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MAX9938F

1µA, 4-Bump UCSP/SOT23, Precision Current-Sense Amplifier

Maxim Integrated 

1µA, 4-Bump UCSP/SOT23,

Precision Current-Sense Amplifier

Applications Information

Choosing the Sense Resistor

Choose RSENSE based on the following criteria:

Voltage Loss

A high RSENSE value causes the power-source voltage

to drop due to IR loss. For minimal voltage loss, use the

lowest RSENSE value.

OUT Swing vs. VRS+ and VSENSE

The MAX9938 is unique since the supply voltage is the

input common-mode voltage (the average voltage at

RS+ and RS-). There is no separate VCC supply voltage

pin. Therefore, the OUT voltage swing is limited by the

minimum voltage at RS+.

VOUT(max) = VRS+ (min) - VSENSE (max) - VOH

and

RSENSE

=

VOUT (max)

G × I LOAD(max)

VSENSE full scale should be less than VOUT/gain at the

minimum RS+ voltage. For best performance with a

3.6V supply voltage, select RSENSE to provide approxi-

mately 120mV (gain of 25V/V), 60mV (gain of 50V/V),

30mV (gain of 100V/V), or 15mV (gain of 200V/V) of

sense voltage for the full-scale current in each applica-

tion. These can be increased by use of a higher mini-

mum input voltage.

Accuracy

In the linear region (VOUT < VOUT(max)), there are two

components to accuracy: input offset voltage (VOS) and

gain error (GE). For the MAX9938, VOS = 500μV (max)

and gain error is 0.5% (max). Use the linear equation:

VOUT = (gain ± GE) x VSENSE ± (gain x VOS)

to calculate total error. A high RSENSE value allows lower

currents to be measured more accurately because off-

sets are less significant when the sense voltage is larger.

Efficiency and Power Dissipation

At high current levels, the I2R losses in RSENSE can be

significant. Take this into consideration when choosing

the resistor value and its power dissipation (wattage)

rating. Also, the sense resistor’s value might drift if it is

allowed to heat up excessively. The precision VOS of

the MAX9938 allows the use of small sense resistors to

reduce power dissipation and reduce hot spots.

Kelvin Connections

Because of the high currents that flow through RSENSE,

take care to eliminate parasitic trace resistance from

causing errors in the sense voltage. Either use a four-

terminal current-sense resistor or use Kelvin (force and

sense) PCB layout techniques.

Optional Output Filter Capacitor

When designing a system that uses a sample-and-hold

stage in the ADC, the sampling capacitor momentarily

loads OUT and causes a drop in the output voltage. If

sampling time is very short (less than a microsecond),

consider using a ceramic capacitor across OUT and

GND to hold VOUT constant during sampling. This also

decreases the small-signal bandwidth of the current-

sense amplifier and reduces noise at OUT.

Input Filters

Some applications of current-sense amplifiers need to

measure currents accurately even in the presence of both

differential and common-mode ripple, as well as a wide

variety of input transient conditions. For example, high-fre-

quency ripple at the output of a switching buck or boost

regulator results in a common-mode voltage at the inputs

of the MAX9938. Alternatively, fast load-current transients,

when measuring at the input of a switching buck or boost

regulator, can cause high-frequency differential sense

voltages to occur at the inputs of the MAX9938, although

the signal of interest is the average DC value. Such high-

frequency differential sense voltages may result in a volt-

age offset at the MAX9938 output.

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