MAX471/MAX472
_______________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:
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
Precision, High-Side
Current-Sense Amplifiers
6 _______________________________________________________________________________________
______________________________________________________________Pin Description
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.
RS-6, 7
Gain Resistor. Connect to load side of current-sense resistor through the gain resistor.RG2—
Power input for MAX472. Connect to sense resistor (R
SENSE
) junction with RG1.V
CC
—
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.
OUT8
Gain Resistor. Connect to battery side of current-sense resistor through the gain resistor.RG1—
Ground or Battery Negative TerminalGND4
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.
SIGN5
No Connect—no internal connectionN.C.—
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.
RS+2, 3
Shutdown. Connect to ground for normal operation. When high, supply current is
less than 5µA.
SHDN1
FUNCTION
MAX471
NAME
—
6
7
8
3
4
5
2
—
PIN
1
MAX472
