ML4813
FUNCTIONAL DESCRIPTION
OSCILLATOR
The ML4813 oscillator charges the external capacitor (C
T
)
with a current (I
SET
) equal to 5/R
SET
. When the capacitor
voltage reaches the upper threshold, the comparator
changes state and the capacitor discharges to the lower
threshold through Q1. While the capacitor is discharging,
Q2 provides a high pulse.
The oscillator period can be described by:
t
OSC
=
t
RAMP
+
t
DEADTIME
10
8
10nF
5
80%
90%
5nF
2nF
1nF
85%
MAXIMUM DUTY CYCLE (%)
RT (kΩ)
3
70%
20nF
(1)
2
where:
t
RAMP
C
=
T
I
SET
(2)
1
10
100
OSCILLATOR FREQUENCY (kHz)
1000
and:
Figure 1. Oscillator Timing Resistance vs. Frequency
t
DEADTIME
=
C
T
8.4mA
-
I
SET
(3)
EXTERNAL
CLOCK
CSYNC
10
A graph showing the relationship between R
T
, C
T
, the
oscillator frequency, and maximum duty cycle is given in
Figure 1. A pulse of a duration shorter than t
DEADTIME
from an external frequency source set to a higher
frequency than f
OSC
can be applied to pin SYNC to
synchronize the oscillator. R
SYNC
and C
SYNC
shorten
longer pulses, as shown in Figure 2.
OUTPUT DRIVER STAGE
The ML4813 output driver is a 1A peak output high speed
totem-pole circuit designed to quickly drive capacitive
loads such as MOSFET gates. See Figure 3 for the output
saturation characteristics for sourcing and sinking current.
ERROR AMPLIFIER
The ML4813 error amplifier is a high open loop gain,
wide bandwidth amplifier. See Figure 4 for the gain and
phase plot.
UN-COMMITTED OP-AMP
The ML4813 contains an uncommitted op amp which is
normally configured as a differencing amplifier to sense
the output voltage. The output voltage in the flyback
configuration is not ground referenced. The op amp in the
ML4813 is a PNP input amplifier similar to the LM324
but with an open emitter class A output stage.
REFERENCE
The reference output voltage versus output current
characteristic is shown in Figure 5.
UNDERVOLTAGE LOCKOUT
On power-up, the ML4813 is in the UVLO condition;
SYNC
ISET
RSYNC
RT
9
RT
16
CT
8.4mA
ISET
+
CT
5.6V
-
CLOCK
TD
RAMP PEAK
V(CT)
RAMP VALLEY
Figure 2. Oscillator Block Diagram
output low and quiescent current low. The ML4813
becomes operational when V
CC
reaches 16V. When V
CC
drops below 10V, the UVLO condition is re-imposed.
During UVLO, the V
REF
pin is off, making it usable as a
"flag" for starting up a down-stream PWM converter.
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
