Datasheet

UCC25705, UCC25706, UCC35705, UCC35706
HIGH-SPEED VOLTAGE MODE PULSE WIDTH MODULATOR
SLUS473B -- NOVEMBER 1999 -- REVISED OCTOBER 2010
7
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FUNCTIONAL DESCRIPTION
component selection for oscillator with duty cycle clamp (MODE = 1)
For a power converter with the following specifications:
D V
IN(min)
=18V
D V
IN(max)
=75V
D V
IN(shutdown)
=15V
D F
OSC
=1MHz
D D
MAX
=0.78atV
IN(min)
In this mode, the on-time is approximately:
D T
ON(max)
= 780 ns
D T
OFF(min)
= 220 ns
D V
FF(min)
=
18
15
=1.20V
(1) Pick C
T
= 220 pF.
(2) Calculate R
T
.
R
T
=
V
IN

min

× T
ON
(
max
)
V
FF

min

× C
T
R
T
= 51.1 kΩ
(3) R
DISCH
R
DISCH
=
30 × R
T
1 +
⎪
⎪
⎪
⎪
⎧
⎩

V
FF(min)
V
IN(min)

×R
T
×C
T
T
OFF(min)
⎪
⎪
⎪
⎪
⎫
⎭
R
DISCH
= 383 kΩ.
I
DISCH
must be between 25 μA and 250 μA over the entire VIN range.
With the calculated values, I
DISCH
ranges from 44 μA to 193 μA, within the allowable range. If I
DISCH
is too high, C
T
must be decreased.
(4) R
FF
R
FF
=
V
FF(min)
× R
DISCH
2 ×

V
IN(min)
− 1

The nearest 1% standard value to the calculated value is 13.7 k.