MP38900/MP38900-B–HIGHEFFICIENCY,FASTTRANSIENTSYNCHRONOUSSTEP-DOWNCONVERTER

MP38900/MP38900-B Rev. 1.1

www.MonolithicPower.com

13

6/25/2012

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© 2012 MPS. All Rights Reserved.

IN

OUT

OUT

OUT

SW

IN

(V

V

) V

I

2L F

V









(2)

It 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

Constant-on-time (COT) control is used in the

MP38900/MP38900-B and there is no dedicated

oscillator in the IC. The input voltage is feed-

forwarded to the on-time one-shot timer through

Hence, the switching frequency is fairly constant

over the input voltage range.

The switching

frequency can be set as follows:

(ns)

t

(V)

V

(V)

V

0.45

(V)

V

)

R7(k

12

10

kHz

f

DELAY2

OUT

IN

IN

6

SW













)

(

(3)

40ns.

MP38900/MP38900-B is optimized to operate at

high switching frequency with high efficiency.

High switching frequency makes it possible to

utilize small sized LC filter components to save

system PCB space.

Jitter and FB Ramp Slope

Figure 5 and Figure 6 show jitter occurring in

both PWM mode and skip mode. When there is

the HS-FET driver deviates from its intended

location and produces jitter. It is necessary to

understand that there is a relationship between a

ripple’s downward slope. The slope steepness of

noise immunity directly.

V

RE F

V

FB

HS D river

J itter

V

S L O PE1

Figure 5: Jitter in PWM Mode

V

FB

HS D river

Jitter

V

REF

V

SL O P E 2

Figure 6: Jitter in Skip Mode

Ramp with Large ESR Cap

In the case of POSCAP or other types of

capacitor with larger ESR is applied as output

capacitor. The ESR ripple dominates the output

ripple, and the slope on the FB is quite ESR

related. Figure 7 shows an equivalent circuit in

PWM mode with the HS-FET off and without an

external ramp circuit. Turn to application

information section for design steps with large

ESR caps.

Figure 7: Simplified Circuit in PWM Mode

without External Ramp Compensation

To realize the stability when no external ramp is

used, usually the ESR value should be chosen

as follow: