UC3842B, 43B UC2842B, 43B

12

MOTOROLA ANALOG IC DEVICE DATA

Figure 24. Adjustable Buffered Reduction of

Clamp Level with Soft–Start

Figure 25. Current Sensing Power MOSFET

Figure 26. Current Waveform Spike Suppression

Figure 27. MOSFET Parasitic Oscillations

+

–

+

–

S

R

+

R

R

R

2R

VClamp [

1.67

R2

R1

) 1

Ipk(max) [

VClamp

RS

5.0V Ref

Q

Bias

Osc

EA

1.0V

5(9)

7(11)

6(10)

5(8)

3(5)

RS

Q1

VCC

Vin

1(1)

2(3)

4(7)

8(14)

R1

VClamp

R2

Where: 0

≤ VClamp ≤ 1.0 V

C

MPSA63

tSoft-Start +* In 1 *

VC

3VClamp

C

R1 R2

R1 ) R2

+

–

5.0V Ref

+

–

S

R

Q

(11)

(10)

(8)

Comp/Latch

(5)

RS

1/4 W

VCC

Vin

K

M

D

SENSEFET

G

S

Power Ground:

To Input Source

Return

Control Circuitry Ground:

To Pin (9)

Virtually lossless current sensing can be achieved with the implementation of a

SENSEFET power switch. For proper operation during over–current conditions, a

reduction of the Ipk(max) clamp level must be implemented. Refer to Figures 22 and 24.

VPin 5 [

RS Ipk rDS(on)

rDM(on) ) RS

If: SENSEFET = MTP10N10M

RS = 200

Then : VPin 5 [ 0.075 Ipk

7(12)

1.0 mA

Comp/Latch

(12)

+

–

5.0V Ref

+

–

S

R

Q

7(11)

6(10)

5(8)

3(5)

RS

Q1

VCC

Vin

S

R

5.0V Ref

Q

7(11)

6(10)

5(8)

3(5)

RS

Q1

VCC

Vin

C

R

The addition of the RC filter will eliminate instability caused by the leading

edge spike on the current waveform.

Series gate resistor Rg will damp any high frequency parasitic oscillations

caused by the MOSFET input capacitance and any series wiring inductance in

the gate–source circuit.

7(12)

7(12)

Rg

Comp/Latch

Comp/Latch

+

–

+

–