MP8765 ─ 24V, HIGH CURRENT SYNCHRONOUS STEP-DOWN CONVERTER

MP8765 Rev. 1.01

www.MonolithicPower.com

11

2/10/2015

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OPERATION

PWM Operation

The MP8765 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

the feedback voltage (VFB) is below the

reference voltage (VREF), which indicates

insufficient output 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

again when VFB drops below VREF. By

repeating 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

mode operation is shown in Figure 2 shown.

on for a fixed interval which is determined by

one-shot on-timer. 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 decrease, the inductor current

decrease too. Once the inductor current touch

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 which is determined by

one-shot on-timer. 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. 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 MP8765 reduces the

switching frequency naturally and then high

efficiency is achieved at light load.

Figure 3—Light Load Operation