Datasheet

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MAX34406
Quad Current-Sense Amplifier
with Overcurrent Threshold Comparators
Efficiency and Power Dissipation
At high current levels, the I
2
R losses in R
SENSE
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 V
OS
of the
device 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 R
SENSE
,
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.
Minimizing Trace Resistance
PCB trace resistance from R
SENSE
to the INx+ inputs
contributes to gain error in the current-sense amplifiers.
Care should be taken to minimize this resistance (shown
as R
TRC
in Figure 1). Total gain including error caused by
trace resistance can be calculated as follows:
OUTx
TRC
R
G
R1 R
=
+
For example, assume a gain of 100V/V, as in the
MAX34406H. From Table 1, R1 = 100I and R
OUTx
= 10kI.
Then every 10mI of PCB trace resistance adds -0.01%
gain error.
Optional Output Filter Capacitor
When designing a system that uses a sample-and-hold
stage in the ADC, the sampling capacitor momentarily
loads OUTx and causes a drop in the output voltage. If
sampling time is very short (less than a microsecond),
consider using a ceramic capacitor across OUTx and
GND to hold V
OUTx
constant during sampling. This also
decreases the small-signal bandwidth of the current-
sense amplifier and reduces noise at OUTx.
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
device’s inputs. Alternatively, the 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 device’s inputs, although
the signal of interest is the average DC value. Such high-
frequency differential sense voltages can result in a volt-
age offset at the device output.
The device allows a method of filtering to help improve
performance in the presence of input common-mode
voltage and input differential voltage transients. Figure 2
shows a differential input filter.
The capacitor C
IN
between INx+ and INx- along with the
resistor R
IN
between the sense resistor and INx- helps
filter against input differential voltages and prevents them
from reaching the device.
The corner frequency of this filter is determined by the
choice of R
IN
, C
IN
, and the value of the input resistance
at INx- (R1). See Table 1 for R1 values at the different
gain options.
The value of R
IN
should be chosen to minimize its effect
on the input offset voltage due to the bias current at INx-.
Figure 1. Input Trace Resistance Figure 2. Differential Input Filter
R
SENSE
INx+
GND
INx-
OUTx
R
TRC
MAX34406
LOAD
R
SENSE
C
IN
INx+
GND
INx-
OUTx
R
IN
MAX34406
LOAD