MAX9938 Maxim, MAX9938 Datasheet - Page 7

• Manufacturer Part Number: MAX9938
• Description: The MAX9938 high-side current-sense amplifier offers precision accuracy specifications of VOS less than 500µV (max) and gain error less than 0
• Manufacturer: Maxim

Choose R
A high R
to drop due to IR loss. For minimal voltage loss, use the
lowest R
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 V
pin. Therefore, the OUT voltage swing is limited by the
minimum voltage at RS+.
and
V
minimum RS+ voltage. For best performance with a
3.6V supply voltage, select R
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.
In the linear region (V
components to accuracy: input offset voltage (V
gain error (GE). For the MAX9938, V
and gain error is 0.5% (max). Use the linear equation:
to calculate total error. A high R
currents to be measured more accurately because off-
sets are less significant when the sense voltage is larger.
SENSE
V
V
OUT
OUT
SENSE
full scale should be less than V
SENSE
SENSE
(max) = V
= (gain ± GE) x V
value.
R
value causes the power-source voltage
Applications Information
based on the following criteria:
SENSE
_______________________________________________________________________________________
Choosing the Sense Resistor
RS+
OUT Swing vs. V
OUT
=
(min) - V
G I
< V
×
V
SENSE
OUT
SENSE
LOAD
OUT
SENSE
SENSE
(max)
Precision Current-Sense Amplifier
(max)), there are two
(max)
± (gain x V
to provide approxi-
OS
CC
value allows lower
RS+
(max) - V
OUT
= 500μV (max)
supply voltage
Voltage Loss
and V
/gain at the
Accuracy
OS
OS
OH
SENSE
)
) and
1µA, 4-Bump UCSP/SOT23,
At high current levels, the I
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 V
the MAX9938 allows the use of small sense resistors to
reduce power dissipation and reduce hot spots.
Because of the high currents that flow through R
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.
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 V
decreases the small-signal bandwidth of the current-
sense amplifier and reduces noise at OUT.
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.
Optional Output Filter Capacitor
OUT
constant during sampling. This also
Efficiency and Power Dissipation
2
R losses in R
Kelvin Connections
Input Filters
SENSE
can be
SENSE
OS
of
7
,