LT1573C 查看數據表（PDF） - Linear Technology

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LT1573C

Low Dropout PNP Regulator Driver

Linear Technology 

LT1573

BLOCK DIAGRA

VIN 6

SHDN 3

GND 4

+

VSHDNTH

C3

–

+

I3

NORMALLY

OFF

S3

NORMALLY

OFF

+

ILATCH

S4

+

ICHRG

2 LATCH

Q4

EXTERNAL

C4

VLATCHTH + CAPACITOR

CEXT

THERMAL

SHUTDOWN

+

I2

NORMALLY

OFF

S2

VOUT

7

C1

VIODTH

VIN

COMP

FB

6

8

1

+

DRIVE 5

IDISCHRG

S1

NORMALLY

ON

Q3

C2

VDRSAT

Q2

Q1

ERROR

AMP

Q5

1.265V

4 GND

R1

R2

FOR FIXED

VOLTAGE

VERSION

1573 BD

FU CTIO AL DESCRIPTIO

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. When the DRIVE pin drops below VDRSAT

(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

LATCH pin is allowed to charge. Discharge current IDISCHRG

is equal to approximately 28µA and charging current ICHRG

is equal to approximately 7µA. If the fault condition goes

away before CEXT charges to the latch threshold, C2 will

switch back low, S1 will close and Q4 will discharge CEXT.

If the fault condition persists long enough for CEXT to

charge up to the latch threshold (VLATCHTH), comparator

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

off. The device will stay latched because latching current

ILATCH is greater than the pull-down current of Q4. Thermal

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

than the input-output differential threshold (VIODTH),

comparator C1 output goes high which closes S2. Current

source I2 then drives base of Q4 which prevents CEXT from

charging. Comparator C3 is used for system shutdown

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

down threshold VSDTH, the comparator C3 output goes

high, shutting down the regulator and closing switch S3.

Current I3 will drive Q4 to discharge CEXT, resetting the

latch.

6

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