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 mini-
mum voltage at RS+.
VOUT (max) = VRS+ (min) - VSENSE (max) - VOH
and
OUT
SENSE
RG I (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 approximately
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 application. These can be
increased by use of a higher minimum 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 offsets
are less significant when the sense voltage is larger.
Efciency 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-
frequency 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 tran-
sients, 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 voltage offset at the MAX9938 output.
MAX9938 nanoPower, 4-Bump UCSP/SOT23,
Precision Current-Sense Amplier
www.maximintegrated.com Maxim Integrated
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