CS51221 datasheet(10/14 Pages) ONSEMI

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CS51221

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Figure 8. The SYNC Pin Generates a Sync Pulse at

the Beginning of Each Switching Cycle.

CH2: GATE Pin, CH3: RTCT, CH4: SYNC Pin

Figure 9. Operation with External Sync.

CH2: SYNC Pin, CH3: GATE Pin, CH4: RTCT Pin

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

IC immediately discharges CT. The GATE signal is turned

on once the RTCT pin reaches the valley voltage. Because of

the steep falling edge, this valley voltage falls below the

regular 1.0 V threshold. However, the RTCT pin voltage is

then quickly raised by a clamp. When the RTCT pin reaches

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

disconnected after a brief delay and CT is charged through

RT.

DESIGN GUIDELINES

Switch Frequency and Maximum Duty Cycle

Calculations

Oscillator timing capacitor, CT, is charged by VREF

through RT and discharged by an internal current source.

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:

tC = charging time;

td = discharging time;

VVALLEY = valley voltage of the oscillator;

VPEAK = peak voltage of the oscillator.

Substituting in typical values for the parameters in the

above formulas, VREF = 3.3 V, VVALLEY = 1.0 V, VPEAK =

2.0 V, Id = 1.0 mA:

tC + 0.57RTCT

td + RTCT ln

1.3

* 0.001RT

2.3

* 0.001RT

D max +

0.57

) In 1.3*0.001RT

2.3

*0.001RT

It is noticed from the equation that for the oscillator to

function properly, RT has to be greater than 2.3 k.

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

is equal to VIN/R. Therefore, the volt−second value is

determined by:

VIN

TON + (VCOMP * VFF(d))

where:

VCOMP = COMP pin voltage;

VFF(d) = FF pin discharge voltage.

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

the VCOMP clamp voltage which is equal to 1.8 V. In

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

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

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