Rainbow Electronics MAX9938 - Manual (Page 6)

MAX9938
1µA, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplifier
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Detailed Description
The MAX9938 unidirectional high-side, current-sense
amplifier features a 1.6V to 28V input common-mode
range. This feature allows the monitoring of current out
of a battery with a voltage as low as 1.6V. The
MAX9938 monitors current through a current-sense
resistor and amplifies the voltage across that resistor.
The MAX9938 is a unidirectional current-sense amplifier
that has a well-established history. An op amp is used
to force the current through an internal gain resistor at
RS+, which has a value of R
1 , such that its voltage drop
equals the voltage drop across an external sense resis-
tor, R
SENSE
. There is an internal resistor at RS- with the
same value as R 1 to minimize offset voltage. The cur-
rent through R 1 is sourced by a high-voltage p-channel
FET. Its source current is the same as its drain current,
which flows through a second gain resistor, R
OUT . This
produces an output voltage, V OUT , whose magnitude is
I LOAD x R SENSE x R OUT /R 1 . The gain accuracy is
based on the matching of the two gain resistors R 1 and
R OUT (see Table 1). Total gain = 25V/V for the
MAX9938T, 50V/V for the MAX9938F, and 100V/V for
the MAX9938H. The output is protected from input
overdrive by use of an output current limiting circuit of
7mA (typical) and a 6V clamp protection circuit.
Applications Information
Choosing the Sense Resistor
Choose R SENSE based on the following criteria:
Voltage Loss
A high R SENSE value causes the power-source voltage
to drop due to IR loss. For minimal voltage loss, use the
lowest R
SENSE value.
GAIN
(V/V)
R 1
( Ω)
R OUT
(k Ω)
100 100 10
50 200 10
25 400 10
Table 1. Internal Gain Setting Resistors
(Typical Values)
Typical Operating Circuit
V BATT = 1.6V TO 28V
R
SENSE
R 1
I
LOAD
R OUT
R 1
GND
OUT
P
MAX9938
RS+ RS-
ADC
LOAD
μC
V
DD
= 3.3V
10k Ω