UCC29421, UCC29422, UCC39421, UCC39422

MULTIMODE HIGHFREQUENCY PWM CONTROLLER

SLUS246C − OCTOBER 1999 − REVISED FEBRUARY 2005

23

POST OFFICE BOX 655303

APPLICATION INFORMATION

changing the low power mode threshold (continued)

Although the LP mode threshold is internally fixed at 0.6 V (referenced to the pin), the point at which the LP

the output voltage of the sense amplifier and fooling the LP mode comparator into thinking the load is higher

than it is. The required bias resistor to transition out of LP mode for a given peak current can be calculated using

the following equation:

R

BIAS +

R

SLOPE

V

OUT

0.03

* I

PEAK

R

SENSE

(17)

UDG−98213

VOUT

COUT

+

300 mV

RSENSE

RSLOPE

RBIAS

DRIVE

CONTROL

LOGIC

+

LP MODE

+

X10

600 mV

PWM

COMP

FB

1.24 V

CHRG

ISENSE

17

18

6

12

UCC39421

VIN

Figure 13. Modifying Low Power (LP) Mode Threshold

Due to the current-sense amplifier gain of 10 and the internal offset of 300 mV, an offset of just 30 mV or more

at the ISENSE pin inhibits the LP mode altogether. Note that inhibiting LP mode does not prevent PFM from

working, as long as the PFM pin is set to a voltage higher than:

10

V

ISENSE )

0.3V

18)

programming the PWM frequency

Some applications may want to remain in a fixed frequency mode of operation, even at light load, rather than

going into PFM mode. This lowers efficiency at light load. One way to improve the efficiency while maintaining

fixed frequency operation is to lower the PWM frequency under light load conditions. This can be easily done,

as shown in Figure 14. By adding a second timing resistor and a small MOSFET switch, the host can switch

between two discrete frequencies at any time.