MP8764 ― 12A, 18V, SYNCHRONOUS STEP-DOWN CONVERTER WITH LATCH-OFF OCP

MP8764 Rev. 1.2

www.MonolithicPower.com

12

6/21/2016

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OPERATION

PWM Operation

The MP8764 is fully integrated synchronous

rectified

step-down

switch

mode

converter.

Constant-on-time (COT) control is employed to

provide fast transient response and easy loop

stabilization. At the beginning of each cycle, the

high-side MOSFET (HS-FET) is turned ON when

voltage. The ON period is determined by the

input voltage and the frequency-set resistor as

follows:

4

.

0

)

(

)

(

1

.

6

)

(

−

Ω

×

=

V

V

k

R

ns

T

IN

FREQ

ON

(1)

After the ON period elapses, the HS-FET is

turned off, or becomes OFF state. It is turned ON

operation this way, the converter regulates the

output voltage. The integrated low-side MOSFET

(LS-FET) is turned on when the HS-FET is in its

OFF state to minimize the conduction loss. There

will be a dead short between input and GND if

both HS-FET and LS-FET are turned on at the

same time. It’s called shoot-through. In order to

avoid

shoot-through,

a

dead-time

(DT)

is

internally generated between HS-FET off and LS-

FET on, or LS-FET off and HS-FET on.

Heavy-Load Operation

Figure 2—Heavy Load Operation

When the output current is high and the inductor

current is always above zero amps, it is called

continuous-conduction-mode (CCM). The CCM

interval which is determined by one-shot on-timer

as equation 1 shown. When the HS-FET is

turned off, the LS-FET is turned on until next

period.

In CCM mode operation, the switching frequency

is fairly constant and it is called PWM mode.

Light-Load Operation

As the load decreases, the inductor current

decreases

too.

When

the

inductor

current

touches zero, the operation is transited from

continuous-conduction-mode

(CCM)

to

discontinuous-conduction-mode (DCM).

The light load operation is shown in Figure 3.

a fixed interval which is determined by one- shot

on-timer as equation 1 shown. When the HS-FET

is turned off, the LS-FET is turned on until the

inductor current reaches zero. In DCM operation,

current is approaching zero. The LS-FET driver

turns into tri-state (high Z) whenever the inductor

current reaches zero. A current modulator takes

over the control of LS-FET and limits the inductor

current to less than -1mA. Hence, the output

capacitors discharge slowly to GND through LS-

FET. As a result, the efficiency at light load

condition is greatly improved. At light load

condition, the HS-FET is not turned ON as

frequently as at heavy load condition. This is

called skip mode.

At light load or no load condition, the output

drops very slowly and the MP8764 reduces the

switching frequency naturally and then high

efficiency is achieved at light load.

Figure 3—Light Load Operation