MAXIM MAX4172 User Manual

19-1184; Rev 0; 12/96

Low-Cost, Precision, High-Side

Current-Sense Amplifier

_______________General Description

The MAX4172 is a low-cost, precision, high-side currentsense amplifier for portable PCs, telephones, and other systems where battery/DC power-line monitoring is critical. High-side power-line monitoring is especially useful in battery-powered systems, since it does not interfere with the battery charger’s ground path. Wide bandwidth and ground-sensing capability make the MAX4172 suitable for closed-loop battery-charger and generalpurpose current-source applications. The 0V to 32V input common-mode range is independent of the supply voltage, which ensures that current-sense feedback remains viable, even when connected to a battery in deep discharge.

To provide a high level of flexibility, the MAX4172 functions with an external sense resistor to set the range of load current to be monitored. It has a current output that can be converted to a ground-referred voltage with a single resistor, accommodating a wide range of battery voltages and currents.

An open-collector power-good output (PG) indicates when the supply voltage reaches an adequate level to guarantee proper operation of the current-sense amplifier. The MAX4172 operates with a 3.0V to 32V supply voltage, and is available in a space-saving, 8-pin µMAX or SO package.

____________________________Features

♦ Low-Cost, High-Side Current-Sense Amplifier ♦ ±0.5% Typical Full-Scale Accuracy Over

Temperature

♦ 3V to 32V Supply Operation ♦ 0V to 32V Input Range—Independent of

Supply Voltage

♦ 800kHz Bandwidth [V

200kHz Bandwidth [V

SENSE SENSE

= 100mV (1C)] = 6.25mV (C/16)]

♦ Available in Space-Saving µMAX and SO

Packages

______________Ordering Information

PART

MAX4172ESA MAX4172EUA -40°C to +85°C

Contact factory for availability.

TEMP. RANGE PIN-PACKAGE

-40°C to +85°C

8 SO

8 µMAX*

MAX4172

________________________Applications

Portable PCs: Notebooks/Subnotebooks/Palmtops Battery-Powered/Portable Equipment Closed-Loop Battery Chargers/Current Sources Smart-Battery Packs Portable/Cellular Phones Portable Test/Measurement Systems Energy Management Systems

__________________Pin Configuration

TOP VIEW

RS+

RSN.C.

N.C.

________________________________________________________________

MAX4172

µMAX/SO

V+ PG

OUT

GND

__________Typical Operating Circuit

UNREGULATED

DC SUPPLY

3V TO 32V

LOW-COST SWITCHING REGULATOR

FEEDBACK LOOP V

OUT

LOW-COST BATTERY CHARGER/CURRENT SOURCE



SENSE

RS+

V+

MAX4172

= 500mV/A

50mΩ

SENSE

GND



RS-

OUT

0V TO 32V

ANALOG OR LOGIC SUPPLY

100k

POWER GOOD

I V

OUT

Maxim Integrated Products

OUT

SENSE

= 

/ 100Ω

LOAD/

BATTERY

For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800

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

ABSOLUTE MAXIMUM RATINGS

V+, RS+, RS-, PG ...................................................-0.3V to +36V

OUT..............................................................-0.3V to (V+ + 0.3V)

Differential Input Voltage, V

Current into Any Pin..........................................................±50mA

Continuous Power Dissipation (T

SO (derate 5.88mW/°C above +70°C)..........................471mW

µMAX (derate 4.10mW/°C above +70°C) .....................330mW

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional

MAX4172

operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.

RS+

- V

............................±700mV

RS-

= +70°C)

ELECTRICAL CHARACTERISTICS

(V+ = +3V to +32V; RS+, RS- = 0V to 32V; TA= T

= +25°C.)

Input Voltage Range Supply Current

Input Offset Voltage mVMAX4172EUA ±0.2 ±1.6

Positive Input Bias Current

Negative Input Bias Current

Voltage

SENSE

Low-Level Current Error

OUT Power-Supply Rejection Ratio

OUT Common-Mode Rejection Ratio

OUT

RS+

RS-

V+

RS-

/ ∆V+

/ ∆V

RS+

to T

MIN

= 0mA

OUT

V+ = 12V, V V

≤ 2.0V

RS+

> 2.0V, I

RS+

≤ 2.0V, I

RS+

> 2.0V

RS+

≤ 2.0V

RS+

SENSE

= 12V (Note 1)

RS+

SENSE

= 12V

RS+

3V ≤ V+ ≤ 32V, V

2.0V < V

; unless otherwise noted. Typical values are at V+ = +12V, RS+ = 12V,

MAX

RS+

OUT

= 6.25mV, V+ = 12V,

= 100mV, V+ = 12V,

< 32V

RS+

Operating Temperature Range

MAX4172E_A....................................................-40°C to +85°C

Storage Temperature Range.............................-65°C to +150°C

Lead Temperature (soldering, 10sec).............................+300°C

CONDITIONS

= 12V

= 0mA

RS+

> 2.0V

MAX4172ESA

-325 42.5

-650 85

MAX4172ESA MAX4172EUA ±15

MAX4172ESA,

TA= -40°C to 0°C

MAX4172EUA,

TA= -40°C to 0°C

MAX4172ESA,

TA= 0°C to +85°C

MAX4172EUA,

TA= 0°C to +85°C

±0.1 ±0.75

0 27 42.5

05085

UNITSMIN TYP MAXSYMBOLPARAMETER

±8.0

±20

±50

±10

±15

V332V+Operating Voltage Range V032V

mA0.8 1.6I

µA

mV150 175Maximum V

µA

µAOutput Current Error

µA/V0.2∆I

µA/V0.03∆I

2 _______________________________________________________________________________________

Low-Cost, Precision, High-Side

Current-Sense Amplifier

ELECTRICAL CHARACTERISTICS (continued)

(V+ = +3V to +32V; RS+, RS- = 0V to 32V; TA= T

= +25°C.)

Maximum Output Voltage (OUT)

Bandwidth

Maximum Output Current

Transconductance

OUT

V+ Threshold for PG Output Low (Note 2)

PG Output Low Voltage

Leakage Current into PG Power-Off Input Leakage

Current (RS+, RS-)

OUT Settling Time to 1%

Note 1: 6.25mV = 1/16 of typical full-scale sense voltage (C/16). Note 2: Valid operation of the MAX4172 is guaranteed by design when PG is low.

to T

MIN

; unless otherwise noted. Typical values are at V+ = +12V, RS+ = 12V,

MAX

CONDITIONS

≤ 1.5mA

OUT

= 100mV

SENSE

= 6.25mV (Note 1)

SENSE

Gm= I V

SENSE

/ (V

OUT

RS+

= 100mV, V

- V

RS+

RS-

> 2.0V

TA= 0°C to +85°C

TA= -40°C to 0°C V+ rising V+ falling 2.67 I

= 1.2mA, V+ = 2.9V, TA= +25°C

SINK

V+ = 2.5V, TA= +25°C

V+ = 0V, V

SENSE

= V

RS+

= 32V µA0.1 1

RS-

= 0mV to 100mV, 10% to 90% = 100mV to 0mV, 90% to 10% ns800OUT Fall Time

= 5mV to 100mV

Rising

Falling

= 150mV MΩ20OUT Output Resistance

800 200

9.8 10 10.2

9.7 10 10.3

2.77

1.3 6

MAX4172

UNITSMIN TYP MAXSYMBOLPARAMETER

VV+ - 1.2

kHz

mA1.5 1.75I

mA/V

V0.4V

µA1

ns400OUT Rise Time

µs

__________________________________________Typical Operating Characteristics

(V+ = +12V, V

1000

950 900 850 800 750 700

650

SUPPLY CURRENT (µA)

600 550 500

010

= 12V, R

RS+

SUPPLY CURRENT

vs. SUPPLY VOLTAGE

TA = +85°C

V+ (V)

= 1kΩ, TA= +25°C, unless otherwise noted.)

OUT

vs. SUPPLY VOLTAGE

0.5

0.4

MAX4172-01

0.3

TA = +25°C

TA = -40°C

OUT

= 0mA

0.2

0.1 0

ERROR (%)

-0.1

-0.2

-0.3

-0.4

-0.5 010

TA = +25°C

OUTPUT ERROR

SENSE

TA = +85°C

TA = -40°C

20 30

V+ (V)

= 100mV

MAX4172-03

1.5

1.0

0.5 0

-0.5

-1.0

ERROR (%)

-1.5

-2.0

-2.5

-3.0 010

C/16 LOAD OUTPUT ERROR

vs. SUPPLY VOLTAGE

V+ (V)

_______________________________________________________________________________________ 3

SENSE

TA = -40°C

TA = +25°C

TA = +85°C

= 6.25mV

MAX4172-02

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

____________________________Typical Operating Characteristics (continued)

(V+ = +12V, V

RS+

= 12V, R

= 1kΩ, TA= +25°C, unless otherwise noted.)

OUT

40 35

MAX4172

30 25

20 15

ERROR (%)

5 0

-5

0.1m 10m 100m1m 1

0.75

0.55

0.35

0.15

-0.05

ERROR (%)

-0.25

-0.45

-0.65 0 6 30 40

ERROR vs. SENSE VOLTAGE

(V)

SENSE

OUTPUT ERROR 

vs. COMMON-MODE VOLTAGE

= 100mV

SENSE

TA = -40°C

TA = +25°C

TA = +85°C

12 18

RS-

(V)

MAX4172-04

MAX4172-06

POWER-SUPPLY REJECTION RATIO

vs. FREQUENCY

= 100mV

SENSE

ERROR (%)

5 0

0.01 0.1 1 10 100 1000 POWER-SUPPLY FREQUENCY (kHz)

1.0Vp-p



5mVp-p

V+ THRESHOLD FOR PG OUTPUT LOW

vs. TEMPERATURE

2.95

2.90

2.85

2.80

2.75

2.70

2.65

2.60

V+ TRIP THRESHOLD (V)

2.55

2.50

2.45

V+ FALLING VOLTAGE

-40 -15 85

V+ RISING VOLTAGE

10 35

TEMPERATURE (°C)



0.5Vp-p



MAX4172-05

MAX4172-07

SENSE

5mV/div

OUT

50mV/div

0mV to 10mV V



GND



GND

TRANSIENT RESPONSE

SENSE

10µs/div

MAX4172-08

SENSE

50mV/div

OUT

500mV/div



GND



GND

0mV to 100mV V

TRANSIENT RESPONSE

SENSE

10µs/div

4 _______________________________________________________________________________________

MAX4172-09

Low-Cost, Precision, High-Side

Current-Sense Amplifier

____________________________Typical Operating Characteristics (continued)

(V+ = +12V, V



OUT

500mV/div

GND

V+

2V/div

GND

SENSE

RS+

= 100mV

= 12V, R

= 1kΩ, TA= +25°C, unless otherwise noted.)

OUT

START-UP DELAY

5µs/div

MAX4172-10

2V/div

V+ to PG POWER-UP DELAY

PG

GND

V+

GND

100kΩ PULL-UP RESISTOR FROM PG TO +4V

MAX4172-11

10µs/div

______________________________________________________________Pin Description

OUT6

Power connection to the external sense resistor. The “+” indicates the direction of current flow.RS+1 Load-side connection for the external sense resistor. The “-” indicates the direction of current flow.RS-2 No Connect. No internal connection. Leave open or connect to GND.N.C.3, 4 GroundGND5

Current Output. OUT is proportional to the magnitude of the sense voltage (V resistor from OUT to ground will result in a voltage equal to 10V/V of sense voltage.

Power Good Open-Collector Logic Output. A low level indicates that V+ is sufficient to power the MAX4172, and adequate time has passed for power-on transients to settle out.

Supply Voltage Input for the MAX4172V+8

FUNCTIONNAME

- V

RS-

). A 1kΩ

RS+

MAX4172

_______________Detailed Description

The MAX4172 is a unidirectional, high-side current-sense amplifier with an input common-mode range that is independent of supply voltage. This feature not only allows the monitoring of current flow into a battery in deep discharge, but also enables high-side current sensing at voltages far in excess of the supply voltage (V+).

The MAX4172 current-sense amplifier’s unique topology simplifies current monitoring and control. The MAX4172’s amplifier operates as shown in Figure 1. The battery/load current flows through the external sense resistor (R

), from the RS+ node to the RS-

SENSE

_______________________________________________________________________________________ 5

node. Current flows through RG1and Q1, and into the current mirror, where it is multiplied by a factor of 50 before appearing at OUT.

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. Since A1’s inverting input is high impedance, no current flows though RG2(neglecting the input bias current), so A1’s negative input is equal to V

SOURCE

- (I

LOAD

x R

). A1’s open-loop

SENSE

gain forces its positive input to essentially the same voltage level as the negative input. Therefore, the drop across RG1equals I

LOAD

x R

. Then, since I

SENSE

RG1

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

flows through RG1, I internal current mirror multiplies I to give I (I

OUT

/ 50) x R

OUT

= 50 x I

= I

= 50 x I

x RG1= I

RG1

. Substituting I

RG1

x R

LOAD

SENSE

x (R

x R

LOAD

by a factor of 50

RG1

OUT

, or:

/ RG1)

SENSE

/ 50 for I

The internal current gain of 50 and the factory-trimmed resistor RG1combine to result in the MAX4172 transconductance (Gm) of 10mA/V. Gmis de-

MAX4172

fined as being equal to I (V (I

- V

RS+

) can be calculated with the following formula:

OUT

RS-

) = I

OUT

= Gmx (V

LOAD

x R

(10mA/V) x (I

OUT

SENSE

RS+

LOAD

/ (V

RS+

- V

RS-

, the output current

- V

) =

RS-

x R

SENSE

)

Current Output

The output voltage equation for the MAX4172 is given below:

= (Gm) x (R

OUT

where V I

= the full-scale current being sensed, R

LOAD

= the desired full-scale output voltage,

OUT

the current-sense resistor, R resistor, and Gm= MAX4172 transconductance (10mA/V).

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

resistor value, but the output volt-

OUT

age must be no greater than V+ - 1.2V. The above equation can be modified to determine the R required for a particular full-scale range:

OUT

= (V

OUT

) / (I

OUT is a high-impedance current source that can be integrated by connecting it to a capacitive load.

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 (typical) 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 typical) 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 systems by ensuring that the components in the control loop have sufficient voltage to operate correctly.

SENSE

LOAD

x R

OUT

x R

x I

OUT

LOAD

= the voltage-setting

SENSE

x Gm)

Output

. The

RG1

). Since

)

SENSE

OUT

= 50 I

RS-V

MAX4172

RG1

LOAD

INPUT

V+

GND

Figure 1. Functional Diagram

SENSE

RS+

SENSE

RG1

CURRENT 

MIRROR

1:50

V+

OUT

__________Applications Information

Suggested Component Values

for Various Applications

The

Typical Operating Circuit

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 R

SENSE

rent levels. Select R

value to monitor higher or lower cur-

SENSE

mulas in the following section.

Choose R

based on the following criteria:

SENSE

• Voltage Loss: A high R power-source voltage to degrade through IR loss. For minimal voltage loss, use the lowest R value.

is useful in a wide variety

using the guidelines and for-

Sense Resistor, R

SENSE

value causes the

TO LOAD/ BATTERY

OUT

SENSE

6 _______________________________________________________________________________________

Low-Cost, Precision, High-Side

Current-Sense Amplifier

Table 1. Suggested Component Values

FULL-SCALE

LOAD CURRENT

(A)

• Accuracy: A high R

CURRENT-SENSE

RESISTOR,

SENSE

(mΩ)

SENSE

10000.1

value allows lower currents to be measured more accurately. This is because offsets become less significant when the sense voltage is larger. For best performance, select R

SENSE

to provide approximately 100mV of sense voltage for the full-scale current in each application.

• Efficiency and Power Dissipation: At high current levels, the I2R losses in R

SENSE

can be significant. Take this into consideration when choosing the resistor value and its power dissipation (wattage) rating. Also, the sense resistor’s value might drift if it is allowed to heat up excessively.

• Inductance: Keep inductance low if I

SENSE

large high-frequency component. Wire-wound resistors have the highest inductance, while metal film is somewhat better. Low-inductance metal-film resistors are also available. Instead of being spiral wrapped around a core, as in metal-film or wirewound resistors, they are a straight band of metal and are available in values under 1Ω.

• Cost: If the cost of R

SENSE

is an issue, you might want to use an alternative solution, as shown in Figure 2. This solution uses the PC board traces to create a sense resistor. Because of the inaccuracies of the copper resistor, the full-scale current value must be adjusted with a potentiometer. Also, copper’s resistance temperature coefficient is fairly high (approximately 0.4%/°C).

In Figure 2, assume that the load current to be measured is 10A, and that you have determined a 0.3-inchwide, 2-ounce copper to be appropriate. The resistivity of 0.1-inch-wide, 2-ounce (70µm thickness) copper is 30mΩ/ft. For 10A, you might want R

SENSE

= 5mΩ for a 50mV drop at full scale. This resistor requires about 2 inches of 0.1-inch-wide copper trace.

OUTPUT

RESISTOR, R

has a

FULL-SCALE

OUT

(kΩ)

3.48

3.48 3.48 3.481001

3.48 3.48

3.48

O.3 in. COPPER O.3 in. COPPER

Figure 2. MAX4172 Connections Showing Use of PC Board

OUTPUT

VOLTAGE, V

3.48

3.48 0.3481010

INPUT

RS+

RS-

(V)

OUT

SENSE

O.1 in. COPPER

SENSE

MAX4172

SCALE FACTOR,

OUT/ISENSE

LOAD/BATTERY

V+

OUT

GND

Current-Sense Adjustment

(Resistor Range, Output Adjust)

Choose R obtain the full-scale voltage you require, given the fullscale I

OUT

ance permits using R minimal error. OUT’s load impedance (e.g., the input of an op amp or ADC) must be much greater than R (e.g., 100 x R accuracy.

after selecting R

OUT

determined by R

) to avoid degrading measurement

OUT

. Choose R

SENSE

. OUT’s high imped-

SENSE

values up to 200kΩ with

OUT

High-Current Measurement

The MAX4172 can achieve high-current measurements by using low-value sense resistors, which can be paralleled to further increase the current-sense limit. As an alternative, PC board traces can be adjusted over a wide range.

34.8

0.696205

3V TO 32V

SUPPLY

OUT

(V/A)

OUT

MAX4172

OUT

_______________________________________________________________________________________ 7

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

Power-Supply Bypassing and Grounding

___________________Chip Information

In most applications, grounding the MAX4172 requires no special precautions. However, in high-current systems, large voltage drops can develop across the ground plane, which can add to or subtract from V

OUT

TRANSISTOR COUNT: 177 SUBSTRATE CONNECTED TO GND

. Use a single-point star ground for the highest currentmeasurement accuracy.

The MAX4172 requires no special bypassing and responds quickly to transient changes in line current. If

MAX4172

the noise at OUT caused by these transients is a problem, you can place a 1µF capacitor at the OUT pin to ground. You can also place a large capacitor at the RS terminal (or load side of the MAX4172) to decouple the load, reducing the current transients. These capacitors are not required for MAX4172 operation or stability. The RS+ and RS- inputs can be filtered by placing a capacitor (e.g., 1µF) between them to average the sensed current.

________________________________________________________Package Information

DIM

0.101mm

0.004 in

A1B

A1

INCHES MILLIMETERS



MIN

A

0.036

0.004

B

0.010

C

0.005

D

0.116

E

0.116 e H

0.188 L

0.016

MAX

0.044

0.008

0.014

0.007

0.120



0°



0.198

0.026 6°

MIN

0.91

0.10

0.25

0.13

2.95

4.78

0.41 0°

0.650.0256



MAX

1.11

0.20

0.36

0.18

3.05

3.05 

5.03

0.66

6°

21-0036D

E H

8-PIN µMAX

MICROMAX SMALL-OUTLINE

PACKAGE

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

___________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600

MAXIM MAX4172 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual