UC3844B, UC3845B, UC2844B, UC2845B

http://onsemi.com

10

If: SENSEFET = MTP10N10M

Figure 20. Adjustable Reduction of Clamp Level

Figure 21. Soft−Start Circuit

Figure 22. Adjustable Buffered Reduction of

Clamp Level with Soft−Start

+

-

5.0V Ref

+

-

S

R

Q

Bias

+

Osc

R

R

R

2R

EA

1.0V

5(9)

7(11)

6(10)

5(8)

3(5)

Q1

1(1)

2(3)

4(7)

8(14)

Ipk(max) [

VClamp

RS

Where: 0

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

+

-

+

-

S

R

+

R

R

R

2R

Ipk(max) [

VClamp

RS

Figure 23. Current Sensing Power MOSFET

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

MPSA63

+

-

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

t power switch. For proper operation during over-current

Refer to Figures 20 and 22.

VPin 5 [

RS Ipk rDS(on)

rDM(on) ) RS

Then : VPin 5 [ 0.075 Ipk

7(12)

7(12)

1.0 mA

Comp/Latch

Comp/Latch

1.0 mA

(12)

T

T

T

T

1.67

R2

R1

) 1

R1R2

R1 ) R2

1.67

R2

R1

) 1

tSoftStart +* In 1 *

VC

3VClamp

C

R1 R2

R1 ) R2