, such as light loads or high VBB voltages, that
cause duty cycles (DC) of less than the minimum value, the
converter enters a pulse-skipping mode to ensure regulation is
maintained.
A charge pump regulator is provided to ensure a sufficient gate
drive is available for all three power switches across the full input
voltage range. This regulator allows operation even at very wide
operating duty cycles. On initial power-up, an internal regulator
is used to provide the bias supply for on-chip control functions.
Each regulator channel utilizes pulse-by-pulse current limiting in
the event of either a short circuit or an overload. If the overload
is applied long enough, the IC temperature may rise sufficiently
to cause the thermal shutdown circuit to operate. The part will
auto-restart under control of the soft start circuit after the thermal
disable condition is removed, and assuming all other conditions
are met. See the Shutdown section for more information.
Power Configuration
The A4490 supports alternative schemes for providing logic sup-
ply voltage on the VDD pin. In addition, the IC can be powered
up and down using either the VBB or ENB pins.
Powering VDD To minimize power dissipation, especially at
high input voltages, it is recommended that an external sup-
ply be applied to the VDD input pin. Typically, this voltage is
derived from one of the three regulated outputs that are set-up for
between 3.3 and 5 V (VREGx).
Another advantage of powering the VDD externally is that the
VBB undervoltage lockout level is lowered. To maximize the
run time of the switchers during a VBB power-down condition,
two alternative undervoltage shutdown conditions are supported,
depending on which VDD-powering configuration has been
implemented. When no external VDD is applied, the minimum
VBB, VBBUV(sd)
, is 4.1 V typical. When an external VDD is
applied, the minimum VBB, VBBCPUV(sd)
, is 3.5 V typical.
One note of caution when deriving VDD from a VREG output:
during initial application of VBB, the internal bias supply auto-
matically starts from the internal regulator because VREG has not
yet reached regulation. This means the startup threshold is deter-
mined by VBBUV(su) (4.3 V typical) because there is no external
VDD. When VREG has begun to supply VDD externally, the
shutdown threshold reduces to VBBCPUV(sd) (3.5 V typical). This
assumes that VREG is present.
Powering Up and Down with VBB Referring to figure 1,
each of the enable inputs (ENBx) are held high by being tied to
the VBB rail via a 100 kΩ resistor and the VDD is supplied from
one of the regulator outputs. When the VBB voltage reaches the
minimum threshold, VBBUV(su) , the charge pump supply (VCP)
ramps up. When VBB + VCP has reached the minimum thresh-
old VBBCPUV(su), the soft start routines are initiated (tSS) for all
three regulator channels (VREGx). When all three regulators
have reached the 85% FBx threshold, the power-on-reset timer
is initiated. After the power-on-reset period, tPOR
, has elapsed,
PORZ goes high, indicating that all the regulators and VBB are in
specification.
Functional Description
Triple Output Step-Down Switching Regulator
A4490
8
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
VBB
V
BBUV(su)
V
BBUV(por)
V
BBCPUV(su)
t
POR
t
SS
V
BBCPUV(sd)
VBB+VCP
VREG1
VREG2
VREG3
PORZ
0V
V
BB
+5.5 V
85%FB1
85%FB3
85%FB2
ENB1
0V
VBB
VREG1
VREG2
VREG3
t
SS
t
SS
t
SS
t
POR
t
POR
t
POR
PORZ
85%FB1
85%FB3
85%FB2
ENB3
ENB2
80%FB2 85%FB2
Figure 1. Timing diagram for powering up and down using the VBB pin
Figure 2. Timing diagram for powering up and down using the ENB pin
Triple Output Step-Down Switching Regulator
A4490
9
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
When the VBB voltage starts to fall below the undervoltage warn-
ing level, VBBUV(por) , of 3.6 V typical, the PORZ flag resets.
This gives advance warning to the system controller that the VBB
voltage is falling. Note that this feature is only guaranteed when
VDD is supplied externally. During this interval, the three switch-
ers continue to operate.
While VBB falls further, the VCP supply also tends to fall, which
degrades the drive voltage to the series switches. In addition,
the higher voltage rails start to fall out of regulation first, as the
corresponding maximum duty cycle (Dmax) for these particular
converters is reached.
The regulators that have the lower output voltages achieve some
level of steady state, before the A4490 powers down when all
of the corresponding VBB undervoltage thresholds have been
reached. For example, it may be possible for a 1 V output to
continue to operate down to a VBB of 3.4 V typical, if the VDD
supply is derived externally. The extent of this effect depends on
a myriad of factors, including input and output filter capacitance,
output loads, gate drive amplitude, MOSFET RDS(on), and so
forth.
Powering Up and Down with Enable Referring to figure 2,
VBB is present and the UVLO start-up thresholds, VBBUV(su)
and VBBCPUV(su) have been reached. Each of the regulators are
enabled in turn. Initially, VREG1 is enabled and is brought-up
under the control of the soft start circuit (tSS). Before VREG1
reaches 85% FB1, VREG2 is enabled and is brought-up under a
separate soft start control.
When both regulators have reached their respective 85% FB
thresholds, the power-on-reset (POR) timer is initiated. Note that
the POR timer is only enabled after all of the enabled regulators
reach their corresponding 85% FB levels. After the power-on-
reset time, tPOR , has elapsed, if the FB levels of VREG1 and
VREG2 are not below their respective 80% FB levels, then the
PORZ signal will go high.
At some point later, if VREG3 is enabled, then the PORZ is
reset and VREG3 is brought-up under the control of the soft start
circuit. When the 85% FB3 threshold is reached, the POR timer
is initiated. After tPOR has elapsed, if all the FB levels are above
their respective 80% FB levels, then the PORZ signal will go
high.
Note that if any regulator channel is not enabled, the channel
will not influence PORZ. To avoid multiple signal changes of the
PORZ signal, it is recommended that the system be designed such
that all three regulator channels are within specification before
tPOR has elapsed.
If any regulator channel drops below 80% FB, the PORZ signal
will be reset. If the voltage then recovers to within 85% FB, the
POR timer is initiated again. Note that a soft start is not initiated
when the feedback voltage drops below the 80% FB level. This is
to allow a rapid auto-restart in the event of an overload or similar
fault. If a soft start is required, it is recommended that on receipt
of the PORZ reset signal, the system controller disables and then
re-enables the relevant regulator channels again. As soon as the
last regulator is disabled the PORZ signal is reset.
Power on Reset The power-on-reset duration, tPOR , is deter-
mined by selecting an appropriate capacitor connected to the
CPOR pin. The value of tPOR can be determined by the following
formula:
tPOR = 2.131 ×105 × CPOR . (1)
The PORZ output goes high when both VBB is above the under-
voltage warning levels, and the FB pins of the regulators that are
enabled are > 85% of the V
REG voltage.
Because the external capacitor is charged via a 5 μA current
source, care must be taken in the layout to avoid additional leak-
age paths. The capacitor should be positioned adjacent to the
CPOR pin, and the ground connection to the A4490 GND pin
should be as short as possible.
It is recommended that the tPOR period be set to exceed the
start-up phases of all three regulators, to avoid the possibility of
multiple triggerings of the PORZ output.
Output Voltage Selection The output voltage on each of the
three regulators is set by the following relationship, shown here
for the VREG1 channel:
R1R2
=,
VFB
VREG1 1
(2)
where R2 (connected between GND and the FB1 pin) should
be a value between 4.7 and 12 kΩ. R1 is connected between the
output rail and the FB1 pin. VREG1 is the set output regulator
voltage. VFB is the reference voltage.
The tolerances of the feedback resistors influence the voltage set-
point. It is therefore important to consider the tolerance selection
when targeting an overall regulation figure.
Triple Output Step-Down Switching Regulator
A4490
10
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
The bias current, IBIAS
, flowing out of the FB1 node into R2, will
introduce a small voltage offset to the output.
Enable Each regulator channel can be individually enabled via
the corresponding ENBx pin. If any channel is required to start-
up automatically after the VBB voltage is applied, that particu-
lar channel should have the ENB pin tied to the VBB rail via a
pull up resistor.
This resistor should be selected to limit the current to less than
the maximum specified value, 1 mA. This prevents the internal
protection clamps from turning on. It is recommended that a
100 kΩ pull-up resistor be used. This would ensure the current
remains below the maximum value when VBB = 36 V.
Soft Start Each regulator channel contains a soft start circuit. A
soft start cycle is initiated when the appropriate regulator enable
input is set to high; the VBB, charge pump, and bias supply volt-
ages are above the minimum values; and no thermal shutdown
condition exists. Note that an overload or short circuit will not
cause a soft start cycle, unless a thermal shutdown event occurs.
During a soft start cycle, the reference voltage is ramped from
0 to 0.8 V typical, which in turn forces the current demand signal
to increase in a linear fashion.
Shutdown All converter channels are disabled in the event of
either a thermal shutdown event or an undervoltage on VBB
(VBBUV(sd) or VBBCPUV(sd)).
As soon as the above fault conditions have been removed, and
assuming the ENB inputs are enabled, the appropriate channels
will auto-restart under control of the soft start.
Current Limit The typical peak current limit for each channel is
specified as 2.5 A minimum, with a duty cycle of 0.9. The mini-
mum current limit occurs at maximum duty cycle (0.9), because
the slope compensation has a maximum effect under this condi-
tion. As the duty cycle reduces, the current limit increases. This
means for applications that operate with a narrow duty cycle, it is
possible to operate with a load current greater than 2.0 A.
Figure 3 illustrates the typical peak current limit versus duty
cycle. For example, it is possible to operate with a peak current
limit of 3.75 A with a duty cycle of 0.3.
As well as ensuring the peak current limit is not exceeded, under
worst case load and input voltage conditions, it is also important
to check the implications on the thermal performance. See the
Thermal Considerations section.
Component Selection
Inductor The inductance value, L, determines the ripple current.
It is important to ensure that the minimum current limit is not
exceeded under worst-case conditions: VBB(min), ILOAD(max),
fSW(min), and L(min).
It is recommended that gapped ferrite solutions be used as
opposed to powdered iron solutions, the latter of which exhibit
relatively high core losses that can have a large impact on long
term reliability.
Inductors are typically specified at two current levels, rms cur-
rent and saturation current. With regard to the rms current, it is
important to understand how the rms current level is specified,
in terms of ambient temperature. Some manufacturers quote an
ambient only, whilst others quote a temperature that includes a
self-induced temperature rise. For example, if an inductor is rated
for 85°C and includes a self-induced temperature rise of 25°C
at maximum load, then the inductor cannot be safely operated
beyond an ambient temperature of 60°C at full load. The rms cur-
rent can be assumed to be simply the maximum load current, with
perhaps some margin to allow for overloads, and so forth.
The first stage of determining the inductor value is to specify a
peak-to-peak ripple current of typically about 20% to 25% of the
maximum load.
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
0.5
1.0
0.5
0
020406080100
Duty Cycle (%)
Current Limit (A)
Figure 3. Current limit versus duty cycle
Triple Output Step-Down Switching Regulator
A4490
11
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
The maximum peak-to-peak ripple current, IRIPPLE , occurs at the
maximum input voltage. Therefore the duty cycle, D, should be
found under these conditions (for the VREG1 channel):
D(min) =.
VREG1+Vf
VBB(max)+Vf
(3)
where Vf is the forward voltage drop of the recirculation diode.
The required inductance can be found:
L
(min)
D(min)
=,
I
RIPPLE
V
BB
(max) V
REG1
f
SW
(min)
1
××
(4)
Note that the manufacturers inductance tolerance should also
be taken into account. This value may be as high as ±20%. The
peak-to-peak current should not exceed 1 A, to avoid instability
in the innermost circuit loops due to insufficient slope compensa-
tion.
The maximum peak current can be found from to ensure that the
saturation current level of the chosen inductor is not exceeded:
Isat ILOAD +
=.
IRIPPLE
2
(5)
Recommended inductor manufacturers and ranges are:
• Taiyo Yuden: NR6045 series for 1.5 A outputs
• Taiyo Yuden: NRG4026 series for 1.0 A outputs
• Sumida: CDH74 series for 1.5 A outputs
Output Capacitor In the interests of size, cost and perfor-
mance, it is highly recommended that ceramic X5R or X7R
capacitor types be used. When using ceramic capacitors another
important consideration is the E-field effects on the actual value
of the capacitor. To minimize the effects of the capacitance
reducing with output voltage, it is recommended that the working
voltage of the capacitor be considerably more than the set output
voltage. As a suggestion, it is recommended that 6.3 V-rated
capacitors should be used for output voltages of 3.3 V and below.
For output voltages of 5 V, a 10 V-rated capacitor should be used.
The output capacitor determines the output voltage ripple and is
used to close the control loop. To guarantee stability, the capaci-
tance has to increase as the output voltage is reduced. This is
actually reasonable from a ripple voltage point of view, as the
ripple voltage is typically specified as a percentage of output
voltage.
The following table outlines what the minimum output capaci-
tance should be for a given output voltage:
Output Voltage
(V)
Minimum Output Capacitance
(μF)
15 3.3
12 4.7
510
3.3 20
1.8 to 2.5 30
<1.8 40
Capacitance values with greater than the above values can be
used with the effect of reducing the bandwidth. This may be nec-
essary in systems that have extremely low ripple/noise require-
ments.
The output ripple is largely determined by the output capacitance
and the effects of ESR and ESL can largely be ignored assuming
good layout practice is observed.
The output voltage ripple can be approximated to:
VRIPPLE ,
IRIPPLE
8 × fSW × COUT
(6)
When using ceramic capacitors, there is generally no need to con-
sider the current carrying capability due to the negligible heating
effects of the ESR. Also, the rms current flowing into the output
capacitor is extremely low.
Input Capacitor Again it is highly recommended that ceramic,
X5R or X7R capacitors be used.
The value of the input capacitance determines the amount of
current ripple (EMI) that appears at the source (VBB supply)
terminals. The amounts of current flowing in and out of the input
capacitor depend on the relative impedances between the input
capacitor impedance and the source impedance. To achieve a low
impedance filter solution it is recommended to place at least two
capacitors in parallel.
Triple Output Step-Down Switching Regulator
A4490
12
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Again, there is generally no need to consider the heating effects
of the rms current flowing through the ESR. Also, the phase-
shifting of the input current drawn by each of the regulators helps
to reduce the overall rms current.
Flyback Diode This diode conducts during the switch off-time.
A Schottky diode is recommended to minimize both the forward
drop and switching losses.
The worst case dissipation occurs at maximum VBB
, when the
duty cycle, D, is a minimum. The average current through the
diode can be found:
IDIODE(av) = ILOAD × (1 – D(min)) . (7)
The forward voltage drop, Vf , can be found from the diode
characteristics by using the actual load current (not the average
current).
The static power dissipation can be found:
PSTAT = ILOAD(av) × Vf . (8)
It is also important to take into account the thermal rating of
the package, RθJA
, and the ambient temperature, to ensure that
enough heatsinking is provided to maintain the diode junction
temperature within the safe operating area for the device.
To minimize the heating effects from the A4490 on the diode and
vice-versa, it is recommended that the diode be mounted on the
reverse side of the printed circuit board.
Support Components POR capacitor (C11), charge pump
capacitor (C1), reservoir capacitor (C2) and VDD filter capacitor
(C12) should be ceramic X5R or X7R.
Thermal Considerations
To ensure the A4490 operates in the safe operating area, which
effectively means restricting the junction temperature to less than
150°C, several checks should be made. The general approach
is to work out what thermal impedance (RJA) is required to
maintain the junction temperature at a given level, for a particular
power dissipation.
Another factor worth considering is that other power dissipating
components on the system PCB may influence the thermal per-
formance of the A4490. For example, the power loss contribution
from the recirculation diode and the sense resistor may cause the
junction temperature of the A4490 to be higher than expected.
The following steps can be used as a guideline for determining a
suitable thermal solution. It should be noted that this process is
usually an iterative one to achieve the optimum solution. These
factors can be considered as follows:
Step 1. Estimate the maximum ambient temperature, TA(max) , of
the application.
Step 2. Define the maximum junction temperature, TJ(max). Note
that the absolute maximum is 150°C.
Step 3. Determine the worst case power dissipation, PD(max).
The evaluation should consider these at maximum load and mini-
mum VBB. Contributors are switch static and dynamic losses, and
control losses. These are described in the following sections
Switch Static Losses The following steps can be used to
determine switch static losses:
Estimate the maximum duty cycle:
D(max) =,
VREG
+ Vf
VBB
(min) + Vf
(9)
where Vf is the forward voltage drop of the Schottky diode under
the given load current.
Estimate the RDS(on) of the each regulator switch at the given
junction temperature:
RDS(on)TJ RDS(on)25C 1+
=.
200
TJ – 25
(10)
Note that if the VBB range is restricted to between 4.5 and 5.5 V,
the RDS(on) increases. For example, the RDS(on) at 25°C with a
VBB greater than 6 V is 450 mΩ typical, as stated in the Electri-
cal Characteristics table. Under the same temperature conditions,
with the VBB = 4.5 V, the RDS(on) is 560 mΩ typical. For VBB
voltages between 4.5 and 6 V, the RDS(on) can be found by linear
approximation. For more information on operating the A4490
between a VBB voltage of 4.5 and 5.5 V, see the Power Configu-
rations section.
The static loss for each switch can be determined:
PSTAT = ILOAD2 × D(max) × RDS(on)TJ , (11)
where ILOAD is the load for that particular regulator channel.
Triple Output Step-Down Switching Regulator
A4490
13
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Switch Dynamic Losses The following can be used to deter-
mine switch dynamic losses:
Both turn on and turn off losses can be estimated:
PDYN VBB
(min)
fSW
30
10–9
=,
ILOAD
2
(12)
where fSW is the switching frequency.
Control Losses The following steps can be used to determine
control losses:
PVBB = IBBON × VBB , (13)
where IBBON is the quiescent current assuming all three regulators
are on.
PVDD = IVDD × VDD , (14)
where IVDD and is the quiescent current on VDD.
Total Losses The total losses can now be estimated:
PTOTAL = PSTAT1 + PSTAT2 + PSTAT2
+PDYN1 +PDYN2 + PDYN3
+PVBB + PVDD . (15)
Thermal Impedance The thermal impedance required for the
solution can now be determined:
RQJA =
.
TJ
TA
PTOTAL
(16)
Example
Selected parameters:
VBB(min) = 6 V
VREG1 = 5 V at 1 A
VREG2 = 3.3 V at 1 A
VREG3 = 1.8 V at 800 mA
TA= 70°C
TJ = 115°C
Vf = 0.4 V
(a) Switch static losses
VREG1 duty cycle, D10.84
==
5+0.4
6+0.4
VREG2 duty cycle, D20.58
==
3.3+0.4
6+0.4
VREG3 duty cycle, D30.34
==
1.8+0.4
6+0.4
The RDS(on) of each switch can be found:
RDS(on)TJ 450×10–3 1+
=0.653 Ω
=
200
115 – 25
The static loss of each switch can be found:
PSTAT1 = 12 × 0.84 × 0.653 = 0.55 W
PSTAT2 = 12 × 0.58 × 0.653 = 0.379 W
PSTAT3 = 0.82 × 0.34 × 0.653 = 0.14 W
(b) Switch dynamic losses
PDYN1 0.045 W
==
1
2
30
10–9
6
500
103
PDYN2 0.045 W
==
1
2
30
10–9
6
500
103
PDYN3 0.036 W
==
0.8
2
30
10–9
6
500
103
(c) Control losses
PVBB = 0.005 × 6 = 0.03 W
PVDD = 0.001 × 3.3 = 0.003 W
(d) The total power dissipation can now be found:
PTOTAL = 0.55 + 0.379 + 0.14 + 0.045
+ 0.045 + 0.036 + 0.03 + 0.003 = 1.228 W
(e) The thermal impedance required for the solution can be
found:
RQJA 36.6 °C/W
==
115
70
1.228
For this particular solution a high thermal efficiency board is
required to ensure the junction temperature is kept below 115°C.
For maximum effectiveness, the PCB pad area underneath the
thermal pad of the A4490 should be exposed copper. Several
thermal vias (say between 4 and 8) should be used to connect
the thermal pad to the internal ground plane. If possible, an
additional thermal copper plane should be applied to the bottom
side of the PCB and connected to the thermal pad of the A4490
through the vias.
Triple Output Step-Down Switching Regulator
A4490
14
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
This calculation assumes no thermal influence from other compo-
nents. If possible, it is advisable to mount the flyback diodes on
the reverse side of the printed circuit board. Ensure low imped-
ance electrical connections are implemented between board
layers.
PCB Layout Guidelines The ground plane is largely dictated
by the thermal requirements described in the previous section.
The ground referenced power components should be referenced
to a star ground, located away from the A4490 to minimize
ground bounce issues.
A small, local, relatively quiet ground plane near the A4490 should
be used for the ground referenced support components, to mini-
mize interference effects of ground noise from the power circuitry.
Figure 4 illustrates the recommended grounding architecture.
To avoid ground bounce and offset issues, it is highly recom-
mended that the ground referenced feedback resistors (R2, R4,
and R6) should be connected as close to the GND connection of
the A4490 as possible.
A local quiet ground plane around these components can be
implemented, however, this ground plane should have a high
impedance connection to the star connection of the power stages.
If a ground plane is used, it is recommended that it does not
overlap the switching nodes (LX1, LX2, and LX3) to avoid the
possibility of noise pick-up. To minimize the possibility of noise
injection issues, it is recommended to isolate the ground plane
around high impedance nodes such as: FBx, ENBx and CPOR.
In terms of grounding the power components, a star connection
should be made to minimize the ground loop impedances. Note
that although a ground plane may be required to meet the thermal
characteristics of the solution it is still imperative to implement
a ground star connection for the power components. The ground
for the charge pump (PGND) should be connected to the thermal
vias.
Figures 5 and 6 below illustrates the importance of keeping the
ground connections as short as possible and forming good star
connections.
Figure 5 also illustrates the current conduction paths during the
on-cycle of the switching FET. The following points should be
noted:
• The capacitor CIN should be placed as close as possible to the
VBB terminals. The capacitance should be split between the VBB
terminals for VREG1 and VREG3 and the VBB terminal for VREG2.
QL
R
LOAD
V
BB
V
REG
D
C
OUT
C
IN
LX
Star Connection
Figure 5. FET on-cycle current conduction paths
R
LOAD
V
BB
V
REG
C
OUT
C
IN
QL
D
LX
Star Connection
Figure 6. FET off-cycle current conduction paths
Cin Cout
D
Star Connection
A4490
Internal Ground Plane
Thermal Vias
Power Circuitry
A4490 Support
Com
p
onents
Local “Quiet’
Ground Plane GND PGND
Figure 4. Ground plane configurations
Triple Output Step-Down Switching Regulator
A4490
15
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
The VBB terminals for VREG1 and VREG2 should be connected
via short and wide traces to the VBB terminal for VREG3.
• Each inductor should be connected as close as possible to the
respective switching FET (LX1, LX2, and LX3) and output
capacitors.
Figure 6 shows the current conduction path during the off-cycle
of the switching FET. The following points should be noted:
• The diode D should be placed as close as possible to both the
switching FET and the inductor.
• Support components: POR capacitor (C11), charge pump ca-
pacitor (C1), reservoir capacitor (C2), and VDD filter capacitor
(C12) should be located as close as possible to their respective
terminal connections. The ground referenced capacitors should
be connected as close to the GND terminal as possible.
Powering Configurations The following three diagrams show
typical configurations for providing power to the application. The
middle diagram corresponds to the typical application shown on
the front page.
VBB
VDD
Only VBB supplied
6 to 34 V
LX
L
V
REG
DC
Comments:
-Simple configuration, only one supply required
-Increased power losses at higher VBB voltages
-V
BB
start-up = 4.3 V (typical), shutdown = 4.1 V
(typical)
VBB
VDD
VDD applied externally (first option)
6 to 34 V
LX
L
V
REG
DC
Comments:
-Reduced power losses at higher VBB voltages
-V
BB
start-up = 4.3 V (typical), shutdown = 3.5 V
(typical). In this case, the start-up threshold
(V
BBUV(su)
) is higher because V
REG
is not present
VBB
VDD
VDD applied externally (second option)
4.5 to 5.5 V
LX
L
V
REG
DC
Comments:
-Power restricted as V
BB
< 6 V, due to increase in
R
DS(on)
of buck switches
-V
BB
start-up = 4.2 V (typical), shutdown = 3.5 V (typical)
,,,+7,7 ; é ’iuuwuu I: c A 2 ‘A c D + C 3 ‘ C l D C nnmnn A
Triple Output Step-Down Switching Regulator
A4490
16
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Package ES, 20-Pin QFN
0.95
C
SEATING
PLANE
C0.08
21X
20
20
2
1
1
2
20
2
1
A
ATerminal #1 mark area
Coplanarity includes exposed thermal pad and terminals
BExposed thermal pad (reference only, terminal #1
identifier appearance at supplier discretion)
For Reference Only
(reference JEDEC MO-220WGGD)
Dimensions in millimeters
Exact case and lead configuration at supplier discretion within limits shown
C
D
D
C
Reference land pattern layout (reference IPC7351
QFN50P400X400X80-21BM)
All pads a minimum of 0.20 mm from all adjacent pads; adjust as necessary
to meet application process requirements and PCB layout tolerances; when
mounting on a multilayer PCB, thermal vias at the exposed thermal pad land
can improve thermal dissipation (reference EIA/JEDEC Standard JESD51-5)
4.10
0.30
0.50
4.10
0.50
0.75 ±0.05
2.60
2.60
0.25 +0.05
–0.07
0.40 +0.15
–0.10
4.00 ±0.15
4.00 ±0.15 2.60
2.60
B
PCB Layout Reference View
Triple Output Step-Down Switching Regulator
A4490
17
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
For the latest version of this document, visit our website:
www.allegromicro.com
Copyright ©2008-2013, Allegro MicroSystems, LLC
Allegro MicroSystems, LLC reserves the right to make, from time to time, such de par tures from the detail spec i fi ca tions as may be required to
permit improvements in the per for mance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that
the information being relied upon is current.
Allegro’s products are not to be used in life support devices or systems, if a failure of an Allegro product can reasonably be expected to cause the
failure of that life support device or system, or to affect the safety or effectiveness of that device or system.
The in for ma tion in clud ed herein is believed to be ac cu rate and reliable. How ev er, Allegro MicroSystems, LLC assumes no re spon si bil i ty for its
use; nor for any in fringe ment of patents or other rights of third parties which may result from its use.
Revision History
Revision Revision Date Description of Revision
Rev. 10 June 26, 2012 Update IDD and undervoltage lockout

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