6

LT1573

–

+

–

+

–

+

+

+

SHDN

GND

COMP

ERROR

AMP

LATCH

DRIVE

NORMALLY

OFF

NORMALLY

ON

NORMALLY

OFF

NORMALLY

OFF

EXTERNAL

CAPACITOR

C1

C2

C4

C3

Q4

+

+

+

+

THERMAL

SHUTDOWN

FB

GND

1573 BD

R1

R2

FOR FIXED

VOLTAGE

VERSION

1.265V

7

6

5

2

6

3

4

4

81

Q5

Q3

Q1

Q2

S3

S4

S2

S1

The basic block diagram of the LT1573 is shown above.

The regulating loop consists of a 1.265V reference, an

error amplifier, a Darlington driver and an external PNP

pass transistor. The 1.265V reference feeds the noninvert-

ing input of the error amplifier. The error amplifier drives

the Darlington connected transistor pair Q1 and Q2. The

collector of Q1 comes out to the DRIVE pin and is used to

drive the base of an external PNP power transistor as

shown in Figure 1. The error amplifier will adjust the drive

current to the external PNP power transistor to maintain

the feedback pin voltage at 1.265V. The LT1573 provides

overcurrent protection by means of a timed latch function.

Base current to the external PNP transistor is limited by

placing a resistor between the base of the transistor and

(the DRIVE pin saturation voltage) the output of the

comparator C2 switches high; S1, which is normally

closed, opens and the external capacitor connected to the

is equal to approximately 28

is equal to approximately 7

µA. If the fault condition goes

C4 will switch high and S4 will close and latch the output

off. The device will stay latched because latching current

shutdown circuitry will close S4 and latch the device off

with no timing delay. Comparator C1 is used to override

the latching function during start-up. If the difference

between the output voltage and the input voltage is less

comparator C1 output goes high which closes S2. Current

charging. Comparator C3 is used for system shutdown

and latch reset. If SHDN pin voltage is higher than shut-

high, shutting down the regulator and closing switch S3.

latch.

BLOCK DIAGRA

FU CTIO AL DESCRIPTIO