8

LT1795

APPLICATIO S I FOR ATIO

Figure 5. Setting Amplifier Supply Current

Level with ON/OFF Control, Version 1

Figure 6. Setting Multiple Amplifier Supply

Current Levels with ON/OFF Control, Version 2

10 SHDN

INTERNAL

LOGIC THRESHOLD

~1.4V

11 SHDNREF

1795 F05

10k

Q1

OFF

(0V)

ON

(3.3V/5V)

Q1: 2N3904 OR EQUIVALENT

10 SHDN

>500k

11 SHDNREF

1795 F06

10k

Q1B

ON

OFF

10k

Q1A

ON

OFF

(3.3V/5V)

(3.3V/5V)

(0V)

(0V)

Figure 7. Setting Amplifier Supply Current Level

with ON/OFF Control, Version 3

10

SHDN

11

SHDNREF

1795 F07

ON

OFF

CONTROL

INTERNAL

LOGIC THRESHOLD

~ 1.4V

(SEE SHDN PIN

CURRENT vs

VOLTAGE

CHARACTERISTIC)

(3.3V/5V)

(0V)

Figure 8. Partial Shutdown

10

SHDN

R1

R2

11

SHDNREF

1795 F08

ON

OFF

CONTROL

INTERNAL

LOGIC THRESHOLD

~ 1.4V

(3.3V/5V)

(0V)

Figure 8 illustrates a partial shutdown with direct logic

control. By keeping the output stage slightly biased on, the

output impedance remains low, preserving the line termi-

nation. The design equations are:

R

V

II

R

VV

VV

I

I

I

H

S

ON

S

OFF

CC

SHDN

SHDN

H

S

ON

S

OFF

S

OFF

1

115

2

115

=

()

(

) () ()









+

()

•

–

•–

/•

–

where

THERMAL CONSIDERATIONS

The LT1795 contains a thermal shutdown feature that

protects against excessive internal (junction) temperature.

If the junction temperature of the device exceeds the

protection threshold, the device will begin cycling between

normal operation and an off state. The cycling is not

harmful to the part. The thermal cycling occurs at a slow

rate, typically 10ms to several seconds, which depends on

the power dissipation and the thermal time constants of the

package and heat sinking. Raising the ambient tempera-

control is desired in addition to reduced quiescent current,

then the circuits in Figures 5 to 7 can be employed.