MPQ8616, 6A/12A, 6V, SYNCHRONOUS STEP-DOWN CONVERTER

MPQ8616 Rev. 1.11

www.MonolithicPower.com

21

3/16/2016

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© 2016 MPS. All Rights Reserved.

ripple

doesn’t

affect

the

noise

immunity

directly.

Figure 3—Jitter in PWM Mode

Ramp with Large ESR Capacitor

In the case of POSCAP or other types of

capacitor with lager ESR is applied as output

capacitor, the ESR ripple dominates the output

ripple, and the slope on the FB is quite ESR

related. Figure 4 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 capacitors.

R1

R2

ESR

POSCAP

SW

Vo

L

FB

Figure 4—Simplified Circuit in PWM Mode

without External Ramp Compensation

To realize the stability when no external ramp is

applied, usually the ESR value should be chosen

as follow:

SW

ON

ESR

OUT

tt

0.7

2

R

C







(3)

Ramp with Small ESR Capacitor

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.

R1

R2

Ceramic

SW

FB

Vo

L

R4

C4

R9

Figure 5—Simplified Circuit in PWM Mode

with External Ramp Compensation

In PWM mode, an equivalent circuit with HS-FET

off

and

the

use

of

an

external

ramp

compensation circuit (R4, C4) is simplified in

Figure 5. The external ramp is derived from the

inductor ripple current. If one chooses C4, R9,

R1 and R2 to meet the following condition:

12

9

SW

4

1

2

RR

11

R

2f

C

20

R

R



















(4)

Where:

R4

C4

FB

C4

II

I

I





(5)

as:

IN

O

12

RAMP

ON

44

1

2

9

VV

R//R

Vt

RC

R //R

R







 





(6)

follows:

OUT

RAMP

SLOPE1

off

4

4

V

V

V

tR

C







(7)

As can be seen from equation 7, if there is

instability in PWM mode, we can reduce either

R4 or C4. If C4 can not be reduced further due to

limitation from equation 4, then we can only

reduce R4. For a stable PWM operation, the

SW

ON

3

ESR

OUT

O

SLOPE1

OUT

OUT

sw

on

tt

RC

0.7 I

10

0.7

2

VV

2L C

t

t





















(8)

Where Io is the load current.