MP8720 – 26V, 10A, SYNCHRONOUS BUCK CONVERTER WITH ADJUSTABLE CLM

MP8720 Rev. 1.01

www.MonolithicPower.com

13

8/1/2017

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OPERATION

Pulse-Width Modulation (PWM) Operation

The MP8720 is a fully integrated, synchronous,

rectified, step-down, switch-mode converter

with

an

adjustable

current

limit

(CLM).

Constant-on-time (COT) control provides fast

transient response and eases loop stabilization.

At the beginning of each cycle, the high-side

MOSFET (HS-FET) is turned on when the

output voltage. The on period is determined by

both the output voltage and the input voltage to

make the switching frequency fairly constant

over the input voltage range.

After the on period elapses, the HS-FET is

turned off or enters an off state. The HS-FET is

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

conduction loss. A dead short occurs between

the input and GND if both the HS-FET and the

LS-FET are turned on at the same time. This is

called shoot-through. To prevent shoot-through,

a dead time (DT) is generated internally

between the HS-FET off and the LS-FET on

period or the LS-FET off and the HS-FET on

period.

Internal compensation is applied for COT

control for stable operation, even when ceramic

capacitors are used as output capacitors. This

internal compensation improves

the jitter

performance without affecting the line or load

regulation.

Heavy-Load Operation

Continuous conduction mode (CCM) occurs

when the output current is high and the inductor

current is always above zero amps (see Figure

turned on for a fixed interval determined by the

one-shot on timer. When the HS-FET is turned

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

period.

In CCM operation, the switching frequency is

fairly constant (pulse-width modulation (PWM)

mode).

Figure 2: CCM Operation

Light-Load Operation

When the load decreases, the inductor current

decreases as well. Once the inductor current

reaches zero, the MP8720 transitions from

CCM to discontinuous conduction mode (DCM).

DCM operation is shown in Figure 3.

on for a fixed interval determined by the one-

shot on timer. When the HS-FET is turned off,

the LS-FET is turned on until the inductor

is approaching zero. The LS-FET driver turns

into tri-state (Hi-Z) when the inductor current

reaches zero. A current modulator takes over

the control of the LS-FET and limits the inductor

current to less than

-1mA. Therefore, the

output capacitors discharge slowly to GND

through the LS-FET. As a result, the efficiency

during light-load condition improves greatly.

The HS-FET does not turn on as frequently

during a light-load condition as it does during a

heavy-load condition (skip mode).

At a light-load or no-load condition, the output

drops very slowly, and the MP8720 reduces the

switching frequency naturally, achieving high

efficiency at light load.

Figure 3: DCM Operation