ML0308 Preliminary

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4/9

Rev. B, July 2005

BLOCK DIAGRAM – ML0308B

Figure 2: Internal Block Diagram of ML0308B

OPERATION

The ML0308 uses a pulse-skipping PFM, current

mode control scheme to provide excellent line and

load

regulation.

Its

operation

can

be

best

explained with reference to figure 2 and figure 1.

The oscillator generates a clock of 1.2MHz with

75% duty ratio for the PFM controller. When CE

pin goes HIGH, the power MOSFET switches at

1.2 MHz to charge the inductor and boost the

output voltage to the target level specified by R1

and R2. The output voltage is set as follow:

V

R

R

R

VOUT

23

.

1

2

2

1

×

+

=

(ML0308B)

When the inductor current reaches the current

limit, comparator A3 will toggle and the control

circuit will turn off the MOSFET immediately. So

the inductor current is limited. Therefore, soft-start

is achieved and potential hazards like current and

voltage overshoot are avoided. When output

voltage reaches and rises over the regulation point,

amplifier A2 will amplify the error. The chip will

adjust its pulse-skipping rate to monitor the output

voltage.