Datasheet

5µs/div
STARTUP DELAY
GND
V
OUT
500mV/div
V+
2V/div
GND
V
SENSE
= 100mV
MAX4172-10
10µs/div
V+ to PG POWER-UP DELAY
GND
PG
2V/div
V+
2V/div
GND
100kΩ 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, V
RS+
= 12V, R
OUT
= 1kΩ, T
A
= +25°C, unless otherwise noted.)
Current Output. OUT is proportional to the magnitude of the sense voltage (V
RS+
- V
RS-
). A 1kΩ
resistor from OUT to ground will result in a voltage equal to 10V/V of sense voltage.
OUT6
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 MAX4172V+8
GroundGND5
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
FUNCTIONNAME
_______________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 (R
SENSE
), from the RS+ node to the RS-
node. Current flows through R
G1
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 R
G2
(neglect-
ing the input bias current), so A1’s negative input is
equal to V
SOURCE
- (I
LOAD
x R
SENSE
). A1’s open-loop
gain forces its positive input to essentially the same
voltage level as the negative input. Therefore, the drop
across R
G1
equals I
LOAD
x R
SENSE
. Then, since I
RG1