ML4813IP View Datasheet(PDF) - Micro Linear Corporation
Part Name
Description
MFG CO.
ML4813IP Datasheet PDF : 15 Pages
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ML4813
APPLICATIONS (Continued)
OFF-LINE START-UP AND BIAS SUPPLY GENERATION
A fast starting circuit is included in Figure 9. MOSFET Q2
quickly charges the ML4813 VCC capacitor (C8) when the
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 39kW, 2W resistor but starts more
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 CDS of
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 (VOUT+ - VOUT-) and the output of
the op amp. For the following discussion, R15' = R15 +
R16 and R14' = R13 + R14. The differencing amplifier
depends on the following relationships:
R14'= R15'
R12 = R25 + (R17 || R18)
Then:
VOUT =
5V + R18
R17 + R18
R15’
R15’+R25
R14’ + 1
R12
(20)
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:
VOUT @ 1000
R18
R17 + R18
(21)
The overvoltage comparator has a threshold that is set for
1.12 x VOUT when OVP and OA OUT are connected
directly. Figure 12 shows the connection for setting an
OVP trip point higher than 1.12 x VOUT, where:
VOVP @ 1.12 VOUT
RA + RB
RB
(22)
12
VOUT+
VOUT-
R12
R14'
R15'
OA+
+
–
OA-
OA OUT
R25
TO
ERROR AMP
AND OVP
R17
R18
Figure 11. Ground Referencing the Op Amp Output
OVP
ML4813
RB
RA
OA OUT
Figure 12. Setting OVP at > 1.12 x VOUT
C13
RD
R24
COMP
VFB ML4813
OVP
OA OUT
Figure 13. Setting OVP at < 1.12 x VOUT
Figure 15 shows OVP set for a voltage lower than 1.12 x
VOUT where:
VOVP @ 1.12 VOUT
RD + R24
RD
(23)
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