MAX472ESA datasheet(6/12 Pages) MAXIM

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_______________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 RSENSE. 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 RSENSE 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 IOUT flows from the emitter of

Q1. Since no current flows through RG2 (Q2 is off), the

negative input of A1 is equal to VSOURCE - (ILOAD x

RSENSE). 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

RSENSE. Then, since IOUT flows through Q1 and RG

(ignoring the extremely low base currents), IOUT x RG1

= ILOAD x RSENSE, or:

IOUT = (ILOAD x RSENSE) / 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

(ROUT) of 2k

Ω yields 1V/A for a full-scale value of +3V

at ±3A. Other full-scale voltages can be set with differ-

ent ROUT values, but the output voltage can be no

greater than VRS+ - 1.5V for the MAX471 or VRG_ - 1.5V

for the MAX472.

VOUT = (RSENSE x ROUT x ILOAD) / RG

where VOUT = the desired full-scale output voltage,

ILOAD = the full-scale current being sensed, RSENSE =

the current-sense resistor, ROUT = the voltage-setting

resistor, and RG = the gain-setting resistor (RG = RG1

= RG2).

The above equation can be modified to determine the

ROUT required for a particular full-scale range:

ROUT = (VOUT x RG) / (ILOAD x RSENSE)

For the MAX471, this reduces to:

ROUT = VOUT / (ILOAD 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

_______________________________________________________________________________________

______________________________________________________________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 (RSENSE) junction with RG1.

VCC

—

Current output that is proportional to the magnitude of the sensed current flowing through

RSENSE. A 2kΩ resistor from this pin to ground will result in a voltage equal to 1V/Amp of

sensed current in the MAX471.

OUT

Gain Resistor. Connect to battery side of current-sense resistor through the gain resistor.

RG1

—

Ground or Battery Negative Terminal

GND

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 VSENSE (see Figure 2). SIGN is

high impedance when SHDN is high. Leave open if SIGN is not needed.

SIGN

No Connect—no internal connection

N.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.

SHDN

FUNCTION

MAX471

NAME

—

MAX472

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