UCC28089

SLUS623A -- SEPTEMBER 2004 -- REVISED AUGUST 2006

7

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APPLICATION INFORMATION

be small enough for the oscillator to have at least 10 μA of current. There are two sets of oscillator programming

equations that model the oscillator over its wide programming range. Measure the charge and the discharge

times at the SYNC pin in order to avoid affecting the oscillator with probe impedances or output driver delays.

The approximate first order equations in the table are adequate for switching frequencies below 50 kHz and/or

discharge times that are greater than 1 μs. The specific requirements for using the first order equations versus

the second order equations are related to the timing capacitor size and the discharge resistor. Keep in mind that

the 1st order equations and 2nd order equations are merely approximations that are typically within +/--20% of

the actual operating point. The frequency, charge and discharge times are relatively insensitive to temperature

equations apply for the operating conditions that are in the Electrical Characteristics table. The oscillator

frequency is set according to the following equations:

Condition

0.169

0.175

1.36 RB CT

(1.37)

5.9

1

TCHARGE + TDISCHARGE

values (100 pF to 0.100 μF).

SYNC: This SYNC pin produces an output pulse from 0 to VDD that can be used to synchronize a secondary

side-buck controller to the free running isolating power stage. The proper timing of this signal enables zero

voltage switching on the primary side MOSFETs. The clean signal also solves a problem of getting a

synchronization signal from the secondary side of the transformer, which can have leakage inductance voltage

spikes that may cause false triggering. The SYNC pulse width is the oscillator discharge time, which is

approximately equal to the dead time. Pulse frequency is the oscillator frequency. During fault conditions, the

SYNC pulses are terminated and the SYNC output is held low for at least 56 oscillator cycles. During soft start,

SYNC precedes the first output pulse by at least one oscillator cycle.

CS: Connect the current sense device to this pin. A voltage threshold of 0.725 V triggers a shutdown sequence.

An over-current fault triggers an immediate shutdown. After the fault clears, a total of 64 oscillator cycles are

required for an entire soft start sequence to occur. First, the outputs and SYNC are kept OFF for at least 56

oscillator cycles. Next, after one or two SYNC pulses, the soft start progressively increases the output duty ratio

over the next five to seven oscillator cycles.