9

LT1573

50

Ω. For circuits that do not require high output current or

up to 200

Ω. For the D45H11, the emitter-base voltage is

a function of base and collector current. Table 2 lists some

useful operating points for the D45H11. These points were

empirically determined using a sampling of devices.

(mA)

(A)

(V)

1

0.2

0.65

7

1

0.75

23

2

0.80

45

3

0.85

66

4

0.90

100

5

0.95

Design Example

Given the following operating requirements:

1. The first step is to determine the required drive current

for the D45H11. Dropout voltage must be less than 1.2V

at 5A output current. From Table 1, a drive current of

100mA will give 0.7V dropout voltage at an output

current of 5A. This satisfies the operating require-

ments.

voltage for this design is 0.95V. The current through

DRIVE pin current equals 119mA, which can be read

from the typical performance characteristics curve to

Eq (1):

The next lowest 5% value is 24

Ω.

Overcurrent Latch-Off

In addition to limiting the base drive current, the resistor

latch-off function. There is a minimum value for this

resistance. It is calculated by Equation 1 with the drive

limits the power dissipation in the LT1573. In some

is possible in circuits where an overload is unlikely and the

is not included in the circuit, the regulator is protected

against the overcurrent condition only by the thermal

certain amount of base drive current is available to the

external PNP transistor. An overcurrent or output short

condition will demand a base drive current greater than the

LT1573 can supply. The internal drive transistor will

saturate. A time-out latch will be triggered by this

overcurrent condition to turn off the regulator system. The

time-out period is determined by an external capacitor

connected between the LATCH and GND pins. The time-

out period is equal to the time it takes for the capacitor to

charge from 0V to the latch threshold which is equal to

current source and is a function of input voltage and

temperature as shown in the typical performance charac-

teristics curve. At 25

°C, the typical latch charging current

ranges from 7.2

µA with 3V input to 8µA with 7V input. If

the overcurrent or output short condition persists longer

than the time-out period, the regulator will be shut down.

Otherwise, the regulator will function normally. In the

latch-off mode, some extra current is drawn from the input

to maintain the latch. The latching current is a function of

input voltage and temperature as shown in the typical

performance characteristic curve. At 25

°C, the typical

latching current ranges from 0.3mA with 3V input to

9.5mA with 7V input. The latch can be reset by recycling

input power, by grounding the LATCH pin or by putting the

device into shutdown.

APPLICATIO S I FOR ATIO