MP8736– HIGHEFFICIENCY,FAST TRANSIENT,6A, 19V SYNCHRONOUSBUCKCONVERTERIN ATINY3X4mmPACKAGE

MP8736 Rev. 1.34

www.MonolithicPower.com

14

4/18/2012

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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

MP8736 and there is no dedicated oscillator in

the IC. The input voltage is feed-forwarded to the

on-time one-shot timer through the resistor R7.

switching frequency is fairly constant over the

input voltage range.

The switching frequency

can be set as follows:

()

()

()

()

()

()

6

SW

IN

DEALY

IN

OUT

10

FkHz

6R7 k

V V

Tns

VV

0.4 V

V

=

×Ω

×+

−

(3)

40ns.

Jitter and FB Ramp Slope

Figure 4 and Figure 5 show jitter occurring in

both PWM mode and skip mode. When there is

HS-FET deviates from its intended location and

produces jitter. It is necessary to understand that

there is a relationship between a system’s

ripple

dominates

in

noise

immunity.

The

the noise immunity directly.

Figure 4—Jitter in PWM Mode

Figure 5—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 6 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.

R1

R2

ESR

POSCAP

SW

FB

Vo

L

Figure 6—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:

SW

ON

ESR

OUT

TT

0.7

2

R

C

+

×π

≥

(4)

Tsw is the switching period.

Ramp with small ESR Cap

When the output capacitors are ceramic ones,

the ESR ripple is not high enough to stabilize the

system, and external ramp compensation is

needed. Skip to application information section

for design steps with small ESR caps.