
UCC28089
SLUS623A -- SEPTEMBER 2004 -- REVISED AUGUST 2006
7
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APPLICATION INFORMATION
The recommended oscillator frequency range is up to 1 MHz. In order to avoid noise issues, RAand RBshould
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
but larger values of CTand RBexhibit the least sensitivity to temperature. Incidentally, the second order
equations apply for the operating conditions that are in the Electrical Characteristics table. The oscillator
frequency is set according to the following equations:
1ST ORDER EQUATIONS 2ND ORDER EQUATIONS
Condition RA> 300 kΩAND CT> 300 pF 32 kΩ<R
A< 300 kΩOR 100 pF < CT< 300 pF
TCHARGE 0.169RA+RBCT0.175RA+RBCT+40 pF+20 ns
TDISCHARGE 1.36 RBCT(1.37)RB+44CT+14 pF+20 ns
fOSC 5.9
RA+8.0 RBCT
1
TCHARGE +TDISCHARGE
Where RAand RBareinOhms;C
Tis in Farads; fOSC is in Hz; tCHARGE and tDISCHARGE are in seconds.
The oscillator is optimized for a CTtiming capacitor range from 100 pF to 1000 nF and RBmore than 100 Ω.
If the shortest discharge time possible is desired, it is permissible to short DIS to CTfor all recommended CT
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.