MP28257 –4A, 4.2V-TO-20V, FAST TRANSIENT, SYNCHRONOUS STEP-DOWN CONVERTER

MP28257 Rev 1.0

www.MonolithicPower.com

9

11/8/2011

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© 2011 MPS. All Rights Reserved.

OPERATION

PWM Operation

The MP28257 is a fully-integrated, synchronous,

rectified, step-down switch converter. The device

uses constant-on-time (COT) control to provide

fast

transient

response

and

easy

loop

stabilization. At the beginning of each cycle, the

high-side

MOSFET

(HS-FET)

turns

ON

indicates insufficient output voltage. The input

voltage and the frequency-set resistor determine

the ON period as follows:

7

ON

IN

9.3 R (k )

t(ns)

40ns

V(V) 0.4







(1)

After the ON period elapses, the HS-FET enters

the OFF state. By cycling the HS-FET between

the ON and OFF states, the converter regulates

the output voltage. The integrated low-side

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

in its OFF state to minimize the conduction loss.

Shoot-through occurs when there is both the HS-

FET and LS-FET are turned on at the same time,

causing a dead short between input and GND.

Shoot-through dramatically reduces efficiency,

and the MP28257 avoids this by internally

generating a dead-time (DT) between when the

HS-FET is off and the LS-FET is on, and when

the LS-FET is off and the HS-FET is on. The

device enters either heavy-load operation or

light-load operation depending on the amplitude

of the output current.

Heavy-Load Operation

Figure 2: Heavy-Load Operation

During heavy-load operation—when the output

current is high—the MP28257 enters continuous-

conduction mode (CCM) where the HS-FET and

LS-FET repeat the on/off operation described for

PWM operation, the inductor current never goes

constant. Figure 2 shows the timing diagram

during this operation.

Light-Load Operation

During light-load operation—when the output

current is low—the MP28257 automatically

reduces the switching frequency to maintain high

efficiency, and the inductor current drops near

zero.. When the inductor current reaches zero,

the LS-FET driver goes into tri-state (high Z). The

current modulator controls the LS-FET and limits

the inductor current to around -1mA as shown in

Figure 3. Hence, the output capacitors discharge

operation greatly improves device efficiency

when the output current is low.

Figure 3: Light-Load Operation

Light-load operation is also called skip mode

because the HS-FET does not turn on as

frequently as during heavy-load conditions. The

frequency at which the HS-FET turns on is a

function of the output current—as the output

current increases, the time period that the current

modulator regulates becomes shorter, and the

HS-FET turns on more frequently. The switching

frequency increases in turn. The output current

reaches the critical level when the current

modulator time is zero, and can be determined

using the following equation:

IN

OUT

OUT

OUT

SW

IN

(V

V

) V

I

2L f

V





 

(2)

The device reverts to PWM mode once the

output current exceeds the critical level. After

that, the switching frequency stays fairly constant

over the output current range.