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LT1573C

Low Dropout PNP Regulator Driver

Linear Technology 

LT1573

APPLICATIO S I FOR ATIO

Thermal Considerations

The thermal characteristics of several components need

to be considered; the LT1573, the pass transistor and

resistor RD. Power dissipation should be calculated based

on the worst-case conditions seen by each component

during normal operation.

1. Power Dissipation of the LT1573: The worst-case

power dissipation in the LT1573 is a function of drive

current, supply voltage and the value of RD. Worst-case

dissipation for the LT1573 occurs when the drive cur-

rent is equal to approximately one half of its maximum

value. The worst-case power dissipation in the LT1573

can be calculated by the following formula:

( )2

PD =

VIN − VBE

4RD

(2)

RD > minimum RD for latch - off function

where,

VIN = the maximum input voltage to the circuit

VBE = the minimum emitter/base voltage of the PNP

pass transistor

Following the previous design example for selecting

resistor RD, the power dissipation of LT1573 is calcu-

lated from Eq (2):

( )2

5.5 − 0.65

( ) PD = 4 24 = 0.25W

For some operating conditions RD may be replaced with

a short. This is possible in applications where the

operating requirements (input voltage and drive cur-

rent) are at the low end and the output will not be

shorted. For RD = 0, the following formula may be used

to calculate the maximum power dissipation in the

LT1573:

PD = (VIN – VBE)(IDRIVE)

(3)

where,

VIN = the maximum input voltage

VBE = the minimum emitter/base voltage of the PNP

IDRIVE = the required maximum drive current

2. Power Dissipation of the Resistor RD: The worst-case

power dissipation in resistor RD needs to be calculated

so that the power rating of the resistor can be deter-

mined. The worst-case power dissipation in this resis-

tor will occur when the drive current is at a maximum.

The power dissipation can be calculated from the fol-

lowing formula:

( )2

PRD =

VIN − VBE − VDRIVE

RD

(4)

where,

VIN = the maximum input voltage

VBE = the minimum emitter/base voltage of the PNP

VDRIVE = the voltage at the LT1573 DRIVE pin

= VSAT of the DRIVE pin in the worst case

Following the previous design example, the power

dissipation of resistor RD is calculated from Eq (4):

( )2

5.5 − 0.65 − 0.39

PRD =

24

= 0.83W

3. Power Dissipation of the PNP Transistor: The worst-

case power dissipation in the PNP pass transistor is

simply equal to:

PPNP = (VIN – VOUT)(IOUT)

(5)

where,

VIN = the maximum input voltage

IOUT = the maximum output current

Following the previous design example, the power

dissipation of PNP transistor is calculated from Eq (5):

PPNP = (5.5 – 3.3)(5) = 11W

The LT1573 series regulators have internal thermal

limiting designed to protect the device during overload

conditions. For continuous normal load conditions,

the maximum 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. For surface mount devices, heat

sinking is accomplished by using the heat spreading

10

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