ml4813 Micro Electronics Corporation, ml4813 Datasheet - Page 12

• Manufacturer Part Number: ml4813
• Description: Flyback Power Factor Controller
• Manufacturer: Micro Electronics Corporation

ML4813
APPLICATIONS
OFF-LINE START-UP AND BIAS SUPPLY GENERATION
A fast starting circuit is included in Figure 9. MOSFET Q2
quickly charges the ML4813 V
supply is initially turned on. This allows the supply to
come on less than 1 second after AC power is applied. A
simpler start-up circuit may be used which replaces the
active circuit with a 39k
slowly (up to 15 seconds under low line conditions).
Systems which do not require quick starting can reduce
cost with the latter start-up method.
POWER FACTOR ENHANCEMENT
Some combinations of line and load may exhibit
distortion of the input current waveform. This distortion is
usually caused by the inductor "ringing" with the C
the power MOSFET, resulting in a non-zero inductor
current at the beginning of the next cycle. This ringing
can be dampened by using R2 and D7 as shown in Figure
9. Applications which can get by with slightly worse
power factor can eliminate these components.
ADJUSTING THE OUTPUT VOLTAGE
The error amplifier creates an error voltage from the
difference between the output voltage presented at OA
OUT and the 5V internal reference. Since the output
voltage is not ground referenced, the ML4813's internal
op amp is connected as an instrumentation amplifier as
shown in Figure 11.
The output voltage is set by resistors which determine the
relationship between (V
the op amp. For the following discussion, R15' = R15 +
R16 and R14' = R13 + R14. The differencing amplifier
depends on the following relationships:
Then:
Since R25 is a low value compared to R15', the second
term reduces to approximately 1. The third term is set at
approximately 200. Equation (20) can be reduced to:
The overvoltage comparator has a threshold that is set for
1.12 x V
directly. Figure 12 shows the connection for setting an
OVP trip point higher than 1.12 x V
12
R14'= R15'
R12 = R25 + (R17 || R18)
V
V
V
OUT
OUT
OVP
OUT
@
=
@
112
1000
.
when OVP and OA OUT are connected
R
5
17
V R
™
+
+
™
V
OUT
R
18
18
R
17
(Continued)
OUT+
™
R
18
™
+

 
R
R
2W resistor but starts more
18
R
A
15
- V
R
CC
+
R
B
’
15
+
R
OUT-
R25
capacitor (C8) when the
B
’
OUT
) and the output of
™
, where:
R
R
14
12
’
+
1
DS
(20)
(21)
(22)
of
Figure 15 shows OVP set for a voltage lower than 1.12 x
V
OUT
V OUT+
V OUT-
Figure 11. Ground Referencing the Op Amp Output
V
OVP
where:
Figure 12. Setting OVP at > 1.12 x V
Figure 13. Setting OVP at < 1.12 x V
@
112
.
R14'
R15'
R12
RD
RB
™
C13
V
OUT
OA+
OA-
™
R24
RA
RD R24
+
–
R25
RD
+
OA OUT
OVP
OA OUT
COMP
V FB
OVP
OA OUT
ML4813
ML4813
R17
R18
OUT
OUT
ERROR AMP
AND OVP
TO
(23)