FS9721_LP3

Rev. 2.1

29/39

15.10

Current Measurement

The difference between Current Measurement（A）and Current Measurement（B） is as follow:

Mode

Mode

Current Measurement

（A）

Current Measurement

（B）

TSTB Open

TSTB connects to VSS

Measureme

nt

Sampling

Resistanc

e

Measureme

nt

Sampling

Resistanc

e

A

10A

0.01Ω

10A

4A

0.01Ω

mA

400mA

40mA

10Ω

400mA

40mA

1Ω

μA

4000μA

400μA

1kΩ

4000μA

400μA

100Ω

Max.

Voltage

Reduction

4V

0.4V

J7

J1

uA/mA

10A

FS9721_LP3

0.5A/250V

10A/250V

COM

SMV

SA

R17

990

R16

9.99

R28

0.01

SGND

R31

10k

TSTB

R19 900k

DT

R18

100k

Diagram 26

Current Measurement（A）

In TSTB open mode, the current signal enters IC from R19.

When doing the current measurement, The sampling resistance of μA mode is R16+R17+R28, the

sampling resistance of mA mode is R16+R28, and the sampling resistance of 10A mode is R28. They are

measured respectively through the mode switch. When measuring μA, J1 is open and J7 is close; when

measuring mA, J7 is open and J1 is close; the large current enters directly via 10A port.

The maximum reduced voltage for μA, mA and 10A modes is 4V. These voltages are input voltage

comparator to compare. If the voltage is smaller than 400mV, it will be sent to ×10 times ampifier to amplify,

then sent to A/D converter; if the voltage is larger than 400mV, the system will work the auto-ranging signal, and

jump up one mode under the control of microprocessor and send the current signal directly to the A/D

converter.

The accuracy of resistance R16, R17, R28, R18 and R19 affects the accuracy of the current measurement.

J7

J1

uA/mA

10A

FS9721_LP3

0.5A/250V

10A/250V

COM

SA

AGND

R17

99

R16

0.99

R28

0.01

SGND

TSTB

Diagram 27

Current Measurement（B）