ML4813CS datasheet(12/15 Pages) MICRO-LINEAR

12 / 15 page

ML4813

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 39k

W, 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:

RR25

OUT =

518

’

(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:

OUT @

1000

(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:

OVP

OUT

112

(22)

APPLICATIONS (Continued)

OA+

OA-

OA OUT

–

VOUT+

VOUT-

ERROR AMP

AND OVP

R14'

R15'

R17

R18

R12

R25

Figure 11. Ground Referencing the Op Amp Output

Figure 12. Setting OVP at > 1.12 x VOUT

Figure 13. Setting OVP at < 1.12 x VOUT

OVP

OA OUT

ML4813

OA OUT

R24

OVP

VFB

COMP

C13

ML4813

Figure 15 shows OVP set for a voltage lower than 1.12 x

VOUT where:

RD R24

OVP

OUT

112

(23)

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