UCC29421, UCC29422, UCC39421, UCC39422

MULTIMODE HIGHFREQUENCY PWM CONTROLLER

SLUS246C − OCTOBER 1999 − REVISED FEBRUARY 2005

7

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

The UCC39421 is a high frequency, synchronous PWM controller optimized for portable, battery-powered

applications where size and efficiency are of critical importance. It includes high-speed, high-current FET

drivers for those converter applications requiring low Rds(on) external MOSFETs. A detailed block diagram is

shown in Figure 2.

optimizing efficiency

The UCC39421 optimizes efficiency and extends battery life with its low quiescent current and its synchronous

rectifier topology. The additional features of low-power (LP) mode and PFM mode maintain high efficiency over

a wide range of load current. These features are discussed in detail.

power saving modes

Since this is a peak current mode controller, the error amplifier output voltage sets the peak inductor current

required to sustain the load. The UCC39421 incorporates two special modes of operation designed to optimize

efficiency over a wide range of load current. This is done by comparing the error amplifier output voltage (on

the COMP pin) to two fixed thresholds (one of which is user programmable). If the error amplifier output voltage

drops below the first threshold, low power mode is entered. If the error-amplifier output voltage drops even

further, below a second user programmable threshold, PFM mode is entered. These modes of operation are

designed to maintain high efficiency at light loads, and are described in detail in the following text. Refer to the

simplified block diagram of Figure 1 for the control logic.

UDG−98108

VOUT

SENSE

+

+

+

0.2 V

1.22

+

+

1=SLEEP

Q

S

R

HOLD AMP

70 mV

PFM

PFM COMP

+

PFM DISABLE COMP

ERROR AMP

1.24 V

0.6 V

LPM COMP

FB

COMP

PFM

1 = LP_MODE

Figure 1. Simplified Block Diagram of Low Power and Pulse Mode Control Logic