MP8763D ― 12A, 18V, SYNCHRONOUS STEP-DOWN CONVERTER

MP8763D Rev. 1.1

www.MonolithicPower.com

13

1/15/2015

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OPERATION

PWM Operation

The MP8763D is a fully integrated synchronous

rectified step-down switch mode converter. It

uses Constant-on-time (COT) control to provide a

fast

transient

response

and

ease

loop

stabilization.

At the beginning of each cycle, the high-side

MOSFET (HS-FET) turns ON when the feedback

voltage. The input voltage and the frequency-set

resistor determine the ON period as follows:

FREQ

ON

IN

6.1 R

(k )

T(ns)

V(V) 0.4

×Ω

=

−

(1)

After the ON period elapses, the HS-FET turns

By repeating this operation, the converter

regulates the output voltage. The integrated low-

side MOSFET (LS-FET) turns on when the HS-

FET is OFF to minimize the conduction loss.

There is a dead short (or shoot-through)

between input and GND if both HS-FET and LS-

FET turn on at the same time. A dead-time (DT)

internally generated between HS-FET OFF and

LS-FET ON, or LS-FET OFF and HS-FET ON

avoids shoot-through.

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). Figure 2

HS-FET

turns

on

for

a

fixed

interval determined by the one-shot on-timer as

per equation 1. When the HS-FET turns off, the

LS-FET turns on until the next period.

In CCM operation, the switching frequency is

fairly constant and is also 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).

Figure 3 shows the light load operation. When

fixed interval determined by the one- shot on-

timer as per equation 1. When the HS-FET turns

off, the LS-FET turns on until the inductor current

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

less than -1mA. Hence, the output capacitors

discharge slowly to GND through LS-FET. As a

result, this mode improves greatly the light load

efficiency. At light load condition, the HS-FET

does not turns 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 MP8763D reduces the

switching frequency naturally and then achieves

high efficiency at light load.

Figure 3—Light Load Operation