ML4812

REV. 1.0 10/10/2000

7

ENABLE

VREF

VREF

GEN.

9V

INTERNAL

BIAS

5V VREF

VCC

–

+

VCC (V)

0

30

20

40

10

25

20

15

10

5

0

25

20

15

10

5

0

TEMPERATURE (degrees)

–60

20

60

140

100

–20

–40

40

80

120

0

OPERATING CURRENT

STARTUP

IREF (mA)

0

40

100

20

60

120

80

0

-4

-8

-12

-16

-20

-24

Figure 10. Reference Load Regulation

Figure 8. Under-Voltage Lockout Block Diagram

Figure 9a. Total Supply Current vs. Supply Voltage

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

analyzed below where it is shown to be maximum duty

cycle dependent.

For the boost converter at steady state:

(1)

input boost inductor will dry out when the following

condition is satisfied:

(2)

or

(3)

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.

V

V

D

OUT

IN

ON

=

-

1

(1)

input boost inductor will dry out when the following

condition is satisfied:

Vt V

D

IN

OUT

ON

()

(

)

<´ -

1

(2)

or

VD

V

INDRY

ON

OUT

=-

´

[(max)]

1

(3)