ML4825
FUNCTIONAL DESCRIPTION
OSCILLATOR
The ML4825 oscillator charges the external capacitor (C
T
)
with a current (I
SET
) equal to 3/R
SET
. When the capacitor
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:
t
OSC
= t
RAMP
+ t
DEADTIME
where:
t
RAMP
=
C (Ramp Valley to Peak)
I
SET
1k
100
1k
10k
100k
100k
100nF
47nF
22nF
R
T
(Ω)
0nF
10k
4.7nF
2.2nF
1nF
470pF
1M
FREQUENCY (Hz)
Figure 2. Oscillator Timing Resistance vs Frequency
and:
t
DEADTIME
=
C (Ramp Valley to Peak)
I
Q1
160
140
I
SET
T
D
(ns)
5
1.0nF
5.1V
3V
I
SET
120
R
T
6
C
T
I
Q1
+
–
100
Q1
4
470pF
80
10k
100k
FREQUENCY (Hz)
1M
Figure 3. Oscillator Deadtime vs Frequency
CLOCK
t
D
RAMP PEAK
C
T
RAMP VALLEY
4.70
2.20
1.00
0.47
0.22
0.10
0.047
0.47
Figure 1. Oscillator Block Diagram
TD (µs)
1.0
2.2
4.7
10.0
22
47
100
CT (nF)
Figure 4. Oscillator Deadtime vs C
T
(3kΩ
≤
R
T
≤
100kΩ)
5
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
