MAX472ESA datasheet(10/12 Pages) MAXIM

10 / 12 page

In Figure 6, assume the load current to be measured is

10A and that you have determined a 0.3 inch wide, 2

ounce copper to be appropriate. The resistivity of 0.1

inch wide, 2 ounce copper is 30m

Ω/ft (see Note 4). For

10A you may want RSENSE = 5m

Ω for a 50mV drop at

full scale. This resistor will require about 2 inches of 0.1

inch wide copper trace.

RG1 and RG2

Once RSENSE is chosen, RG1 and RG2 can be chosen

to define the current-gain ratio (RSENSE/RG). Choose

RG = RG1 = RG2 based on the following criteria:

a) 1

Ω Input Resistance. The minimum RG value is lim-

ited by the 1

Ω input resistance, and also by the out-

put current limitation (see below). As RG is reduced,

the input resistance becomes a larger portion of the

total gain-setting resistance. With RG = 50

Ω, the

input resistance produces a 2% difference between

the expected and actual current-gain ratio. This is a

gain error that does not affect linearity and can be

removed by adjusting RG or ROUT.

b) Efficiency. As RG is reduced, IOUT gets larger for a

given load current. Power dissipated in ROUT is not

going to the load, and therefore reduces overall effi-

ciency. This is significant only when the sense cur-

rent is small.

c) Maximum Output Current Limitation. IOUT is limit-

ed to 1.5mA, requiring RG

≥ VSENSE / 1.5mA. For

VSENSE = 60mV, RG must be

≥ 40Ω.

d) Headroom. The MAX472 requires a minimum of

1.5V between the lower of the voltage at RG1 or

RG2 (VRG_) and VOUT. As RG becomes larger, the

voltage drop across RG also becomes larger for a

given IOUT. This voltage drop further limits the maxi-

mum full-scale VOUT. Assuming the drop across

RSENSE is small and VCC is connected to either side

of RSENSE, VOUT (max) = VCC - (1.5V + IOUT (max) x

RG).

e) Output Offset Error at Low Load Currents. Large

RG values reduce IOUT for a given load current. As

IOUT gets smaller, the 2.5µA max output offset-error

current becomes a larger part of the overall output

current. Keeping the gain high by choosing a low

value for RG minimizes this offset error.

f) Input Bias Current and Input Bias Current

Mismatching. The size of RG also affects the errors

introduced by the input bias and input bias mis-

matching currents. After selecting the ratio, check to

make sure RG is small enough that IB and IOS do

not add any appreciable errors. The full-scale error

is given by:

% Error =

(RG1 - RG2) x IB + IOS x RG x 100

IFS x RSENSE

where RG1 and RG2 are the gain resistors, IB is the

bias current, IOS is the bias-current mismatch, IFS is the

full-scale current, and RSENSE is the sense resistor.

Assuming a 5A load current, 10m

Ω RSENSE, and 100Ω

RG, the current-gain ratio is 100µA/A, yielding a full-

scale IOUT of 500µA. Using the maximum values for IB

(20µA) and IOS (2µA), and 1% resistors for RG1 and

RG2 (RG1 - RG2 = 2

Ω), the worst-case error at full

scale calculates to:

Ω x 20µA + 100Ω x 2µA

= 0.48%

Ω x 5A

The error may be reduced by: a) better matching of

RG1 and RG2, b) increasing RSENSE, or c) decreasing

RG.

Current-Sense Adjustment

(Resistor Range, Output Adjust)

Choose ROUT after selecting RSENSE, RG1, and RG2.

Choose ROUT to obtain the full-scale voltage you

Precision, High-Side

Current-Sense Amplifiers

______________________________________________________________________________________

Note 4: Printed Circuit Design, by Gerald L. Ginsberg; McGraw-Hill, Inc.; page 185.

36V

RSENSE

MAX472

0.1" COPPER

0.3" COPPER

RG2

RG1

1.5k

POWER

SOURCE

BATTERY

TO LOAD/CHARGER

RG1

N.C.

SHDN

SIGN

OUT

VCC

GND

RG2

Figure 6. MAX472 Connections Showing Use of PC Board

Trace

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