Datasheet

UCC25705, UCC25706, UCC35705, UCC35706
HIGH-SPEED VOLTAGE MODE PULSE WIDTH MODULATOR
SLUS473B -- NOVEMBER 1999 -- REVISED OCTOBER 2010
8
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FUNCTIONAL DESCRIPTION
oscillator without duty cycle clamp (MODE = 0)
In this mode, the timing capacitor is discharged through a low impedance directly to ground. The DISCH pin is
externally grounded. A comparator connected to DISCH senses the ground connection and disables both the
discharge current source and VFF current source. A resistor divider is now required to set VFF.
45
RC DISCH
3
VFF
R
T
C
T
V
IN
Figure 3. Ocsillator Without Clamp (MODE = 0)
In this mode, the on-time is approximately:
T
ON
=∝ × R
T
× C
T
where ∝=
V
FF
V
IN
Theoff-timeis:
T
OFF
≈ 75 ns
The frequency is:
f =
1
∝×R
T
× C
T
+ 75 ns