UCC29421, UCC29422, UCC39421, UCC39422

MULTIMODE HIGHFREQUENCY PWM CONTROLLER

SLUS246C − OCTOBER 1999 − REVISED FEBRUARY 2005

21

POST OFFICE BOX 655303

APPLICATION INFORMATION

start up (continued)

If input voltages below 2.5 V are expected, it is important to use a low voltage logic N-channel MOSFET (with

a threshold voltage around 1 V or less) to ensure start-up at full load.

A block diagram of the low voltage start-up logic is shown in Figure 12.

UDG−98121

DRIVER

5−

µsec

DELAY

2.5−

µsec

DELAY

MUX

VIN

VPUMP

2.5 V < VPUMP

LPM < VCOMP

NORMAL PWM

B

A

VOUT

VIN

2.5 V < VPUMP

2.5 V < VOUT

VPUMP

UCC39421

+

S

R

Q

Q

A/B

+

RSENSE

CFLY

COUT

CPUMP

L

DBODY

36 mV

−

−

Figure 12. Symplified Diagram of Low Voltage Start−Up and Charge Pump Control Logic

anticross-conduction and adaptive synchronous rectifier commutation logic

When operating in the continuous conduction mode (CCM), the charge MOSFET and the synchronous rectifier

MOSFET are simply driven out of phase, so that when one is on the other is off. There is a built-in time delay

of about 30 ns to prevent any cross-conduction.

In the event that the converter is operating in the discontinuous conduction mode (DCM), the synchronous

rectifier needs to be turned off sooner, when the rectifier current drops to zero. Otherwise, the output begins

to discharge as the current reverses and goes back through the rectifier to the input. (This obviously cannot

happen when using a conventional diode rectifier). To prevent this, the UCC39421 incorporates a high-speed

comparator that senses the voltage on the synchronous rectifier (using the RSEN input) for purposes of

commutation. In the boost and SEPIC topologies, the synchronous rectifier is turned off when the voltage on

decoupled.