ml4823 Fairchild Semiconductor, ml4823 Datasheet - Page 5

• Manufacturer Part Number: ml4823
• Description: High Frequency Power Supply Controller
• Manufacturer: Fairchild Semiconductor

FUNCTIONAL DESCRIPTION
OSCILLATOR
The ML4823 oscillator charges the external capacitor (C
with a current (I
voltage reaches the upper threshold (Ramp Peak), the
comparator changes state and the capacitor discharges to
the lower threshold (Ramp Valley) through Q1. While the
capacitor is discharging, Q2 provides a high pulse.
REV. 1.0 10/12/2000
RAMP VALLEY
CLOCK OUT
Figure 2. Oscillator Timing Resistance vs Frequency
RAMP PEAK
I
R
SET
T
100k
10k
1k
Figure 1. Oscillator Block Diagram
0.1k
C
R
I
3V
T
T
Q1
100nF
C
T
SET
Q1
47nF
) equal to 3/R
1k
22nF
10nF
10k
FREQ (Hz)
4.7nF
I
SET
+
–
SET
100k
2.2nF
. When the capacitor
1nF
5.1V
1M
T
470pF
400 A
D
10M
Q
2
CLOCK
OUT
T
)
The oscillator period can be described by the following
relationship:
where: T
and:
Figure 4. Oscillator Deadtime vs C
160
140
120
100
80
10k
Figure 3. Oscillator Deadtime vs Frequency
T
T
OSC
RAMP
DEADTIME
30
25
20
15
10
5
0
0.47
= T
= C (Ramp Valley to Peak)/I
1.0
RAMP
= C (Ramp Valley to Peak)/I
470pF
1.0nF
2.2
+ T
100k
DEADTIME
4.7
C
T
FREQ (Hz)
(nF)
10.0
T
22
(3ký - R
47
SET
ML4823
1M
100
Q1
T
- 100ký)
5