LT1795(RevA) 查看數據表（PDF） - Linear Technology
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LT1795 Datasheet PDF : 12 Pages
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LT1795
APPLICATIO S I FOR ATIO
quiescent current to 33mA with VS = ±15V. If ON/OFF
control is desired in addition to reduced quiescent current,
then the circuits in Figures 5 to 7 can be employed.
V+
OFF
ON
(0V)
(3.3V/5V)
RSHDN
RB
10k
Q1
Q1: 2N3904 OR EQUIVALENT
10 SHDN
11 SHDNREF
INTERNAL
LOGIC THRESHOLD
~1.4V
1795 F05
Figure 5. Setting Amplifier Supply Current
Level with ON/OFF Control, Version 1
V+
RPULLUP
>500k
10 SHDN
11 SHDNREF
RSHDN1
RSHDN2
ON
RB1
10k
ON
RB2
10k
OFF
Q1A OFF
Q1B
(0V)
(0V)
(3.3V/5V)
(3.3V/5V)
Q1A, Q1B: ROHM IMX1 or FMG4A (W/INTERNAL RB)
1795 F06
Figure 6. Setting Multiple Amplifier Supply
Current Levels with ON/OFF Control, Version 2
ON
OFF
REXT
(0V)
IPROG
(3.3V/5V)
IPROG ≅ 0.5mA
FOR REXT = 0Ω
(SEE SHDN PIN
CURRENT vs
VOLTAGE
CHARACTERISTIC)
SHDN
10
SHDNREF
11
INTERNAL
LOGIC THRESHOLD
ISY ~ 1.4V
CONTROL
1795 F07
Figure 7. Setting Amplifier Supply Current Level
with ON/OFF Control, Version 3
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:
( ) ( ) R1= 115•VH
IS
–
ON
IS
OFF
( ) 115 • VCC – VSHDN
( ) ( ) ( ) ( ) R2 =
VSHDN / VH
•
IS
–
ON
IS
OFF
+
IS
OFF
where
VH = Logic High Level
(IS)ON = Supply Current Fully On
(IS)OFF = Supply Current Partially On
VSHDN = Shutdown Pin Voltage ≈1.4V
VCC = Positive Supply Voltage
VCC
ON
R1
OFF
(0V)
(3.3V/5V)
R2
10
SHDN
INTERNAL
LOGIC THRESHOLD
ISY ~ 1.4V
CONTROL
11
SHDNREF
1795 F08
Figure 8. Partial Shutdown
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-
1795fa
8
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