MP8758–18V, HIGH CURRENT SYNCHRONOUS BUCK CONVERTER

MP8758 Rev. 1.0

www.MonolithicPower.com

11

1/13/2015

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OPERATION

PWM Operation

The MP8758 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 both the

output voltage and input voltage to make the

switching frequency fairy constant over input

voltage range.

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.

An internal compensation is applied for COT

control to make a more stable operation even

when ceramic capacitors are used as output

capacitors, this internal compensation will then

improve the jitter performance without affect the

line or load regulation.

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. The one shot timer is controlled by input

and output voltage so that the switching

frequency could be fairly fixed at 500kHz for

different input/output conditions. When the HS-

MOSFET is turned off, the LS-MOSFET 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

With the load decreases, the inductor current

decreases too. Once the inductor current touches

zero, the operation is transition from continuous-

conduction-mode

(CCM)

to

discontinuous-

conduction-mode (DCM).

The light load operation is shown in Figure 3.

on for a fixed interval. When the HS-MOSFET is

turned off, the LS-MOSFET 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. 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 MP8758 reduces the

switching frequency naturally and then high

efficiency is achieved at light load.

Figure 3—Light Load Operation