UC3842B, UC3843B, UC2842B, UC2843B

http://onsemi.com

13

+

-

+

-

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)

Q1

1(1)

2(3)

4(7)

8(14)

Where: 0

C

MPSA63

tSoftStart +* In 1 *

VC

3 VClamp

C

R1 R2

R1 ) R2

7(12)

1.0 mA

Comp/Latch

5.0V Ref

+

-

S

R

Q

Bias

+

1.0mA

Osc

R

R

R

2R

EA

1.0V

5(9)

1(1)

2(3)

4(7)

8(14)

C

1.0M

Figure 25. Soft−Start Circuit

Figure 26. Adjustable Buffered Reduction of

Clamp Level with Soft−Start

+

-

5.0V Ref

+

-

S

R

Q

(11)

(10)

(8)

Comp/Latch

(5)

1/4 W

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

VPin 5 [

RS Ipk rDS(on)

rDM(on) ) RS

If: SENSEFET = MTP10N10M

Then : VPin5 [ 0.075 Ipk

(12)

Figure 27. Current Sensing Power MOSFET

+

-

5.0V Ref

+

-

S

R

Q

7(11)

6(10)

5(8)

3(5)

Q1

C

R

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

edge spike on the current waveform.

7(12)

Comp/Latch

Figure 28. Current Waveform Spike Suppression