UC3842A, 43A UC2842A, 43A

11

MOTOROLA ANALOG IC DEVICE DATA

Figure 20. External Clock Synchronization

Figure 21. External Duty Cycle Clamp and

Multi Unit Synchronization

Figure 22. Adjustable Reduction of Clamp Level

Figure 23. Soft–Start Circuit

Figure 24. Adjustable Buffered Reduction of

Clamp Level with Soft–Start

Figure 25. Current Sensing Power MOSFET

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.

The diode clamp is required if the Sync amplitude is large enough to

cause the bottom side of CT to go more than 300 mV below ground.

External

Sync

Input

47

5(9)

R

R

Bias

Osc

Vref

RT

8(14)

4(7)

2(3)

1(1)

0.01

CT

2R

R

EA

+

–

+

5(9)

R

R

Bias

Osc

8(14)

4(7)

2(3)

1(1)

2R

R

EA

+

–

+

7

5.0k

3

8

6

5

1

C

R

S

MC1455

2

RA

+

–

+

–

4

Q

5.0k

RB

To Additional

UCX84XA’s

f =

1.44

(RA + 2RB)C

Dmax =

RB

RA + 2RB

5(9)

R

R

Bias

Osc

8(14)

4(7)

2(3)

1(1)

2R

R

EA

+

–

+

Q1

RS

3(5)

5(8)

1.0V

–

R

S

Q

Comp/Latch

5.0Vref

VClamp

Vin

VCC

7(11)

6(10)

–

+

+

–

+

–

+

7(12)

+

–

R1 R2

R2

VClamp =

1.67

+ 1

+ 0.33 x 10 – 3Ipk(max) =

VClamp

RS

Where: 0

≤ VClamp ≤ 1.0 V

R2

R1

1.0mA

R1

R1 + R2

5(9)

R

R

Bias

Osc

8(14)

4(7)

2(3)

1(1)

2R

R

EA

+

–

+

1.0V

–

R

S

Q

5.0Vref

–

+

+

–

+

C

tSoft–Start  3600C in µF

1.0M

1.0mA

5(9)

R

R

Bias

Osc

8(14)

4(7)

2(3)

1(1)

2R

R

EA

+

–

+

Q1

RS

3(5)

5(8)

1.0V

–

R

S

Q

Comp/Latch

5.0Vref

VClamp

Vin

VCC

7(11)

6(10)

–

+

+

–

+

–

+

7(12)

+

–

MPSA63

R1

R2

C

tSoftstart = – In

1 –

VC

R1 R2

C

R2

VClamp =

1.67

+ 1

Ipk(max) = VClamp

RS

Where: 0

≤ VClamp ≤ 1.0 V

1.0mA

R1

3VClamp

R1 + R2

RS

1/4 W

(5)

(8)

–

R

S

Q

Comp/Latch

5.0Vref

Vin

VCC

(11)

(10)

–

+

+

–

+

–

+

(12)

+

–

Power Ground

To Input Source

Return

VPin 5 =

If: SENSEFET = MTP10N10M

RS = 200

Then: Vpin 5 = 0.075 Ipk

SENSEFET

RS Ipk rDS(on)

M

G

D

S

K

Control CIrcuitry

Ground:

To Pin (9)

rDM(on) + RS