LT1795 Linear Technology, LT1795 Datasheet - Page 8

• Manufacturer Part Number: LT1795
• Description: Dual 500mA/50MHz Current Feedback Line Driver Amplifier
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
quiescent current to 33mA with V
control is desired in addition to reduced quiescent current,
then the circuits in Figures 5 to 7 can be employed.
LT1795
8
ON
Q1: 2N3904 OR EQUIVALENT
CHARACTERISTIC)
Q1A, Q1B: ROHM IMX1 or FMG4A (W/INTERNAL R
(0V)
OFF
(0V)
(SEE SHDN PIN
FOR R
I
PROG
CURRENT vs
(3.3V/5V)
(3.3V/5V)
Figure 7. Setting Amplifier Supply Current Level
with ON/OFF Control, Version 3
ON
OFF
(0V)
OFF
Figure 6. Setting Multiple Amplifier Supply
Current Levels with ON/OFF Control, Version 2
VOLTAGE
EXT
(3.3V/5V)
Figure 5. Setting Amplifier Supply Current
Level with ON/OFF Control, Version 1
0.5mA
ON
= 0
R
10k
R
B1
SHDN1
10k
R
R
B
I
R
PROG
SHDN
EXT
Q1A
U
V
Q1
+
SHDNREF
SHDN
10
11
OFF
(0V)
U
(3.3V/5V)
ON
10 SHDN
11 SHDNREF
CONTROL
R
R
10k
R
I
PULLUP
B2
SY
B
S
SHDN2
>500k
W
1795 F07
)
1795 F05
= 15V. If ON/OFF
V
INTERNAL
LOGIC THRESHOLD
~ 1.4V
Q1B
+
INTERNAL
LOGIC THRESHOLD
~1.4V
10 SHDN
11 SHDNREF
U
1795 F06
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:
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-
V
(I
(I
V
V
R
R
H
SHDN
CC
S
S
1
2
)
)
OFF
(0V)
= Logic High Level
ON
OFF
= Positive Supply Voltage
(3.3V/5V)
I
= Supply Current Fully On
ON
S
V
= Shutdown Pin Voltage 1.4V
= Supply Current Partially On
SHDN
ON
115
–
R1
•
/
Figure 8. Partial Shutdown
V
V
I
S
H
H
115
SHDNREF
OFF
•
SHDN
V
10
11
CC
•
R2
I
S
V
CC
ON
–
CONTROL
–
V
I
SY
SHDN
I
S
1795 F08
OFF
INTERNAL
LOGIC THRESHOLD
~ 1.4V
I
S
OFF