8

LT1573

Current Limit

For regulator circuits using the LT1573, current limiting is

achieved by limiting the base drive current to the external

PNP pass transistor. This means that the actual system

current limit will be a function of both the current limit of

the LT1573 and the Beta of the external PNP. Motorola

provides the following Beta information for the D45H11.

4A. For other PNP transistors, the user should first find out

the Beta information from the external PNP transistor

manufacturer to determine the appropriate LT1573 base

drive current limit. The current limit of the regulator

system then can be achieved by selecting the appropriate

user should first choose the value of the drive current that

will give the required value of output current and dropout

voltage. For a circuit using the D45H11 as a pass transistor

this can be done using Table 1. For circuits using transis-

tors other than D45H11, the user must characterize the

transistor to determine the drive current requirements for

the specified output current and dropout voltage. In gen-

eral, it is recommended that the user choose the lowest

value of drive current that will satisfy the output current

requirements. This will minimize the stress on circuit

components during overload conditions.

(1)

where,

pass transistor

output current with dropout voltage as low as 0.35V. The

40MHz which enables the regulator, composed of this

PNP transistor and the LT1573, to handle the load changes

of several amps in a few hundred nanoseconds with a

minimum amount of output capacitance.

The following sections describe how specifications can be

determined for the basic regulator based on the LT1573

and D45H11 from Motorola. To determine the specifica-

tions for regulators formed by the LT1573 and other PNP

transistors, a similar method can be used.

Dropout Voltage

The dropout voltage of an LT1573-based regulator circuit

external PNP transistor when it is driven with a base

current equal to the available drive current of the LT1573.

The LT1573 is guaranteed to sink 250mA of base current

(440mA typ). The available drive current of the LT1573 can

with the DRIVE pin. Table 1 lists some useful operating

points for the D45H11. These points were empirically

determined using a sampling of devices.

Table 1. D45H11 Dropout Voltage

TYPICAL

DRIVE CURRENT

OUTPUT CURRENT

DROPOUT VOLTAGE

(mA)

(A)

(V)

20

1

0.20

20

2

0.50

40

2

0.25

40

3

0.50

60

3

0.25

60

4

0.70

80

4

0.45

100

4

0.35

100

5

0.70

150

5

0.40

200

5

0.35

150

6

0.65

200

6

0.45

250

7

0.50

APPLICATIO S I FOR ATIO