5
s/div
STARTUP DELAY
GND
VOUT
500mV/div
V+
2V/div
GND
VSENSE = 100mV
MAX4172-10
10
s/div
V+ to PG POWER-UP DELAY
GND
PG
2V/div
V+
2V/div
GND
100k
PULLUP RESISTOR FROM PG TO +4V
MAX4172-11
______________________________________________________________Pin Description
MAX4172
Low-Cost, Precision, High-Side
Current-Sense Amplifier
5
Maxim Integrated
____________________________Typical Operating Characteristics (continued)
(V+ = +12V, VRS+ = 12V, ROUT = 1k, TA = +25°C, unless otherwise noted.)
Current Output. OUT is proportional to the magnitude of the sense voltage (VRS+ - VRS-). A 1k
resistor from OUT to ground will result in a voltage equal to 10V/V of sense voltage.
OUT
6
Power Good Open-Collector Logic Output. A low level indicates that V+ is sufficient to power the
MAX4172, and adequate time has passed for power-on transients to settle out.
PG
7
Supply Voltage Input for the MAX4172
V+
8
Ground
GND
5
No Connect. No internal connection. Leave open or connect to GND.
N.C.
3, 4
PIN
Load-side connection for the external sense resistor. The “-” indicates the direction of current flow.
RS-
2
Power connection to the external sense resistor. The “+” indicates the direction of current flow.
RS+
1
FUNCTION
NAME
_______________Detailed Description
The MAX4172 is a unidirectional, high-side current-sense
amplifier with an input common-mode range that is inde-
pendent of supply voltage. This feature not only allows
the monitoring of current flow into a battery in deep dis-
charge, but also enables high-side current sensing at
voltages far in excess of the supply voltage (V+).
The MAX4172 current-sense amplifier’s unique topolo-
gy simplifies current monitoring and control. The
MAX4172’s amplifier operates as shown in Figure 1.
The battery/load current flows through the external
sense resistor (RSENSE), from the RS+ node to the RS-
node. Current flows through RG1 and Q1, and into the
current mirror, where it is multiplied by a factor of 50
before appearing at OUT.
To analyze the circuit of Figure 1, assume that current
flows from RS+ to RS-, and that OUT is connected to
GND through a resistor. Since A1’s inverting input is
high impedance, no current flows though RG2 (neglect-
ing the input bias current), so A1’s negative input is
equal to VSOURCE - (ILOAD x RSENSE). A1’s open-loop
gain forces its positive input to essentially the same
voltage level as the negative input. Therefore, the drop
across RG1 equals ILOAD x RSENSE. Then, since IRG1