Precision, High-Side
Current-Sense Amplifiers
MAX471/MAX472
______________________________________________________________Pin Description
PIN
NAME
MAX471
1
2, 3
—
—
4
5
6, 7
—
—
8
MAX472
1
—
2
3
4
5
—
6
7
8
SHDN
RS+
N.C.
RG1
GND
SIGN
RS-
RG2
V
CC
OUT
Shutdown. Connect to ground for normal operation. When high, supply current is
less than 5µA.
Battery (or power) side of the internal current-sense resistor. The “+” indicates direction of
flow for SIGN output only. Connect pins 2 and 3 together at the package.
No Connect—no internal connection
Gain Resistor. Connect to battery side of current-sense resistor through the gain resistor.
Ground or Battery Negative Terminal
An open-collector logic output. For the MAX471, a low level indicates current is flowing from
RS- to RS+. For the MAX472, a low level indicates a negative V
SENSE
(see Figure 2). SIGN is
high impedance when SHDN is high. Leave open if SIGN is not needed.
Load side of the internal current-sense resistor. The “-” indicates direction of flow for SIGN
output only. Connect pins 6 and 7 together at the package.
Gain Resistor. Connect to load side of current-sense resistor through the gain resistor.
Power input for MAX472. Connect to sense resistor (R
SENSE
) junction with RG1.
Current output that is proportional to the magnitude of the sensed current flowing through
R
SENSE
. A 2kΩ resistor from this pin to ground will result in a voltage equal to 1V/Amp of
sensed current in the MAX471.
FUNCTION
_______________Detailed Description
The MAX471 and MAX472 current-sense amplifier’s
unique topology allows a simple design to accurately
monitor current flow. The MAX471/MAX472 contain two
amplifiers operating as shown in Figures 1 and 2. The
battery/load current flows from RS+ to RS- (or vice
versa) through R
SENSE
. Current flows through either
RG1 and Q1 or RG2 and Q2, depending on the sense-
resistor current direction. Internal circuitry, not shown in
Figures 1 and 2, prevents Q1 and Q2 from turning on at
the same time. The MAX472 is identical to the
MAX471, except that R
SENSE
and gain-setting resistors
RG1 and RG2 are external (Figure 2).
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. In this case, amplifier A1 is
active and output current I
OUT
flows from the emitter of
Q1. Since no current flows through RG2 (Q2 is off), the
negative input of A1 is equal to V
SOURCE
- (I
LOAD
x
R
SENSE
). The open-loop gain of A1 forces its positive
input to essentially the same level as the negative input.
Therefore, the drop across RG1 equals I
LOAD
x
R
SENSE
. Then, since I
OUT
flows through Q1 and RG
(ignoring the extremely low base currents), I
OUT
x RG1
= I
LOAD
x R
SENSE
, or:
6
I
OUT
= (I
LOAD
x R
SENSE
) / RG1
Current Output
The output voltage equation for the MAX471/MAX472 is
given below. In the MAX471, the current-gain ratio has
been preset to 500µA/A so that an output resistor
(R
OUT
) of 2kΩ yields 1V/A for a full-scale value of +3V
at ±3A. Other full-scale voltages can be set with differ-
ent R
OUT
values, but the output voltage can be no
greater than V
RS+
- 1.5V for the MAX471 or V
RG_
- 1.5V
for the MAX472.
V
OUT
= (R
SENSE
x R
OUT
x I
LOAD
) / RG
where V
OUT
= the desired full-scale output voltage,
I
LOAD
= the full-scale current being sensed, R
SENSE
=
the current-sense resistor, R
OUT
= the voltage-setting
resistor, and RG = the gain-setting resistor (RG = RG1
= RG2).
The above equation can be modified to determine the
R
OUT
required for a particular full-scale range:
R
OUT
= (V
OUT
x RG) / (I
LOAD
x R
SENSE
)
For the MAX471, this reduces to:
R
OUT
= V
OUT
/ (I
LOAD
x 500µA/A)
OUT is a high-impedance current-source output that
can be connected to other MAX471/MAX472 OUT pins
_______________________________________________________________________________________
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