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
DELAY
resistor as shown below. This resistor
value should be no less than 1kΩ. Expressing R
DELAY
in
kΩ the delay, in ns is:
T
DELAY
=
33
×
R
DELAY
+
45
(1)
I
SET
11
R
T
1.7V
2
C
T
5.5mA
Q1
I
SET
5V
CLOCK
OUT
13
250Ω
5V
+
–
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
Φ
Comparator trips in voltage mode.
Figure 5. Ocillator Block Diagram
Φ
MOD
OUTPUT
RAMP
S
Q
OSC
R
Q
R
3
For frequencies of less than 500kHz, oscillator frequency
can be set by using the following formulae:
f
OSC
=
1
(2)
0.52 C
T
R
T
+
500 C
T
Figure 3. Voltage Feed-Forward Circuit.
The collector of Q
R
in figure 3 is high only during a power
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
IN
to
provide input voltage feed forward.
OSCILLATOR
The ML4818 oscillator charges the external capacitor, C
T
,
with a current (I
SET
) equal to 5/R
T
. When the C
T
voltage
reaches the upper threshold (Ramp Peak), the comparator
changes state, turning on the current sink which
discharges C
T
to the lower threshold (Ramp Valley). The
C
T
pin is clamped to Ramp Valley by Q1 (Figure 5) to
prevent inaccuracy due to undershoot on C
T
.
To use the CLOCK output for driving external synch-
ronization circuitry, a pull-down resistor is required from
CLOCK to GND.
ERROR AMPLIFIER
The ML4818 error amplifier is a 2.5MHz bandwidth,
8.5V/µs 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.
CLOCK
RAMP PEAK
C
T
RAMP VALLEY
T
C
T
D
Figure 4. Ocillator Timing Diagram
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
