8

LT1121/LT1121-3.3/LT1121-5

S

APPLICATI

I FOR ATIO

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 (30

µA). In shutdown the

quiescent current drops to only 16

µA. 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.

Adjustable Operation

The adjustable version of the LT1121 has an output

voltage range of 3.75V to 20V. 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

output current change of 1mA to 150mA is –12mV typical

12

375

12

38

V

V

mV

mV

.

––









×

Thermal Considerations

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

2. Ground pin current multiplied by the input voltage:

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 limit-

ing designed to protect the device during overload condi-

tions. For continuous normal load conditions the maxi-

mum junction 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

Figure 1. Adjustable Operation

IN

LT1121

GND

SHDN

1121 • F01

+

ADJ

OUT

R2

R1

R2

R1