UCC29421, UCC29422, UCC39421, UCC39422

MULTIMODE HIGHFREQUENCY PWM CONTROLLER

SLUS246C − OCTOBER 1999 − REVISED FEBRUARY 2005

13

POST OFFICE BOX 655303

APPLICATION INFORMATION

boost using N- and P-channel MOSFETs

For output voltages greater than the input and greater than about 3.0 V, a P-channel may be used for the

synchronous rectifier. This configuration is shown in Figure 5. In this case, the VPUMP pin should be connected

to VOUT. This configuration can be used for a 3.3 V output if a low voltage logic MOSFET is used.

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

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

the following:

I

PEAK +

I

OUT

(1

* D)

) di

2

(5)

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

(6)

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

D

+

V

O *

V

IN

V

O

(7)

Substituting equations (6) and (7) into equation (5) yields:

I

PEAK +

I

OUT

1

*

V

O*

V

IN

V

O

)

V

IN

2

f

L

V

O *

V

IN

V

O

(8)

Note that in these equations, the voltage drop across the rectifier has been neglected.