MP8719 – 26V, 12A, HIGH-CURRENT, SYNC BUCK CONVERTER WITH ±1A LDO

PRELIMINARY SPECIFICATIONS SUBJECT TO CHANGE

MP8719 Rev. 1.01

www.MonolithicPower.com

11

8/3/2017

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OPERATION

Pulse-Width Modulation (PWM) Operation

The MP8719 is a fully integrated, synchronous,

rectified, step-down, switch-mode converter

with ±1A of LDO current. 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

indicates an insufficient 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

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, which is

determined by the one-shot on timer. When the

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

until the next period.

Figure 2: CCM Operation

In CCM operation, the switching frequency is

fairly constant (PWM mode).

Light-Load Operation

When the load decreases, the inductor current

decreases as well. Once the inductor current

reaches zero, the MP8719 transitions from

CCM to discontinuous conduction mode (DCM).

DCM operation is shown in Figure 3.

on for a fixed interval, which is determined by

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

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

inductor

current

reaches

zero.

In

DCM

the inductor current 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,

efficiency during light-load condition is improved

greatly. The HS-FET does not turn on as

frequently during light-load condition as it does

during heavy-load condition (skip mode).

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

drops very slowly, and the MP8719 reduces the

switching frequency naturally, achieving high

efficiency at light load.

Figure 3: DCM Operation