NCP1294

http://onsemi.com

10

Figure 8. The SYNC Pin Generates a Sync Pulse at

the Beginning of Each Switching Cycle.

Figure 9. Operation with External Sync.

An external pulse signal can feed to the bidirectional

SYNC pin to synchronize the switch frequency. For reliable

operation, the sync frequency should be approximately 20%

higher than free running IC frequency. As show in Figure 9,

when the SYNC pin is triggered by an incoming signal, the

the steep falling edge, this valley voltage falls below the

the 0.95 V (typ) Valley Clamp Voltage, the clamp is

DESIGN GUIDELINES

Switch Frequency and Maximum Duty Cycle

Calculations

During the discharge time, the internal clock signal sets the

Gate output to the low state, thus providing a user selectable

maximum duty cycle clamp. Charge and discharge times are

determined by following general formulas;

tC + RTCT ln

(VREF * VVALLEY)

(VREF * VPEAK)

td + RTCT ln

(VREF * VPEAK * IdRT)

(VREF * VVALLEY * IdRT)

where:

Substituting in typical values for the parameters in the

tC + 0.57RTCT

td + RTCT ln

1.3 * 0.001RT

2.3 * 0.001RT

D max +

0.57

2.3*0.001RT

It is noticed from the equation that for the oscillator to

Select RC for Feed Forward Ramp

If the line voltage is much greater than the FF pin Peak

Voltage, the charge current can be treated as a constant and

determined by:

VIN

TON + (VCOMP * VFF(d))

R

C

where:

As shown in the equation, the volt−second clamp is set by

Forward or Flyback circuits, the volt−second clamp value is

designed to prevent transformers from saturation.

In a buck or forward converter, volt−second is equal to

VIN

TON +

VOUT

TS

n

n = transformer turns ratio, which is a constant determined

by the regulated output voltage, switching period and

transformer turns ration (use 1.0 for buck converter). It is

interesting to notice from the aforementioned two equations