ML4818CS Micro Linear, ML4818CS Datasheet - Page 6

• Manufacturer Part Number: ML4818CS
• Description: Phase Modulation/Soft Switching Controller
• Manufacturer: Micro Linear

ML4818
The ML4818 can also be used in current mode by sensing
load current on the RAMP input (pin 3).
The four output delay timers are programmed via an
external R
value should be no less than 1k . Expressing R
k the delay, in ns is:
The ML4818 contains special logic circuits to provide for
voltage mode feed-forward and lock out long pulses into
the internal logic. This prevents instability from occuring
when the
The collector of Q
cycle. When the power cycle terminates, RAMP is pulled
low. In voltage mode operation, a capacitor is connected
from RAMP to GND with a resistor from RAMP to V
provide input voltage feed forward.
OSCILLATOR
The ML4818 oscillator charges the external capacitor, C
with a current (I
reaches the upper threshold (Ramp Peak), the comparator
changes state, turning on the current sink which
discharges C
C
prevent inaccuracy due to undershoot on C
To use the CLOCK output for driving external synch-
ronization circuitry, a pull-down resistor is required from
CLOCK to GND.
6
T
RAMP VALLEY
pin is clamped to Ramp Valley by Q1 (Figure 5) to
RAMP PEAK
CLOCK
Figure 3. Voltage Feed-Forward Circuit.
DELAY
OSC
OUTPUT
Figure 4. Ocillator Timing Diagram
Comparator trips in voltage mode.
T
MOD
C
to the lower threshold (Ramp Valley). The
T
SET
resistor as shown below. This resistor
T
DELAY
R
) equal to 5/R
S
R
in figure 3 is high only during a power
T
C
T
33
D
Q
R
DELAY
T
. When the C
45
Q
RAMP
R
T
.
T
DELAY
3
voltage
IN
in
to
(1)
T
,
For frequencies of less than 500kHz, oscillator frequency
can be set by using the following formulae:
ERROR AMPLIFIER
The ML4818 error amplifier is a 2.5MHz bandwidth,
8.5V/ s slew rate op-amp with provision for limiting the
positive output voltage swing (output inhibit line) to
implement the soft start function. The error amplifier
output source current is limited to 4.5mA.
I
R
SET
T
C
T
11
2
5.5mA
Figure 5. Ocillator Block Diagram
1.7V
f
OSC
Q1
0 52
.
C R
T T
I
SET
1
+
–
5V
500
C
T
5V
CLOCK
250
OUT
(2)
13