Datasheet

Detailed Description
The MAX9928F/MAX9929F micropower uni-/bidirection-
al, 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 during
short-circuit/fault conditions, and also enables highside
current sensing at voltages far in excess of the supply
voltage (V
CC
). The MAX9928F/MAX9929F operate from
a 2.5V to 5.5V single supply and draw a low 20μA quies-
cent 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 RG1 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 noninverting
input is high impedance and the voltage on RS- terminal
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 mirror 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 compara-
tor has a built-in offset skew of -1.2mV so that random
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 characteristic 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
reconfigures the internal structure of the amplifier to work
with negative V
SENSE
voltages (V
RS-
> V
RS+
) and the
precision V
OS
trim is no longer effective and the result-
ing V
OS
is slightly impacted. See details in the Electrical
Characteristics Note 2. The user can choose the direction
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 cur-
rent 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., battery
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
NAME FUNCTION
µMAX UCSP
1 B3 RS- Negative Current-Sense Input. Load-side connection for the external sense resistor.
2 B2 SIGN
SIGN Output. Indicates polarity of V
SENSE
.
SIGN = H indicates V
RS+
> V
RS-
SIGN = L indicates V
RS+
< V
RS-
3 B1 RS+ Positive Current-Sense Input. Power-side connection to the external sense resistor.
4, 5 — N.C. No Connection. Not internally connected.
6 A1 V
CC
Supply Voltage Input. Bypass to GND with a 0.1µF capacitor.
7 A2 GND Circuit Ground
8 A3 OUT
Current-Sense Output. MAX9928: Current output (I
OUT
is proportional to |V
SENSE
|). MAX9929:
Voltage output (V
OUT
is proportional to |V
SENSE
|).
MAX9928/MAX9929 -0.1V to +28V Input Range, Micropower,
Uni-/Bidirectional, Current-Sense Ampliers
www.maximintegrated.com
Maxim Integrated │ 8
Pin Conguration