MP8763D ― 12A, 18V, SYNCHRONOUS STEP-DOWN CONVERTER

MP8763D Rev. 1.1

www.MonolithicPower.com

14

1/15/2015

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As the output current increases from the light

load condition, the current modulator regulates

the operating period that becomes shorter. The

HS-FET turns ON more frequently. Hence, the

switching frequency increases correspondingly.

The output current reaches the critical level when

the current modulator time decreases to zero.

Determine the critical output current level as

follows:

IN

SW

OUT

OUT

IN

OUT

V

F

L

2

V

)

V

V

(

I

×

×

×

×

−

=

(2)

The IC turns into 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

Selecting the switching frequency requires

trading off between efficiency and component

size.

Low

frequency

operation

increases

efficiency by reducing MOSFET switching losses,

but requires larger inductor and capacitor values

to minimize the output voltage ripple.

For MP8763D，set the on time using the FREQ

pin to set the frequency for steady state

operation at CCM.

The MP8763D uses adaptive constant-on-time

(COT) control, though the IC lacks a dedicated

oscillator. Connect the FREQ pin to the IN pin

voltage is feed-forwarded to the one-shot on-time

timer. When operating in steady state at CCM,

frequency is fairly constant over the input voltage

range. Set the switching frequency as follows:

6

SW

FREQ

IN

DELAY

IN

OUT

10

F(kHz)

6.1 R

(k )

V(V)

T(ns)

V(V) 0.4

V

(V)

=

×Ω

×+

−

(3)

5ns. After adding load, the frequency may be

affected a little because power MOSFET voltage

drop will affect the duty cycle.

Typically, the MP8763D is set to 200kHz to

1MHz applications. It is optimized to operate at

high switching frequencies at high efficiency:

high switching frequencies allow for physically

smaller LC filter components to reduce the PCB

footprint.

Jitter and FB Ramp Slope

Figure 4 and Figure 5 show jitter occurring in

both PWM mode and skip mode. When there is

ON time deviates from its intended point and

produces jitter and influences system stability.

noise immunity though its magnitude has no

direct effect.

Figure 4—Jitter in PWM Mode

Figure 5—Jitter in Skip Mode

Ramp with a Large ESR Capacitor

Using POSCAPs or other large-ESR capacitors

as the output capacitor results in the ESR ripple

dominating the output ripple. The ESR also

shows the simplified equivalent circuit in PWM

mode with the HS-FET off and without an

external ramp circuit.