LT1521 datasheet(10/18 Pages) AD

10 / 18 page

LT1521/LT1521-3

LT1521-3.3/LT1521-5

Rev. C

For more information www.analog.com

APPLICATIONS INFORMATION

VIN

VOUT

LT1521

GND

SHDN

OUT

ADJ

LT1521 • F01

VOUT = 3.75V 1 +

+ (IADJ • R2)

VADJ = 3.75V

IADJ = 50nA AT 25°C

OUTPUT RANGE = 3.75V TO 20V

)

Figure 2. Adjustable Operation

The LT1521 is a 300mA low dropout regulator with

micropower quiescent current and shutdown. The device

is capable of supplying 300mA at a dropout of 0.5V and

operates with very low quiescent current (12µA). In shut-

down, the quiescent current drops to only 6µA. In addition

to the low quiescent current, the LT1521 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 LT1521 acts like it has a diode in series with

its output and prevents reverse current flow.

Adjustable Operation

The adjustable version of the LT1521 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 2. 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, 50nA at 25°C, flows through R2 into the

adjust pin. The output voltage can be calculated using the

formula in Figure 2. 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 positive temperature coefficient of

approximately 15ppm/°C. The adjust pin bias current has

a negative temperature coefficient. These effects 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 voltages 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 300mA is –20mV typical

at VOUT = 3.75V. At VOUT = 12V, load regulation would be:

(12V/3.75V)(–20mV) = –64mV

Thermal Considerations

The power handling capability of the device will be limited

by the maximum rated junction temperature (125°C). The

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.

TheLT1521seriesregulatorshaveinternalthermallimiting

designed to protect the device during overload conditions.

Forcontinuousnormalloadconditionsthemaximumjunc-

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.

For surface mount devices, heat sinking 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.

The following tables list thermal resistance for each pack-

age. Measured values of thermal resistance for several

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