Datasheet

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MAX34406
Quad Current-Sense Amplifier
with Overcurrent Threshold Comparators
Detailed Description
The MAX34406 quad-channel, unidirectional, high-side,
current-sense amplifier features a 2.0V to 28V input
common-mode range. This feature allows the monitoring
of current out of a voltage supply as low as 2.0V. The
device monitors current through a current-sense resistor
and amplifies the voltage across that resistor.
Current-sense amplifier output voltages (OUT1 to OUT4)
are compared to a fixed 1.0V reference; if V
OUTx
exceeds 1.0V, the corresponding overcurrent warning
output (OC1 to OC4) is asserted. If the enable input
(ENA) is logic-high, SHTDN asserts when any of the four
overcurrent outputs go logic-high. Assertion of SHTDN on
overcurrent can be delayed and/or filtered by attaching
an external capacitor to CDLY. Once SHTDN is latched
high impedance, it remains so until ENA is toggled.
The unidirectional current-sense amplifiers used in each
channel of the device have a well established history. For
each channel, an op amp is used to force the current
through an internal gain resistor at IN+, which has a value
of R1, such that its voltage drop equals the voltage drop
across an external sense resistor, R
SENSE
. There is an
internal resistor at IN- with the same value as R1 to mini-
mize offset voltage. The current through R1 is sourced by
a high-voltage p-channel FET. Its source current is the
same as its drain current, which flows through a second
gain resistor, R
OUTx
. This produces an output voltage,
V
OUTx
, whose magnitude is I
LOAD
x R
SENSE
x R
OUTx
/R1.
The gain accuracy is based on the matching of the two
gain resistors, R1 and R
OUTx
(Table 1). Total gain = 25V/V
for the MAX34406T, 50V/V for the MAX34406F, 100V/V
for the MAX34406H, and 200V/V for the MAX34406W.
The output is protected from input overdrive by use of a
6V clamp-protection circuit.
Applications Information
Choosing the Sense Resistor
Choose R
SENSE
based on the criteria detailed in the fol-
lowing sections.
Voltage Loss
A high R
SENSE
value causes the power-source voltage
to drop due to IR loss. For minimal voltage loss, use the
lowest R
SENSE
value.
OUTx Swing vs. V
INx+
and V
SENSE
The device is unique because the supply voltage for
the current-sense amplifier in each channel is the input
common-mode voltage for that channel (the average
voltage at INx+ and INx-). There are no separate supply
voltage pins for the current-sense amplifiers. Therefore,
the OUTx voltage swing for a given channel is limited by
the minimum voltage at IN+ for that channel.
V
OUTx(MAX)
= V
INx+(MIN)
- V
SENSE(MAX)
- V
OH
and
OUTx
SENSE
LOAD
V (MAX)
R
G I (MAX)
=
×
V
SENSE
full scale should be less than V
OUTx
/gain at the
minimum INx+ voltage. For best performance with a 3.6V
supply voltage, select R
SENSE
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 (V
OUTx
< V
OUTx(MAX)
), there are
two components to accuracy: input offset voltage (V
OS
)
and gain error (GE). For all variants of the device,
V
OS
= P 600FV (max); gain error is 0.6% (max) for the
MAX34406T/F/H or 0.8% (max) for the MAX34406W. Use
the linear equation to calculate total error:
OS
SENSE
V
Error (%) GE 100
V

=±×


where GE is gain error, V
SENSE
is the voltage across
the sense resistor R
SENSE
, and V
OS
is offset voltage. A
high R
SENSE
value allows lower currents to be measured
more accurately because offsets are less significant
when the sense voltage is larger.
Table 1. Internal Gain Setting Resistors
(Typical Values)
GAIN (V/V)
R1 (I) R
OUTx
(kI)
200 100 20
100 100 10
50 200 10
25 400 10