LT1074/LT1076

10

Figure 5. Shutdown Circuitry

Figure 6. Undervoltage Lockout

Figure 7. Adding Hysteresis

DESCRIPTIO S

PI

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:

RR

VV

V

TP

SH

SH

12

=

−

()

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

–

+

C1

10 A

µ

300 A

µ

–

+

SHUTDOWN

PIN

C2

I

PIN

6V

Q1

TO TOTAL

REGULATOR

SHUTDOWN

2.3V

0.3V

LT1074•PD07

LIM

EXTERNAL

Hysteresis in undervoltage lockout may be accomplished

shutdown pin as shown in Figure 7. D1 prevents the

shutdown divider from altering current limit.

LT1074•PD08

R1

R2

5k

V

SHUT

GND

IN

LT1074

LT1074•PD09

R1

R2

R3

V

SHUT

IN

LT1074

OPTIONAL CURRENT

LIMIT RESISTOR

D1*

*1N4148

Trip Po

V

V

R

R

TP

int

.

==

+









235

1

1

2

VV

R

R

R

R

V

R

R

UTP

SH

=+









−









1

1

2

1

3

08

1

3

.

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

R

VV R

VV

R

R

SH

UTP

SH

3

08

1

1

1

2

=

−

()( )

−+









.

Example: An undervoltage lockout is required such that

Rk

VV

V

k

Rk

12 34

15

2 35

235

12 5

3

235 0 8 125

20 2 35 1

12 5

232

39

=

()

−

()

=

=

−

()( )

−+









=

.

.

.

.

..

.

.

.

.

.