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MAX9937AXKT

Current-Sense Amplifier with Reverse-Battery Protection

Maxim Integrated 

MAX9937

Current-Sense Amplifier with

Reverse-Battery Protection

Pin Description

PIN

NAME

FUNCTION

1

VCC

Power Supply. Bypass to GND with a

0.1µF capacitor.

2

GND Ground

3

OUT Current Output

4

RSN

Load-Side Connection Through

External RRSN Resistor

5

RSP

Supply-Side Connection Through

External RRSP Resistor

Detailed Description

The MAX9937 unidirectional high-side, current-sense

amplifier features a 4V to 28V input common-mode volt-

age range that is independent of supply voltage (VCC =

2.7V to 5.5V). The MAX9937 monitors the current through

a current-sense resistor by converting the sense voltage

to a current output (OUT). Gain is set by the ratio of an

output resistor (ROUT) and an input resistor (RRSP).

High-side current monitoring with the MAX9937 does not

interfere with the ground path of the load, making it useful

for a variety of battery/ECU monitoring.

Robust input ESD structure allows input common-mode

voltages to exceed the 28V maximum operating input

range for short durations, making the MAX9937 ideal

for applications that need to withstand short-duration

load-dump conditions. The MAX9937 is able to withstand

reverse-battery conditions by a suitable choice of input

resistors (RRSN, RRSP). See the Input Common-Mode

Voltages > 28V and < 0V section.

Current-Sense Amplifier Operation

The MAX9937 current-sense amplifier operation is

best understood as a specialized op-amp circuit with a

p-channel FET in the feedback path. The op amp forces

a current through an external gain resistor at RSP (RRSP,

see the Typical Application Circuit) so that its voltage

drop equals the voltage drop across the external sense

resistor, RSENSE, making the voltage at RSP the same as

RSN. An external resistor at RSN (RRSN) has the same

value as RRSP to minimize input offset voltage due to

input bias currents.

The current through RRSP is now sourced by the

high-voltage p-channel FET into an external resistor

(ROUT) at OUT. This produces an output voltage whose

magnitude is given by the following equations:

VSE= NSE ILOAD × R SENSE

= VOUT

VSENSE

×

R OUT

R RSP

The gain accuracy is primarily determined by the match-

ing of the two gain resistors, RRSP and ROUT. The volt-

age gain error of the MAX9937 is less than 1.5%.

Total gain = 20V/V with ROUT = 10kΩ

and RRSP = 500Ω.

Low temperature drift of input bias currents and input

offset currents minimizes their impact on total input offset

voltage of the current-sense amplifier.

Applications Information

Choosing RSENSE

To measure lower currents more accurately, use a high

value for RSENSE. The high value develops a higher

sense voltage that reduces the effect of offset voltage

errors of the internal op amp. In applications monitoring

very high currents, however, RSENSE must be able to

dissipate the I2R losses. If the resistor’s rated power

dissipation is exceeded, its value may drift or it may fail

altogether, causing large differential voltages to develop

between RSP and RSN.

To minimize the effect of input offset voltage by produc-

tion calibration, see the Skewed Input Offset Voltage for

Production Calibration section. This can help reduce the

size of the sense resistor in high-current applications, as

well as measure wide-dynamic-range currents without

sacrificing accuracy.

If ISENSE has a large high-frequency component, mini-

mize the inductance of RSENSE and use input differential

filters (see the Flexible EMI Filtering section). Low-

inductance metal-film resistors are best suited for these

applications.

Calculation of Total Input Offset Voltage

Because of the use of op-amp style architecture, cal-

culation of total input offset voltage involves the same

methodology as is used for any standard op-amp cir-

cuit. Interaction of the input bias currents and tolerance

of the external resistors, combined with the core input

offset voltage of the op amp, are important to consider.

Finally, RSS (root-sum-of-squares) calculation for all

these uncorrelated sources of error gives the final input

offset voltage.

(VOS−FI= NAL )2 (VOS )2 + (IB × ∆RRS )2 + (∆IB ×RRS )2

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