The LT1121 is a micropower low dropout regulator with
shutdown, capable of supplying up to 150mA of output
current at a dropout voltage of 0.4V. The device operates
with very low quiescent current (30µA). In shutdown the
quiescent current drops to only 16µA. In addition to the
low quiescent current the LT1121 incorporates several
protection features which make it ideal for use in battery-
powered systems. The device is protected against both
reverse input voltages and reverse output voltages. In
battery backup applications where the output can be held
up by a backup battery when the input is pulled to ground,
the LT1121 acts like it has a diode in series with its output
and prevents reverse current flow.
The adjustable version of the LT1121 has an output voltage
range of 3.75V to 30V. The output voltage is set by the
ratio of two external resistors as shown in Figure 1. The
device servos the output voltage to maintain the voltage
at the adjust pin at 3.75V. The current in R1 is then equal
to 3.75V/R1. The current in R2 is equal to the sum of the
current in R1 and the adjust pin bias current. The adjust
pin bias current, 150nA at 25°C, flows through R2 into the
adjust pin. The output voltage can be calculated according
to the formula in Figure 1. The value of R1 should be less
than 400k to minimize errors in the output voltage caused
by the adjust pin bias current. Note that in shutdown the
output is turned off and the divider current will be zero.
Curves of Adjust Pin Voltage vs Temperature and Adjust
Pin Bias Current vs Temperature appear in the Typical
Performance Characteristics. The reference voltage at the
adjust pin has a slight positive temperature coefficient of
approximately 15ppm/°C. The adjust pin bias current has
a negative temperature coefficient. These effects are small
and will tend to cancel each other.
The adjustable device is specified with the adjust pin tied
to the output pin. This sets the output voltage to 3.75V.
Specifications for output voltage greater than 3.75V will
be proportional to the ratio of the desired output voltage
to 3.75V (VOUT/3.75V). For example: load regulation for an
output current change of 1mA to 150mA is –12mV typical
at VOUT = 3.75V. At VOUT = 12V, load regulation would be:
Power handling capability will be limited by maximum
rated junction temperature (125°C). Power dissipated by
the device will be made up of two components:
1. Output current multiplied by the input/output voltage
differential: IOUT • (VIN – VOUT), and
2. Ground pin current multiplied by the input voltage:
IGND • VIN.
The ground pin current can be found by examining the
Ground Pin Current curves in the Typical Performance
Characteristics. Power dissipation will be equal to the
sum of the two components listed above.
The LT1121 series regulators have internal thermal limiting
designed to protect the device during overload conditions.
For continuous normal load conditions the maximum junc-
tion temperature rating of 125°C must not be exceeded.
It is important to give careful consideration to all sources
of thermal resistance from junction to ambient. Additional
heat sources mounted nearby must also be considered.
Heat sinking, for surface mount devices, is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Copper board stiffeners and plated
through holes can also be used to spread the heat gener-
ated by power devices. Tables 1 through 5 list thermal
resistances for each package. Measured values of thermal
resistance for several different board sizes and copper
areas are listed for each package. All measurements were
taken in still air, on 3/32" FR-4 board with 1oz copper. All
Figure 1. Adjustable Operation
1121 • F01
VOUT = 3.75V 1 + + IADJ • R2
VADJ = 3.75V
IADJ = 150nA AT 25°C
OUTPUT RANGE = 3.75V TO 30V