LT1074CK datasheet(10/16 Pages) LINER

10 / 16 page

LT1074/LT1076

Figure 5. Shutdown Circuitry

Figure 6. Undervoltage Lockout

Figure 7. Adding Hysteresis

DESCRIPTIO S

Undervoltage Lockout

Undervoltage lockout point is set by R1 and R2 in Figure 6.

To avoid errors due to the 10

µA shutdown pin current, R2

is usually set at 5k, and R1 is found from:

−

()

VTP = Desired undervoltage lockout voltage

VSH = Threshold for lockout on the

shutdown pin = 2.45V

If quiescent supply current is critical, R2 may be increased

up to 15k

Ω, but the denominator in the formula for R2

should replace VSH with VSH – (10µA)(R2).

–

10 A

300 A

–

SHUTDOWN

VIN

TO TOTAL

REGULATOR

SHUTDOWN

2.3V

0.3V

LT1074•PD07

LIM

EXTERNAL

CLIM

Hysteresis in undervoltage lockout may be accomplished

by connecting a resistor (R3) from the ILIM pin to the

shutdown pin as shown in Figure 7. D1 prevents the

shutdown divider from altering current limit.

LT1074•PD08

SHUT

GND

LT1074

LT1074•PD09

SHUT

ILIM

LT1074

OPTIONAL CURRENT

LIMIT RESISTOR

D1*

*1N4148

Trip Po

int









235

If R3 is added, the lower trip point (VIN descending) will be

the same. The upper trip point (VUTP) will be:

UTP









−









If R1 and R2 are chosen, R3 is given by:

VV R

UTP

−

()( )

−+









Example: An undervoltage lockout is required such that

the output will not start until VIN = 20V, but will continue

to operate until VIN drops to 15V. Let R2 = 2.32k.

12 34

2 35

235

12 5

235 0 8 125

20 2 35 1

12 5

232

()

−

()

−

()( )

−+









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