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MAX9937AXKT

Current-Sense Amplifier with Reverse-Battery Protection

Maxim Integrated 

MAX9937

Current-Sense Amplifier with

Reverse-Battery Protection

VBAT = 4V

TO 28V

RS+ RSENSE RS-

RRSP

499Ω

CIN*

220nF

RRSN

499Ω

RSP

GND

RSN

VCC

BIAS

BLOCK

LOAD

5V

MICROCONTROLLER

MAX9937

OUT

ADC

GAIN = VOUT = ROUT

VSENSE RRSP

ROUT

10kΩ

COUT*

1nF

*FILTER CAPACITORS ARE OPTIONAL.

Figure 1. Typical Application Circuit with Optional External Filtering

In this case, RRS = RRSP = RRSN, ∆RRS depends on the

tolerance of the RRS resistors used, and ∆IB = input offset

current of the amplifier.

The temperature drift of these parameters similarly add

up to give a final result.

Shown below is an example calculation of VOS at +25°C:

With VOS = ±1.2mV (max), IB = 5.6µA (max), ∆IB = ±12%

(max) of 5.6µA (max) = ±0.67µA (max), and RRS = 500Ω

with ±1% tolerance (i.e., ∆RRS = ±5Ω max)

VOS-FINAL = 1.25mV (max).

Flexible EMI Filtering

Real-world applications of current-sense amplifiers need

to measure currents precisely in the presence of a wide

variety of input transients. For example, fast load-current

transients when measuring at the input of a switching

buck regulator can cause high-frequency differential

sense voltages to occur at inputs of the MAX9937,

although the signal of interest is the average DC value.

Alternately, parasitic voltage pickup on a disconnected or

long cable can cause common-mode voltage transients to

occur at inputs of the MAX9937, which are required to be

rejected effectively.

The MAX9937 allows two methods of filtering to help

improve performance in the presence of input com-

mon-mode voltage and input differential-voltage tran-

sients (see Figure 1).

The capacitor CIN between RS+ and RSN helps filter

against input differential voltages, and prevents them from

reaching the MAX9937. The corner frequency of this filter

is determined by the choice of RRSN and CIN.

f C−IN

=

1

2πR RSN

×

CIN

Similarly, capacitor COUT from OUT to ground helps filter

the output voltage, thus providing not only differential

filtering, but also filtering for input common-mode tran-

sients that have made it past the MAX9937. The corner

frequency of this filter is similarly determined by choice of

ROUT and COUT. Note: The MAX9937 is a current-output

device, and has the ability to drive an infinite amount of

load capacitance.

f C−OUT

=

1

2πROUT × COUT

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