ML4812CP(2001) Ver la hoja de datos (PDF) - Fairchild Semiconductor

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componentes Descripción

Fabricante

ML4812CP
(Rev.:2001)

Power Factor Controller

Fairchild Semiconductor 

ML4812

Typical Applications

Input Inductor (L1) Selection

The central component in the regulator is the input boost

inductor. The value of this inductor controls various critical

operational aspects of the regulator. If the value is too low,

the input current distortion will be high and will result in low

power factor and increased noise at the input. This will

require more input filtering. In addition, when the value of

the inductor is low the inductor dries out (runs out of current)

at low currents. Thus the power factor will decrease at lower

power levels and/or higher line voltages. If the inductor

value is too high, then for a given operating current the

required size of the inductor core will be large and/or the

required number of turns will be high. So a balance must be

reached between distortion and core size.

One more condition where the inductor can dry out is ana-

lyzed below where it is shown to be maximum duty cycle

dependent.

For the boost converter at steady state:

VOUT

=

------V----I--N--------

1 – DON

(1)

Where DON is the duty cycle [TON/(TON + TOFF)]. The

input boost inductor will dry out when the following condi-

tion is satisfied:

VIN(t) < VOUT × (1 – DON)

(2)

or

VINDRY = [1 – DON(max)] × VOUT

(3)

VINDRY: voltage where the inductor dries out.

VOUT: output DC voltage.

Effectively, the above relationship shows that the resetting

volt-seconds are more than setting volt-seconds. In energy

transfer terms this means that less energy is stored in the

inductor during the ON time than it is asked to deliver during

the OFF time. The net result is that the inductor dries out.

ENABLE

VREF

VREF

GEN.

9V

–

INTERNAL

BIAS

+

5V VREF

VCC

Figure 8. Under-Voltage Lockout Block Diagram

8

PRODUCT SPECIFICATION

25

20

15

10

5

0

0

10

20

30

40

VCC (V)

Figure 9a. Total Supply Current vs. Supply Voltage

25

20

OPERATING CURRENT

15

10

5

STARTUP

0

–60 –40 –20 0 20 40 60 80 100 120 140

TEMPERATURE (degrees)

Figure 9b. Supply Current (ICC) vs. Temperature

0

-4

-8

-12

-16

-20

-24

0

20

40

60

80 100 120

IREF (mA)

Figure 10. Reference Load Regulation

REV. 1.0.4 5/31/01

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