UCC29421, UCC29422, UCC39421, UCC39422

MULTIMODE HIGHFREQUENCY PWM CONTROLLER

SLUS246C − OCTOBER 1999 − REVISED FEBRUARY 2005

24

POST OFFICE BOX 655303

APPLICATION INFORMATION

non-synchronous boost for higher output voltage applications

The UCC39421 can also be used in non-synchronous applications to provide output voltages greater than 8

volts from low voltage inputs. An example of a 12-V boost application is shown in Figure 16. Since none of the

IC pins are exposed to the boosted voltage, the output voltage is limited only by the ratings of the external

MOSFET, rectifier, and filter capacitor. At these higher output voltages, good efficiency is maintained since the

rectifier drop is small compared to the output voltage. Note that PFM mode can still be used to maintain high

efficiency at light load. Typical efficiency causes are shown in Figure 15.

Since all the power supply pins (VIN, VOUT, VPUMP) operate off the input voltage, it must be >2.5 V and high

enough to assure proper gate drive to the charge FET.

UDG−98216

1

3

2

4

5

7

6

8

16

14

15

13

12

10

11

9

ISENSE

VOUT

RSEN

VGRECT

PGND

CHRG

VPUMP

CP

VIN

RSEL

RT

GND

PFM

FB

COMP

SYNC/SD

UCC39421

R2

R1

VOUT

RT1

2N7002

RT2

FREQUENCY

CONTROL

Figure 14. Changing the PWM Frequency