MP9447–HIGH-EFFICIENCY, FAST-TRANSIENT, SYNCHRONOUS, STEP-DOWN CONVERTER

MP9447 Rev. 1.2

www.MonolithicPower.com

10

1/5/2016

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© 2016 MPS. All Rights Reserved.

OPERATION

PWM Operation

The MP9447 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 as:



 

FREQ

ON

DELAY

IN

96 R

k

tns

t

(ns)

V





(1)

After the ON period elapses, the HS-FET turns

regulates the output voltage. The integrated low-

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

FET is OFF to minimize conduction loss. A dead

short occurs between input and GND if both the

HS-FET and the LS-FET turn on at the same

time (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 2: Heavy-Load Operation

In continuous-conduction mode (CCM), when the

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

repeatedly turn ON/OFF as shown in MPS. All

Rights Reserved. The inductor current never

goes to zero. In CCM, the switching frequency

Light-Load Operation

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

drops very slowly and the MP9447 reduces the

switching frequency automatically to maintain

high efficiency. Figure 3 shows the 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. Hence,

the output capacitors discharge slowly to GND

through the LS-FET to greatly improve the light-

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

not turn ON as frequently as at heavy loads. This

is called skip mode.

t

ON

Figure 3: Light-Load Operation

As the output current increases from light-load

condition, the current modulator’s regulatory time

period becomes shorter. The HS-FET turns ON

more frequently, thus increasing the switching

frequency increases. The output current reaches

its critical level when the current modulator time

is zero. The critical output current level is:

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. After that, the switching

frequency stays fairly constant over the output

current range.

Switching Frequency

The input voltage is feed-forwarded to the on-

switching frequency is fairly constant over the

input voltage range. The switching frequency can

be set as: