Datasheet
LT1999-10/LT1999-20/
LT1999-50
18
1999fb
applicaTions inForMaTion
V
OUT
V
REF
V
SHDN
LT1999
4k
0.8k
160k
160k
2µA
0.8k
4k
SHDN
V
+
V
+
V
+
+
–
+
–
8
1
2
3
4
7
6
5
R
G
V
+
V
+
V
S
5V
1999 F05
0.1µF
0.1µF
FUSE
STEERING
DIODE
LOAD
I
LOAD
R
SENSE
OFFON
5V
V
OUT
V
+IN
V
–IN
V
REF
V
SHDN
Figure 5. Using the LT1999 to Monitor a Fuse
Fuse Monitor
The inputs can be overdriven without fear of damaging
the LT1999. This makes the LT1999 ideal for monitoring
fuses if either +IN or –IN are shorted to ground while the
other is at the full common mode supply voltage (see
Figure 5). If the fuse in Figure 5 opens with the +IN tied
to the positive supply, the load will pull –IN
to GND. The
output will be forced to the positive V
+
supply rail. If it is
desired that the output be near ground if the fuse opens,
it is a simple matter of swapping the inputs. Precautions
should be followed: First, when the inputs are stressed
differentially due to the fuse blowing open, a large voltage
drop will be placed across the +IN to –IN pins, dissipating
power in the precision on-chip input resistors. Precaution
should be taken to prevent junction temperatures from
exceeding the Absolute Maximum ratings (see Note 3 in the
Electrical Characteristics section). Secondly, if the load is
inductive, and the fuse blows open without a clamp diode,
energy stored in the inductive load will be dissipated in
the LT1999, which could cause damage. A simple steering
diode as shown in Figure 5 will prevent this from happen-
ing, and will protect the LT1999 from damage.
Finally, the user should be aware that in fuse monitoring
applications with the sense voltage (V
SENSE
= V
+
IN
– V
–
IN
)
being driven in excess of –25V, the output of the LT1999
will undergo phase reversal (see Figure 6).
Figure 6. A Plot of the LT1999’s Output Voltage vs V
SENSE
(V
SENSE
= V
+IN
– V
–
IN
).
In Applications Where the Sense Voltage Is Driven in Excess of –25V, the Output
of the LT1999 Will Undergo Phase Reversal
V
SENSE
(V)
–60
V
OUT
(1V/DIV)
–30–45 –15 30
1999 F06
6015 450
V
OUT
PHASE REVERSAL FOR V
SENSE
< –25V
V
REF
= 2.5V