MAX4172ESA datasheet(6/8 Pages) MAXIM

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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 MAX4172

transconductance (Gm) of 10mA/V. Gm is de-

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

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

resistor, and Gm = MAX4172 transconductance

(10mA/V).

The full-scale output voltage range can be set by

changing the ROUT resistor value, but the output volt-

age 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 sup-

ply. A logic low on the

PG output indicates that V+ is

sufficient to power the MAX4172. This level is tempera-

ture dependent (see

Typical Operating Characteristics

graphs), and is typically 2.7V at room temperature. The

internal PG comparator has a 100mV (typical) hystere-

sis to prevent possible oscillations caused by repeated

toggling of the

PG output, making the device ideal for

power-management systems lacking soft-start capabili-

ty. An internal delay (15µs typical) in the

PG compara-

tor allows adequate time for power-on transients to

settle out. The

PG status indicator greatly simplifies the

design of closed-loop 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 vari-

ous applications required to sense currents from

100mA to 10A.

Adjust the RSENSE value to monitor higher or lower cur-

rent levels. Select RSENSE using the guidelines and for-

mulas in the following section.

Sense Resistor, RSENSE

Choose RSENSE based on the following criteria:

• Voltage Loss: A high RSENSE value causes the

power-source voltage to degrade through IR loss.

For minimal voltage loss, use the lowest RSENSE

value.

Low-Cost, Precision, High-Side

Current-Sense Amplifier

_______________________________________________________________________________________

RG1

RS+

RSENSE

INPUT

RG2

TO LOAD/

BATTERY

RS-

VSENSE

1:50

CURRENT

MIRROR

MAX4172

OUT

VTH

IOUT = 50 IRG1

IRG1

V+

GND

ILOAD

Figure 1. Functional Diagram

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