Datasheet

 
SLUS318B – APRIL 1999 – REVISED JANUARY 2002
9
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APPLICATION INFORMATION
configuring the oscillator
The UCC3626 oscillator is designed to operate at frequencies up to 250 kHz and provide a triangle waveform
on CT with a peak-to-peak amplitude of 5 V for improved noise immunity. The current used to program CT is
derived from the R_TACH resistor according to the following equation:
I
OSC
+
25
R_TACH
Amps
The oscillator frequency is set by R_TACH and CT according to the following relationship:
f
OSC
+
2.5
R_TACH CT
Hz
Timing resistor values should be between 25 kΩ and 500 kΩ, while capacitor values should be between 100 pF
and 1 µF. Figure 3 provides a graph of oscillator frequency for various combinations of timing components. As
with any high-frequency oscillator, timing components should be located as close as possible to the device pins
when laying out the printed-circuit board. It is also important to reference the timing capacitor directly to the
ground pin on the UCC3626 rather than daisy chaining it to another trace or the ground plane. This technique
prevents switching current spikes in the local ground from causing jitter in the oscillator.
synchronizing the oscillator
A common system specification is to have all oscillators synchronized to a master clock. The UCC3626 provides
a SYNCH input for this purpose. The SYNCH input is designed to interface with a digital clock pulse generated
by the master oscillator. A positive-going edge on this input causes the UCC3626 oscillator to begin discharging.
In order for the slave oscillator to function properly, it must be programmed for a frequency slightly lower than
that of the master. Also, a resistor equal to R_TACH must be placed in parallel with CT. Figure 4 illustrates the
waveforms for a slave oscillator programmed to 20 kHz with a master frequency of 30 kHz. The SYNCH pin
must be grounded when not used.
Figure 3
1.E+03
1.E+04
1.E+05
1.E+06
1.E–10
1.E–09
1.E–08
1.E–07
R_TACH = 25 kΩ
R_TACH = 100 kΩ
R_TACH = 500 kΩ
R_TACH = 250 kΩ
C
T
– Oscillator Timing Capacitance – F
f
OSC
– PWM Frequency – Hz
OSCILLATOR FREQUENCY
vs
TIMING CAPACITANCE
(1)
(2)