AN606

Vishay Siliconix

Document Number: 71991

17-Dec-03

www.vishay.com

3

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

−50 −25

0

25

50

75

100 125 150 175

On-Resistance vs. Junction Temperature

DESIGN EQUATIONS

The following three equations enable circuit design and

analysis.

Current flowing out of sense terminal

r

Current-sensing ratio

Drain-source current

Drain-source voltage

Mirror active resistance

Application Aspects and Design Examples

The current-sense ratio r, even though fixed by design, is

dependent

on

manufacturing

process

variations.

result, a practical design can realize an accuracy of 15% —

20% for current sensing. Accordingly, the current-sensing

MOSFET is most suitable for supervisory functions such as

overcurrent and/or short-circuit protection.

Three keys to a successful design are to:

1. have an adequate margin between the normal

operating-current value and the trip-current value;

3. use a fast comparator with hysterisis to control and protect

the MOSFET.

Typical schematic configurations for implementing the current

sense are shown in Figure 4 and Figure 5.

The Virtual Earth Sensing Scheme, Figure 4, is suitable for

applications aiming at higher noise immunity and speed. This

approach also improves measurement accuracy by

eliminating the sense resistor. However, a dual power supply

and inverted (negative) output signal are the price designers

pay for deriving these benefits.

The Resistor Sensing Scheme shown in Figure 5 is a quite

simple and economical approach. The accuracy of current

measurement is affected by the introduction of an external

temperature sensitivity of the current-sense signal.

G

S

Sense

Kelvin

Load

R1

Stray

Resistance

R2

R3

−

+

D

G

S

Sense

Kelvin

Load

−

+

D

−V

+V

FIGURE 4. Virtual Earth Sensing Scheme

FIGURE 5. Resistor Sensing Scheme