LT1074 データシートの表示（PDF） - Linear Technology

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LT1074

Step-Down Switching Regulator

Linear Technology 

LT1074/LT1076

PI DESCRIPTIO S

10µ A

SHUTDOWN

PIN

–

C1

2.3V +

–

C2

0.3V +

VIN

300 µA

ILIM

PIN

Q1

6V

TO TOTAL

REGULATOR

SHUTDOWN

LT1074•PD07

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.

EXTERNAL

CLIM

R1

VIN

SHUT

R3 D1*

LT1074

I LIM

R2

OPTIONAL CURRENT

LIMIT RESISTOR

*1N4148

LT1074•PD09

Figure 7. Adding Hysteresis

Figure 5. Shutdown Circuitry

Undervoltage Lockout

Trip

Po int

=

VTP

=



2.35V 1+

R1

R2

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

To avoid errors due to the 10µA shutdown pin current, R2

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

( ) VTP − VSH

R1= R2

VSH

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 15kΩ, but the denominator in the formula for R2

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

R1

VIN

SHUT

LT1074

R2

5k

GND

LT1074•PD08

Figure 6. Undervoltage Lockout

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

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

VUTP

=

VSH1RR21

+

R1

R3

−



0.8V

R1

R3

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

( )( ) VSH − 0.8V R1

R3 =

VUTP

−



VSH1+

R1

R2

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.

( ) ( ) 15V − 2.35V

R1= 2.34k

= 12.5k

2.35V

( )( ) 2.35 − 0.8 12.5

R3 =

= 3.9k

20

−



2.351+

12.5

2.32

1074fd

10

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