MP4473–HIGH-EFFICIENCY, FAST-TRANSIENT, SYNCHRONOUS, STEP-DOWN CONVERTER

MP4473 Rev. 1.0

www.MonolithicPower.com

12

12/15/2014

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© 2014 MPS. All Rights Reserved.

OPERATION

PWM Operation

The MP4473 is a fully integrated, synchronous,

rectified, step-down, switch-mode converter. At

the beginning of each cycle, the high-side

MOSFET (HS-FET) turns ON when the feedback

voltage. The ON period is determined by the

input voltage and the frequency-set resistor,

which can be calculated as follows:

()

( )

FREQ

ON

DELAY

IN

96 R

k

tns

t

(ns)

V

×Ω

=+

(1)

After the ON period elapses, the HS-FET turns

regulates the output voltage. To minimize

conduction

loss,

the

integrated,

low-side

MOSFET (LS-FET) turns ON when the HS-FET

is OFF. A dead short occurs between the input

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

turn on simultaneously (shoot through). An

internal dead-time (DT), generated between HS-

FET OFF and LS-FET ON or LS-FET OFF and

HS-FET ON, prevents shoot through.

Heavy-Load Operation

t

ON

Figure 1: Heavy-Load Operation

In continuous-conduction mode (CCM), when the

output current is HIGH, the HS-FET and LS-FET

repeatedly turn ON/OFF (see MPS. All Rights

Reserved. The inductor current never reaches

fairly constant.

Light-Load Operation

At light-load or no-load conditions, the output

drops very slowly; the MP4473 reduces the

switching frequency automatically to maintain

high efficiency. Figure 2 shows light-load

inductor current approaches zero. The LS-FET

driver enters a tri-state (high Z) whenever the

inductor

current

reaches

zero.

A

current

modulator takes control of the LS-FET and limits

the inductor current to less than -1mA. This

causes the output capacitors to discharge slowly

to GND through the LS-FET to improve light-

load efficiency. At light loads, the HS-FET does

not turn ON as frequently as with heavy loads.

This is called skip mode.

t

ON

Figure 2: Light-Load Operation

As the output current increases from the light-

load

conditions,

the

current

modulator’s

regulatory time period becomes shorter. The HS-

FET turns ON more frequently, thus increasing

the switching frequency. The output current

reaches a critical level when the current

modulator time is zero. The critical output-current

level can be calculated as follows:

IN

OUT

OUT

OUT

SW

IN

(V

V

) V

I

2L F

V

−×

=

××

×

(2)

It enters PWM mode once the output current

exceeds the critical level; then the switching

frequency stays fairly constant over the output-

current range.

Switching Frequency

The input voltage is feed-forwarded to the on-

switching frequency to remain fairly constant over

the input-voltage range. The switching frequency

can be calculated as follows: