UC3844, 45 UC2844, 45

10

MOTOROLA ANALOG IC DEVICE DATA

Figure 19. Adjustable Reduction of Clamp Level

Figure 20. Soft–Start Circuit

Figure 21. Adjustable Buffered Reduction of

Clamp Level with Soft–Start

Figure 22. Current Sensing Power MOSFET

Virtually lossless current sensing can be achieved with the implement 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 19 and 21.

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 10–3

Ipk(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

1.0mA

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

+ 0.33 x 10–3

R1 R2

R1 + R2

Figure 23. Current Waveform Spike Suppression

The addition of the RC filter will eliminate

instability caused by the leading edge spike on

the current waveform.

Q1

RS

3(5)

5(8)

–

R

S

Q

Comp/Latch

5.0Vref

Vin

VCC

7(11)

6(10)

–

+

+

–

+

–

+

7(12)

+

–

R

C

T

T

T

T

T