ML4813
FUNCTIONAL DESCRIPTION
90%
10
8
OSCILLATOR
5nF
2nF
85%
The ML4813 oscillator charges the external capacitor (C )
with a current (I ) equal to 5/R . When the capacitor
1nF
T
SET
SET
10nF
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.
5
80%
70%
3
2
The oscillator period can be described by:
20nF
(1)
(2)
tOSC = tRAMP + tDEADTIME
where:
1
10
CT
100
OSCILLATOR FREQUENCY (kHz)
1000
tRAMP
and:
tDEADTIME
=
ISET
Figure 1. Oscillator Timing Resistance vs. Frequency
CT
=
(3)
8.4mA - ISET
EXTERNAL
CLOCK
A graph showing the relationship between R , C , the
T
T
oscillator frequency, and maximum duty cycle is given in
Figure 1. A pulse of a duration shorter than t
C
DEADTIME
SYNC
SYNC
from an external frequency source set to a higher
10
frequency than f
can be applied to pin SYNC to
OSC
I
synchronize the oscillator. R
and C
shorten
SET
SYNC
SYNC
R
T
R
SYNC
longer pulses, as shown in Figure 2.
9
R
T
I
SET
OUTPUT DRIVER STAGE
C
T
16
+
-
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.
C
T
5.6V
8.4mA
ERROR AMPLIFIER
The ML4813 error amplifier is a high open loop gain,
wide bandwidth amplifier. See Figure 4 for the gain and
phase plot.
CLOCK
UN-COMMITTED OP-AMP
T
D
RAMP PEAK
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.
V(C )
T
RAMP VALLEY
Figure 2. Oscillator Block Diagram
REFERENCE
The reference output voltage versus output current
characteristic is shown in Figure 5.
output low and quiescent current low. The ML4813
becomes operational when V reaches 16V. When V
CC
CC
drops below 10V, the UVLO condition is re-imposed.
UNDERVOLTAGE LOCKOUT
During UVLO, the V
pin is off, making it usable as a
REF
On power-up, the ML4813 is in the UVLO condition;
"flag" for starting up a down-stream PWM converter.
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
