ADOG datasheet(7/9 Pages) MAXIM

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Single/Dual/Quad, High-Side Current-Sense

Amplifiers with Internal Gain

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Detailed Description

The MAX4376/MAX4377/MAX4378 high-side current-

sense amplifiers feature a 0 to +28V input common-mode

range that is independent of supply voltage. This feature

allows the monitoring of current out of a battery in deep

discharge and also enables high-side current sensing at

voltages greater than the supply voltage (VCC).

The MAX4376/MAX4377/MAX4378 operate as follows:

current from the source flows through RSENSE to the

load (Figure 1). Since the internal sense amplifier’s

inverting input has high impedance, negligible current

flows through RG2 (neglecting the input bias current).

Therefore, the sense amplifier’s inverting-input voltage

equals VSOURCE - (ILOAD)(RSENSE).

The amplifier’s open-loop gain forces its noninverting

input to the same voltage as the inverting input.

Therefore, the drop across RG1 equals (ILOAD)

(RSENSE). Since IRG1 flows through RG1, IRG1 =

(ILOAD)(RSENSE)/RG1. The internal current mirror multi-

plies IRG1 by a current gain factor,

β, to give IRGD = β x

IRG1. Solving IRGD =

β x (ILOAD)(RSENSE)/RG1.

Therefore:

VOUT =

β x (RGD/RG1)(RSENSE x ILOAD) x amp gain

where amp gain is 2, 5, or 10.

The part’s gain equals (

β x RGD / RG1) x amp gain.

Therefore:

VOUT = (GAIN)(RSENSE)(ILOAD)

where GAIN = 20 for MAX437_T.

GAIN = 50 for MAX437_F.

GAIN = 100 for MAX437_H.

Set the full-scale output range by selecting RSENSE and

the appropriate gain version of the MAX4376/

MAX4377/MAX4378.

Applications Information

Recommended Component Values

The MAX4376/MAX4377/MAX4378 sense a wide variety

of currents with different sense resistor values. Table 1

lists common resistor values for typical operation of the

MAX4376/MAX4377/MAX4378.

Choosing RSENSE

To measure lower currents more accurately, use a high

value for RSENSE. The high value develops a higher

sense voltage that reduces offset voltage errors of the

internal op amp.

In applications monitoring very high currents, RSENSE

must be able to dissipate the I2R losses. If the resistor’s

rated power dissipation is exceeded, its value may drift

or it may fail altogether, causing a differential voltage

across the terminals in excess of the absolute maxi-

mum ratings.

If ISENSE has a large high-frequency component, mini-

mize the inductance of RSENSE. Wire-wound resistors

have the highest inductance, metal-film resistors are

somewhat better, and low-inductance metal-film resis-

tors are best suited for these applications.

Bidirectional Current-Sense Amplifier

Systems such as laptop computers and other devices

that have internal charge circuitry require a precise

bidirectional current-sense amplifier to monitor accu-

rately the battery’s current regardless of polarity. Figure

2 shows the MAX4377 used as a bidirectional current

Pin Description

MAX4376

MAX4377

MAX4378

SOT23-5

SO-8

µMAX-8/

SO-8

SO-14/

TSSOP-14

NAME

FUNCTION

1, 7

1, 7, 8, 14

OUT, OUT_

Output Voltage. VOUT_ is proportional to the magnitude

of the sense voltage (VRS+ - VRS-). VOUT_ is

approximately zero when VRS - > VRS - + (no phase

reversal).

GND

Ground

3184

VCC

Supply Voltage

3, 5

3, 5, 10, 12

RS+, RS_+

Power connection to the external sense resistor

2, 6

2, 6, 9, 13

RS-, RS_-

Load-side connection to the external sense resistor

—

2, 5, 7

——

N.C.

No Connection. Not internally connected.

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