LT1638/LT1639
18
16389fg
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1998
LT 1011 REV G • PRINTED IN USA
RELATED PARTS
TYPICAL APPLICATION
PART NUMBER DESCRIPTION COMMENTS
LT1078/LT1079
LT2078/LT2079
Dual/Quad 55μA Max, Single Supply, Precision Op Amps Input/Output Common Mode Includes Ground, 70μV V
OS(MAX)
and 2.5μV/°C Drift (Max), 200kHz GBW, 0.07V/μs Slew Rate
LT1178/LT1179
LT2178/LT2179
Dual/Quad 17μA Max, Single Supply, Precison Op Amps Input/Output Common Mode Includes Ground, 70μV V
OS(MAX)
and 4μV/°C Drift (Max), 85kHz GBW, 0.04V/μs Slew Rate
LT1366/LT1367 Dual/Quad Precision, Rail-to-Rail Input and Output Op Amps 475μV V
OS(MAX)
, 500V/mV A
VOL(MIN)
, 400kHz GBW
LT1490/LT1491 Dual/Quad Over-The-Top Micropower, Rail-to-Rail Input and
Output Op Amps
Single Supply Input Range: –0.4V to 44V, Micropower 50μA
per Amplifi er, Rail-to-Rail Input and Output, 200kHz GBW
LT1636 Single Over-The-Top Micropower Rail-to-Rail Input and Output
Op Amp
55μA Supply Current, V
CM
Extends 44V above V
EE
,
Independent of V
CC
; MSOP Package, Shutdown Function
The battery monitor in Figure 5 also demonstrates the
LT1638’s ability to operate with its inputs above the
positive rail. In this application, a conventional amplifi er
would be limited to a battery voltage between 5V and
ground, but the LT1638 can handle battery voltages as
high as 44V. When the battery is charging, Amp B senses
the voltage drop across R
S
. The output of Amp B causes
Q2 to drain suffi cient current through R
B
to balance the
input of Amp B. Likewise, Amp A and Q1 form a closed
loop when the battery is discharging. The current through
Q1 or Q2 is proportional to the current in R
S
and this cur-
rent fl ows into R
G
and is converted into a voltage. Amp
D buffers and amplifi es the voltage across R
G
. Amp C
compares the output of Amp A and Amp B to determine
the polarity of current through R
S
. The scale factor for
V
OUT
with S1 open is 1V/A. With S1 closed the scale
factor is 1V/100mA and currents as low as 500μA can
be measured.
R
A
, 2k
Q2
2N3904
S1
S1 = OPEN, GAIN = 1
S1 = CLOSED, GAIN = 10
R
A
= R
B
V
S
= 5V, 0V
10k
90.9k
V
OUT
LOGIC
1638/39 F05
LOGIC HIGH (5V) = CHARGING
LOGIC LOW (0V) = DISCHARGING
R
G
10k
Q1
2N3904
R
S
, 0.2Ω
CHARGER
VOLTAGE
R
A',
2k
R
B'
,
2k
R
B
, 2k
V
BATT
= 12V
I
BATT
+
LOAD
V
OUT
(R
S
)(R
G
/R
A
)(GAIN)
V
OUT
GAIN
I
BATT
= = AMPS
–
+
A
1/4 LT1639
–
+
B
1/4 LT1639
–
+
C
1/4 LT1639
–
+
D
1/4 LT1639
Figure 5. Battery Monitor
