MAX9937(2008) データシートの表示（PDF） - Maxim Integrated

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MAX9937
(Rev.:2008)

Automotive Current-Sense Amplifier with Reverse-Battery Protection

Maxim Integrated 

Automotive Current-Sense Amplifier

with Reverse-Battery Protection

Pin Description

PIN

NAME

FUNCTION

1

VCC

Power Supply. Bypass to GND with a

0.1µF capacitor.

2

GND Ground

3

OUT Current Output

4

RSN

Load-Side Connection Through

External RRSN Resistor

5

RSP

Supply-Side Connection Through

External RRSP Resistor

Detailed Description

The MAX9937 unidirectional high-side, current-sense

amplifier features a 4V to 28V input common-mode volt-

age range that is independent of supply voltage (VCC =

2.7V to 5.5V). The MAX9937 monitors the current through

a current-sense resistor by converting the sense voltage

to a current output (OUT). Gain is set by the ratio of an

output resistor (ROUT) and an input resistor (RRSP). High-

side current monitoring with the MAX9937 does not inter-

fere with the ground path of the load, making it useful for

a variety of automotive battery/ECU monitoring.

Robust input ESD structure allows input common-mode

voltages to exceed the 28V maximum operating input

range for short durations, making the MAX9937 ideal

for applications that need to withstand short-duration

load-dump conditions. The MAX9937 is able to with-

stand reverse-battery conditions by a suitable choice of

input resistors (RRSN, RRSP). See the Input Common-

Mode Voltages > 28V and < 0V section.

Current-Sense Amplifier Operation

The MAX9937 current-sense amplifier operation is best

understood as a specialized op-amp circuit with a

p-channel FET in the feedback path. The op amp

forces a current through an external gain resistor at

RSP (RRSP, see the Typical Application Circuit) so that

its voltage drop equals the voltage drop across the

external sense resistor, RSENSE, making the voltage at

RSP the same as RSN. An external resistor at RSN

(RRSN) has the same value as RRSP to minimize input

offset voltage due to input bias currents.

The current through RRSP is now sourced by the high-

voltage p-channel FET into an external resistor (ROUT)

at OUT. This produces an output voltage whose magni-

tude is given by the following equations:

VSENSE = ILOAD × RSENSE

VOUT

= VSENSE ×

ROUT

RRSP

The gain accuracy is primarily determined by the

matching of the two gain resistors, RRSP and ROUT. The

voltage gain error of the MAX9937 is less than 1.5%.

Total gain = 20V/V with ROUT = 10kΩ

and RRSP = 500Ω.

Low temperature drift of input bias currents and input

offset currents minimizes their impact on total input off-

set voltage of the current-sense amplifier.

Applications Information

Choosing RSENSE

To measure lower currents more accurately, use a high

value for RSENSE. The high value develops a higher

sense voltage that reduces the effect of offset voltage

errors of the internal op amp. In applications monitoring

very high currents, however, 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 large differential voltages to devel-

op between RSP and RSN.

To minimize the effect of input offset voltage by produc-

tion calibration, see the Skewed Input Offset Voltage for

Production Calibration section. This can help reduce

the size of the sense resistor in high-current applica-

tions, as well as measure wide-dynamic-range currents

without sacrificing accuracy.

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

mize the inductance of RSENSE and use input differen-

tial filters (see the Flexible EMI Filtering section).

Low-inductance metal-film resistors are best suited for

these applications.

Calculation of Total Input Offset Voltage

Because of the use of op-amp style architecture, calcu-

lation of total input offset voltage involves the same

methodology as is used for any standard op-amp cir-

cuit. Interaction of the input bias currents and tolerance

of the external resistors, combined with the core input

offset voltage of the op amp, are important to consider.

Finally, RSS (root-sum-of-squares) calculation for all

these uncorrelated sources of error gives the final input

offset voltage.

(VOS−FINAL)2 = (VOS)2 + (IB × ΔRRS)2 + (ΔIB × RRS)2

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