MAX9928/MAX9929
-0.1V to +28V Input Range, Micropower,
Uni-/Bidirectional, Current-Sense Amplifiers
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Detailed Description
The MAX9928F/MAX9929F micropower uni-/bidirectional,
current-sense amplifiers feature -0.1V to +28V input
common-mode range that is independent of the supply
voltage. This wide input voltage range feature allows the
monitoring of the current flow out of a power supply dur-
ing short-circuit/fault conditions, and also enables high-
side current sensing at voltages far in excess of the
supply voltage (V
CC
). The MAX9928F/MAX9929F oper-
ate from a 2.5V to 5.5V single supply and draw a low
20µA quiescent supply current.
Current flows through the sense resistor, generating a
sense voltage V
SENSE
(Figure 1). The comparator sens-
es the direction of the sense voltage and configures the
amplifier for either positive or negative sense voltages
by controlling the S1 and S2 switches.
For positive V
SENSE
voltage, the amplifier’s inverting
input is high impedance and equals V
IN
- V
SENSE.
The
amplifier’s output drives the base of Q1, forcing its non-
inverting input terminal to (V
IN
- V
SENSE
); this causes a
current to flow through R
G1
equal to |V
SENSE
|/R
G1
.
Transistor Q2 and the current mirror amplify the current
by a factor of M.
For negative V
SENSE
voltage, the amplifier’s noninvert-
ing input is high impedance and the voltage on RS- ter-
minal equals V
IN
+ V
SENSE.
The amplifier’s output
drives the base of Q1 forcing its inverting input terminal
to match the voltage at the noninverting input terminal;
this causes a current to flow through R
G2
equal to
|V
SENSE
|/R
G2
. Again, transistor Q2 and the current mir-
ror amplify the current by a factor of M.
+V
SENSE
vs. -V
SENSE
The amplifier is configured for either positive V
SENSE
or
negative V
SENSE
by the SIGN comparator. The com-
parator has a built-in offset skew of -1.2mV so that ran-
dom offsets in the comparator do not affect the
precision of I
OUT
(V
OUT
) with positive V
SENSE
. The
comparator has a small amount of hysteresis (typically
0.6mV) to prevent its output from oscillating at the
crossover sense voltage. The ideal transfer characteris-
tic of I
OUT
(V
OUT
) and the output of the comparator
(SIGN) is shown in Figure 2.
The amplifier V
OS
is only trimmed for the positive V
SENSE
voltages (V
RS+
> V
RS-
). The SIGN comparator reconfig-
ures the internal structure of the amplifier to work with
negative V
SENSE
voltages (V
RS-
> V
RS+
) and the preci-
sion V
OS
trim is no longer effective and the resulting V
OS
is slightly impacted. See details in the
Electrical
Characteristics
Note 2. The user can choose the direc-
tion that needs the best precision to be the direction
where V
RS+
> V
RS-
. For example, when monitoring Li+
battery currents, the discharge current should be V
RS+
>
V
RS-
to give the best accuracy over the largest dynamic
range. When the battery charger is plugged in, the
charge current flows in the opposite direction and is
usually much larger, and a higher V
OS
error can be
tolerated. See the
Typical Operating Circuit
.
For applications with unidirectional currents (e.g., bat-
tery discharge only), the SIGN output can be ignored.
Note that as V
SENSE
increases, the output current (I
OUT
for the MAX9928 or V
OUT
/10kΩ for the MAX9929) also
increases. This additional current is supplied from V
CC
.
Pin/Bump Description
SIGN Output. Indicates polarity of V
.
Supply Voltage Input. Bypass to GND with a 0.1µF capacitor.
Current-Sense Output. MAX9928: Current output (I
|). MAX9929:
|).
