Datasheet

At frequencies below the output corner frequency, the
MAX9937 itself provides excellent 100dB (DC) com-
mon-mode rejection. At higher frequencies, as the CMRR
of the MAX9937 degrades, the output filter formed by
R
OUT
and C
OUT
helps boost the common-mode rejection
of the circuit.
Input Common-Mode Voltages
> 28V and < 0V
Short-duration overvoltages on the battery line are isolat-
ed from the RSP and RSN pins of the MAX9937 by the
use of input resistors R
RSP
and R
RSN
. The input ESD
clamp structure is designed so that the device can with-
stand short-duration (< 1s) overvoltages up to 40V when
using resistors R
RSP
and R
RSN
of 500Ω or greater as
shown in the Typical Application Circuit.
Approximately 40mA flows out of each ESD diode during
this condition (20V/500Ω). This current is less than the
50mA absolute maximum specification for the RSN and
RSP pins.
Skewed Input Offset Voltage
for Production Calibration
Due to low temperature drift of input bias current and
input offset voltage in the MAX9937, the part can be used
to provide powerful application and system benefits not
normally attainable from other current-sense amplifiers
on the market. For example, input resistors R
RSP
and
R
RSN
can be intentionally mismatched so as to introduce
an external, controlled input offset voltage into the circuit.
Doing so allows microcontroller firmware to trim out input
offset voltages completely by using production-line cal-
ibration during the manufacturing process or in system
operation as long as a zero load- current condition is
forced. Only minimal temperature-drift-based errors in the
resistor and in the bias currents then remain.
V
OS-FINAL
= V
OS
+ I
B-
x R
RSN
- I
B
+ x R
RSP
while gain = R
OUT
/R
RSP
.
Since gain can be fixed by choosing R
OUT
and R
RSP
,
a positive offset voltage can be induced by varying the
value of R
RSN
compared to R
RSP
.
For example:
R
OUT
= 10kΩ, R
RSP
= 500Ω fixes gain = 20V/V. Now,
choosing R
RSN
= 2.5kΩ, and knowing ∆I
B
= ±12% of I
B
,
the additional V
OS
becomes:
∆V
OS
(max) = (5.6µA x 2500) ± (0.12 x 5.6µA x 2500) -
(5.6µA x 500) = 11.2mV ± 1.7mV
∆V
OS
(min) = (0.8µA x 2500) ± (0.12 x 0.8µA x 2500) -
(0.8µA x 500) = 1.6mV ± 0.24mV
Since the minimum extra V
OS
introduced into the part
is greater than the maximum V
OS
of the current-sense
amplifier (= 1mV), the output of the current-sense amplifi-
er is always greater than zero even at zero sense voltage,
thus allowing the current-sense amplifier to be calibrated
at zero input current.
Operation with V
CC
= 0V (Shutdown)
The input terminals go into a high-impedance mode
when V
CC
= 0, as shown by the input bias current in
shutdown 1µA specification. Due to the low 20µA sup-
ply current, this then becomes a convenient way to put
the amplifier in shutdown simply by using a digital I/O
port of a microcontroller to power up/down the current-
sense amplifier. This can be especially useful in certain
battery-operated applications that need to implement
flexible power-management schemes.
PACKAGE
TYPE
PACKAGE
CODE
DOCUMENT NO.
LAND
PATTERN NO.
5 SC70 X5+1 21-0076 90-0188
GND
RSNOUT
1 5 RSPV
CC
MAX9937
SC70
TOP VIEW
2
3 4
+
www.maximintegrated.com
Maxim Integrated │ 10
MAX9937 Current-Sense Amplier with
Reverse-Battery Protection
Package Information
For the latest package outline information and land patterns
(footprints), go to www.maximintegrated.com/packages. Note
that a “+”, “#”, or “-” in the package code indicates RoHS status
only. Package drawings may show a different suffix character, but
the drawing pertains to the package regardless of RoHS status.
Chip Information
PROCESS: BiCMOS
Pin Conguration