ML4880

5

Figure 2. One Shot and Synchronous Rectifier Timing Diagram

The oscillator/one shot block generates a constant ON-

time and a minimum OFF-time. The OFF-time is extended

for as long as the output of the current comparator stays

low. Note that the inductor current flows in the current

sense resistor during the OFF-time. Therefore, 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 at voltages above 7V

input. This results in an inductor current ripple of:

∆I

TV

V

L

L

ON

IN

OUT

=

×−

()

(1)

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

wide input voltage range.

The transconductance amplifier generates a current from

the voltage difference between the reference and the

across Rgm that adds to the negative voltage that is

developed across the current sense resistor. When the

current level in the inductor drops low enough (a less

negative sense voltage) to cause the voltage at the non-

inverting input of the current comparator to go positive,

the comparator trips and starts a new ON cycle. In other

words, 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.

This technique allows the feedback transconductance

amplifier’s output current to steer the current level in the

inductor. The higher the transconductance amplifier’s

output current, the higher the inductor current. For

example, when the output voltage drops due to a load

increase, the transconductance amplifier will increase its

which in turn raises the inductor current trip level,

shortening the OFF-time. At some level of increasing the

output load, the transconductance amplifier can no longer

continue to increase its output current. When this occurs,

inductor current cannot increase. If the inductor current

tries to increase, the voltage developed across the current

sense resistor would become more negative, causing the

non-inverting input of the current comparator to be

negative, which extends the OFF-time and reduces the

inductor current.

When the output voltage is too high, the

transconductance amplifier’s output current will

eventually become negative. However, since the inductor

current flows in only one direction (assuming no shoot

through current) the non-inverting input of the current

comparator will also stay negative. This extends the OFF-

time allowing the inductor current to decrease to zero and

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 three operation modes can be defined by

Discontinuous

current mode

Continuous

current mode

The synchronous rectifier comparator and the two NOR

gates make up the synchronous rectifier control circuit.

The synchronous control does not influence the operation

of the main control loop, and operation with a Schottky

diode in place of the synchronous rectifier is possible, but

at a lower conversion efficiency. The synchronous rectifier

(N DRV) is turned on during the minimum OFF-time or

remain on until a new ON-time is started or until the

above –18mV, the current in the inductor has gone to zero