MP8763 — 12A, 18V, SYNCHRONOUS STEP-DOWN CONVERTER

MP8763 Rev. 1.3

www.MonolithicPower.com

13

6/21/2016

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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 the MP8763, set the ON time using the

FREQ pin, thus setting the frequency for steady-

state operation at CCM.

The MP8763 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 in CCM,

frequency is fairly constant over the input voltage

range. Set switching frequency as follows:

)

(

)

(

)

(

4

.

0

)

(

)

(

1

.

6

10

)

(

6

ns

V

V

V

V

V

V

k

R

kHz

f

DELAY

OUT

IN

IN

FREQ

SW

τ

+

×

−

Ω

×

=

(3)

After adding load, the frequency may be affected

a little because power MOSFET voltage drop will

affect the duty cycle.

Typically, the MP8763 is set between 200kHz

and 1MHz. It is optimized to operate efficiently at

high switching frequencies, which 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 appoint,

introducing jitter that influences the system’s

stability.

The

dominates

the

noise

immunity

though

its

magnitude has no direct effect.

Figure 4: PWM-Mode Jitter

Figure 5: Skip-Mode Jitter

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.

R1

R2

ESR

POSCAP

SW

L

FB

Figure 6: Simplified PWM-Mode Circuit without

External Ramp Compensation

To realize the stability without an external ramp,

select the ESR value as follows:

OUT

ON

SW

ESR

C

.

R

2

7

0

τ

+

π

×

τ

≥

(4)