ML4863

5

FUNCTIONAL DESCRIPTION

The ML4863 utilizes a flyback topology with constant on-

time control. The circuit determines the length of the off-

time by waiting for the inductor current to drop to a level

the current programming is somewhat unconventional

because the valley of the current ripple is programmed

instead of the peak. The controller automatically enters

burst mode when the programmed current falls below

zero. Constant on-time control therefore features a

transition into and out of burst mode which does not

require additional control circuitry.

The control circuit is made up of four distinctive blocks;

the constant on-time oscillator, the current programming

comparator, the feedback transconductance amplifier, and

the synchronous rectifier controller. A simplified circuit

diagram is shown in Figure 1.

OSCILLATOR & COMPARATOR

The oscillator has a constant on-time and a minimum off-

time. The off-time is extended as long as the output of the

current programming comparator is low. Note that in

constant on-time control, a discharge (off-time) cycle is

needed for the inductor current to be sensed. The

minimum off-time is required to account for the finite

circuit delays in sensing the inductor output current.

TRANSCONDUCTANCE AMPLIFIER

The feedback transconductance amplifier generates a

current from the voltage difference between the output

and the reference. This current produces a voltage across

drops low enough to cause the voltage at the non-inverting

input of the current programming comparator to go

positive, the comparator trips and the converter starts a

new on-cycle. The current programming comparator

controls the length of the off-time by waiting until the

current in the secondary decreases to the value specified

by the feedback transconductance amplifier.

In this way, the feedback transconductance amplifier‘s

output current steers the current level in the inductor.

When the output voltage drops due to a load increase, it

will increase the output current of the feedback amplifier

raises the secondary current trip level. However, when the

output voltage is too high, the feedback amplifier’s output

current will eventually become negative. Because the

output current of the inductor can never go negative by

virtue of the diode, the non-inverting input of the

comparator will also stay negative. This causes the

converter to stop operation until the output voltage drops

enough to increase the output current of the feedback

transconductance amplifier above zero.

Figure 1. Schematic of the ML4863 Controller and Power Stage

RP

CP

FEEDBACK

TRANSCONDUCTANCE

AMPLIFIER

Rgm

CURRENT

PROGRAMMING

COMPARATOR

ONE SHOT

2.5µs

ONE SHOT

CONSTANT ON-TIME

MINIMUM OFF-TIME

OSCILLATOR

VIN

1:1

C

VOUT

RSENSE

BLANKING

A2

ML4863

RECTIFIER

COMPARATOR

IS

SENSE

OUT 2

OUT 1

–

+

COMP

+

–

COMP

+

–

VREF

VFB

500ns

4

7

6

2