Datasheet

12
LT1638/LT1639
16389fc
TYPICAL APPLICATIONS
U
–
+
1/2 LT1638
5V
V
+
200Ω
200Ω
0.2Ω
2k
0V TO 4.3V
1638/39 F03
V
OUT
= (2Ω)(I
LOAD
)
Q1
2N3904
LOAD
I
LOAD
Figure 4. Current Source
Figure 3. Positive Supply Rail Current Sense
SI PLIFIED SCHE ATIC
WW
Q10
D5
Q9
Q1
Q7
R2
1k
R3
1k
R4
8k
Q8
Q5
–IN
+IN
Q11 Q12
D4
ONE AMPLIFIER
D2
Q2
D1
Q6
Q13 Q14
R1
6k
R5
8k
Q4
10µA
+
Q15
Q19
D3
Q3
Q16 Q18
Q22
V
+
Q17 Q20
Q21
OUT
V
–
1638/39 SS
–
+
1/2 LT1638
R1
1.2V
R1
I
OUT
=
LT1634-1.2
I
OUT
1638/39 F04
V
CC
V
CC
The application in Figure 3 utilizes the Over-The-Top
capabilities of the LT1638. The 0.2Ω resistor senses the
load current while the op amp and NPN transistor form a
closed loop making the collector current of Q1
propor
tional to the load current. As a convenient monitor,
the 2k load resistor converts the current into a voltage. The
positive supply rail, V
+
, is not limited to the 5V supply of
the op amp and could be as high as 44V.
The Figure 4 application uses the LT1638 in conjunction
with the LT1634 micropower shunt reference. The supply
current of the op amp also biases the reference. The drop
across resistor R1 is fixed at 1.2V generating an output
current equal to 1.2V/R1.