UCC29421, UCC29422, UCC39421, UCC39422

MULTIMODE HIGHFREQUENCY PWM CONTROLLER

SLUS246C − OCTOBER 1999 − REVISED FEBRUARY 2005

16

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

flyback topology using n-channel MOSFETs (continued)

1

3

2

4

5

7

6

8

16

14

15

13

12

10

11

9

0.1

µF

Q1B

Si9802

(N)

1 k

0.1

µF

R

SENSE

0.05

Ω

R

SLOPE 1.5 k

ISENSE

VOUT

RSEN

VGRECT

PGND

CHRG

VPUMP

CP

VIN

RSEL

RT

GND

PFM

FB

COMP

SYNC/SD

UCC39421

C

POLE

R

COMP

C

COMP

RT

100 k

R1

20 k

1%

R2

41 k

1%

R3

100 k

1%

L1A

COILTRONICS

CTX5−2

CIN

100

µF

10V

+1.8 V TO 4.2 V

+VIN

RG1

4.7

RG2 4.7

C

OUT

VIN

V

OUT

2.5 V

0.1

µF

Q1A

Si9802

(N)

L1B

10

470 pF

+

+

R

BIAS 180 k

D1

1N4148

UDG−98211

Figure 7. Flyback Converter Using Charge Pump Input for Low Voltage Operation

relating peak inductor current to average output current for the flyback converter

For a continuous mode flyback converter, the average output current is related to the peak inductor current by

the following:

I

PEAK +

I

OUT

(1

* D)

) di

2

(9)

where D is the duty cycle and the inductor ripple current, dI, is defined as:

di

+

t

ON

V

IN

L

+

D

V

IN

f

L

(10)

where f is the switching frequency and L is the inductor value. The duty cycle is defined as: