ML4825 Micro Linear, ML4825 Datasheet - Page 5

• Manufacturer Part Number: ML4825
• Description: High Frequency Power Supply Controller
• Manufacturer: Micro Linear

FUNCTIONAL DESCRIPTION
OSCILLATOR
The ML4825 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.
The oscillator period can be described by the following
relationship:
where:
and:
t
t
t
OSC
RAMP
DEADTIME
I
SET
R
C
RAMP VALLEY
= t
T
T
RAMP PEAK
Figure 1. Oscillator Block Diagram
RAMP
C Ramp Valley to Peak
5
6
CLOCK
(
I
SET
Q1
3V
C Ramp Valley to Peak
+ t
) equal to 3/R
(
DEADTIME
Q1
C
T
I
SET
I
t
D
Q
1
I
SET
+
–
SET
. When the capacitor
5.1V
)
)
4
T
)
Figure 4. Oscillator Deadtime vs C
Figure 2. Oscillator Timing Resistance vs Frequency
Figure 3. Oscillator Deadtime vs Frequency
0.047
100k
4.70
2.20
1.00
0.47
0.22
0.10
10k
160
140
120
100
1k
80
0.47
100
10k
100nF
1.0
47nF
1k
1.0nF
470pF
2.2
22nF
FREQUENCY (Hz)
FREQUENCY (Hz)
100k
4.7
10k
0nF
C T (nF)
10.0
4.7nF
T
100k
(3k
2.2nF
22
1M
1nF
ML4825
47
1M
470pF
R
T
100
100k )
5