Aug 1998 Battery Charger IC Doubles as Current Sensor

DESIGN IDEAS
Battery Charger IC Doubles
as Current Sensor
5V
C2
0.33µF
sible to again mirror this signal to
provide a ground referenced output.
Circuit operation is as follows: The
LT1620 operates by producing a voltage between the V CC pin and the AVG
pin that is 10× the voltage across sense
resistor R5. C2 filters this voltage. An
internal op amp has its noninverting
input at the AVG pin (pin 8), its inverting input at the PROG pin (pin 7) and
its output at the IOUT pin (pin 2). With
the circuit connected as shown in Figure 1, this amplifier will force enough
current through R4 to make the voltage drop on R4 equal to the voltage
across C2. This current is mirrored
through R3 and is filtered by C3, producing a clean, ground-referenced, DC
output voltage. Resistor R2 cancels a
C1
1000pF
LT1620
7
AVG
PROG
1
6
VCC
SENSE
2
4
IN–
IOUT
3
5
IN+
GND
R5
0.02Ω
INPUT
R4
10k
1%
R2
1.2M
Q1
TP0610T
R3
31.6k
IL
4.0
3.5
R1
100k
8
LOAD
small built-in offset in the LT1620’s
amplifiers. The output voltage obeys
the following relationship: V O = IL (R5 •
R3 • 10)/R4. Changing the value of R3
selects different scale factors.
The circuit yields excellent linearity
over a wide range of loads and input
voltages. The curve shown in Figure 2
was measured with the sense resistor
referenced to a 5V input source. The
curve looks the same even at inputs
over 25V, so only one curve is presented. Maximum input voltage is 36V.
There is a small offset at no load, but in
a typical microprocessor-based data
acquisition system, only a simple
2-point calibration is needed to obtain
absolute accuracy.
OUTPUT
C3
0.33µF
OUTPUT VOLTAGE (V)
It’s always fun to find applications
for an IC that its designer never
intended. The circuit shown in Figure 1
is such a design. In many cases, a
circuit is required to provide a groundreferenced output voltage that is
proportional to a measured current.
Frequently, the current must be measured with a shunt in the positive rail
that may be well above ground and,
worse yet, may vary considerably with
time. The LT1620 was originally
intended as a contr oller for a
synchronous buck regulator in battery-charger applications. The normal
operating mode for this IC is to mirror
a current signal down to a 5V reference
supply. By adding a single small-signal
MOSFET and a few resistors, it is pos-
by Craig Varga
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
Figure 1. Current sensor schematic
New Device Cameos
LT1671: A 60ns, 450µA,
Single-Supply Comparator
with Complementary Outputs
and Output Latch
The LT1671 is a low power (450µ A),
fast (60ns), single-supply comparator
designed to operate on either single
5V or ±5V supplies. It has a maximum
offset voltage of 2.5mV, complementary TTL compatible outputs and
output-latch capability. The wide
input-voltage range extends from the
bottom supply rail to within 1.5V of
Linear Technology Magazine • August 1998
the top supply rail. The LT1671 is
made with Linear Technology’s new
6GHz complementary bipolar technology, which results in a dramatically
improved speed/power product
compared to industry-standard comparators developed in slower NPN-only
technologies.
These features combine to make
the LT1671 well suited for applications such as high performance crystal
oscillators, single-supply voltage-tofrequency converters and high speed,
1
2
3
4
LOAD CURRENT (A)
5
6
Figure 2. Transfer function
high accuracy level detectors. The
LT1671 is offered in SO-8 and is pin
compatible with the industry-standard
LT1016 and LT1116 comparators.
The LTC1596-1:
Ultra-Accurate, Low Power,
16-Bit Multiplying,
Current-Output DAC
Clears to Midscale
The latest addition to LTC’s family of
16-bit current-output DACs is the
multiplying LTC1596-1. Based on the
LTC1596, this DAC features the same
true 16-bit performance (DNL and INL,
1LSB maximum), the low glitch im37
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