UCC29421, UCC29422, UCC39421, UCC39422

MULTIMODE HIGHFREQUENCY PWM CONTROLLER

SLUS246C − OCTOBER 1999 − REVISED FEBRUARY 2005

6

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

RESET: This is the open drain output of the reset comparator. (UCC39422 only) and is active low.

RT: A resistor from this pin to ground programs the frequency of the pulse width modulator.

Frequency (MHz)

^

50

R

T

(k

W)

SYNC/SD: This pin has two functions. It may be used to synchronize the UCC39421’s switching frequency to

an external clock, or to shutdown the IC entirely. In shutdown, the quiescent current is reduced to just a few

microamps (both external FETs are turned off). To shutdown the converter, this pin must be held high (above

2.0 V) for a minimum of 29

µs. If not used, this pin should be grounded.

To synchronize the internal oscillator to an external source, the SYNC/SD pin must be driven with a clock pulse,

with a minimum amplitude of 2.0 V. The internal circuitry syncs to the rising edge of the external clock. The clock

pulse width is not critical (must be 50 ns minimum).

Note: When coming out of shutdown (or during power-up), the SYNC/SD pin must be held low for a minimum

of 200

µs before applying an external clock to ensure startup.

VPUMP: This is the output of the charge pump. For applications requiring a charge pump, connect a 1-

µF

capacitor from this pin to ground. Otherwise, connect this pin to the higher of VIN or VOUT, and decouple with

a 0.1-

µF capacitor.

VOUT: Connect this pin to the output voltage. This input is used for sensing the voltage across the synchronous

rectifier and for supplying power to internal circuitry and should be decoupled with a 0.1-

µF capacitor.

VIN: This is the input power pin of the device. Connect this pin to the input voltage source. A 0.1-

µF decoupling

capacitor should be connected between this pin and ground.

VDET: This is the non-inverting input to an uncommitted comparator. This input may be used for detecting a

low-battery condition. (UCC39422 only)