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MAX4172EUA

Low-Cost, Precision, High-Side Current-Sense Amplifier

Maxim Integrated 

MAX4172

Low-Cost, Precision, High-Side

Current-Sense Amplifier

ILOAD x RSENSE. Then, since IRG1 flows through RG1,

IRG1 x RG1 = ILOAD x RSENSE. The internal current mirror

multiplies IRG1 by a factor of 50 to give IOUT = 50 x IRG1.

Substituting IOUT/50 for IRG1, (IOUT/50) x RG1 = ILOAD

x RSENSE, or:

IOUT = 50 x ILOAD x (RSENSE/RG1)

The internal current gain of 50 and the factory-trimmed

resistor RG1 combine to result in the device’s transcon-

ductance (Gm) of 10mA/V. Gm is defined as being equal

to IOUT/(VRS+ - VRS-). Since (VRS+ - VRS-) = ILOAD x

RSENSE, the output current (IOUT) can be calculated with

the following formula:

IOUT = Gm x (VRS+ - VRS-) =

(10mA/V) x (ILOAD x RSENSE)

Current Output

The output voltage equation for the MAX4172 is given

below:

VOUT = (Gm) x (RSENSE x ROUT x ILOAD)

where VOUT = the desired full-scale output voltage, ILOAD

equals the full-scale current being sensed, RSENSE

equals the current-sense resistor, ROUT equals the

voltage-setting resistor, and Gm equals the device’s

transconductance (10mA/V).

The full-scale output voltage range can be set by changing

the ROUT resistor value, but the output voltage must be

no greater than V+ - 1.2V. The above equation can be

modified to determine the ROUT required for a particular

full-scale range:

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

OUT is a high-impedance current source that can be

integrated by connecting it to a capacitive load.

PG Output

The PG output is an open-collector logic output that

indicates the status of the MAX4172’s V+ power supply. A

logic low on the PG output indicates that V+ is sufficient to

power the MAX4172. This level is temperature dependent

(see Typical Operating Characteristics graphs), and is

typically 2.7V at room temperature. The internal PG

comparator has a 100mV (typ) hysteresis to prevent

possible oscillations caused by repeated toggling of the

PG output, making the device ideal for power-management

systems lacking soft-start capability. An internal delay

(15μs, typ) in the PG comparator allows adequate time

for power-on transients to settle out. The PG status

indicator greatly simplifies the design of closed-loop

INPUT

RSENSE

ILOAD

RS+ VSENSE RS-

TO LOAD/

BATTERY

RG1

RG2

IRG1

A1

Q1

V+

MAX4172

1:50

CURRENT

IOUT = 50 IRG1

OUT

MIRROR

V+

PG

VTH

GND

Figure 1. Functional Diagram

systems by ensuring that the components in the control

loop have sufficient voltage to operate correctly.

Applications Information

Suggested Component Values

for Various Applications

The Typical Operating Circuit is useful in a wide variety of

applications. Table 1 shows suggested component values

and indicates the resulting scale factors for various applications

required to sense currents from 100mA to 10A.

Adjust the RSENSE value to monitor higher/lower current

levels. Select RSENSE using the guidelines and formulas

in the following section.

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