ML4880

6

or the buck regulator is operating in a discontinuous

current mode (DCM). Therefore, the synchronous rectifier

comparator is used only for DCM operation. A timing

diagram is shown in Figure 2.

FLYBACK CONTROLLER

A block diagram of the flyback regulator is shown in

Figure 3. The circuit also utilizes a constant ON-time PFM

control architecture. As with the buck controllers, the

flyback controller determines the OFF-time by waiting for

the inductor current to drop to a level set by the feedback

of the inductor current ripple is programmed instead of

the peak, and features an automatic transition into and out

of burst mode which does not require additional control

circuitry.

The flyback controller’s oscillator/one shot block has a

constant ON-time and a minimum OFF-time similar to

that of the buck controllers. The OFF-time is extended for

as long as the current comparator’s output stays low.

Again, since the inductor current flows in the current

sense resistor during the OFF-time, a minimum OFF-time

is required to allow for the finite circuit delays in sensing

the inductor current. The ON-time is triggered when the

current comparator’s output goes high. However, unlike

conventional fixed ON-time controllers, the ML4880’s

one shot has an inverse relationship with the input voltage

as shown in Figure 4. Figure 5 plots the inductor voltage-

ON-time product. Note that the volt-second product is

nearly constant over the input voltage range. This results

in an inductor current ripple of:

∆I

TV

L

L

ON

IN

=

×

(2)

It is important to note that the ripple current does not vary

wide input voltage range.

The transconductance amplifier and current comparator

operate in the same manner as the buck regulator. The

current programming comparator controls the length of

the OFF-time by waiting until the inductor current

decreases to a value determined by the transconductance

amplifier before starting a new ON-time.

Like the buck controllers, when the flyback’s output

voltage drops due to a load increase, the

transconductance amplifier will increase its output current

causes the current comparator’s output to go positive

earlier or shortens the OFF-time. Conversely, when the

output voltage is too high, the transconductance

amplifier’s output current will eventually become

negative. This holds the current comparator’s output low

and extends the OFF-time causing the converter to stop

operation until the output voltage drops enough to

increase the output current of the transconductance amp

above zero.

In summary, the flyback has three operation modes that

can be defined by the voltage at the SENSE pin at the end

of the OFF-time:

Discontinuous

current mode

Continuous

current mode

–140mV > VSENSE > –250mV Current limit

OSCILLATOR/

ONE SHOT

–

+

+

–

1.25V

N DRV

Rgm

CURRENT

COMPARATOR

TRANSCONDUCTANCE

AMPLIFIER

Figure 3. Flyback Controller Functional Diagram