MP28248 – 3A, 3.3V-20V INPUT, FAST TRANSIENT SYNCHRONOUS STEP-DOWN CONVERTER IN 2X3MM QFN

MP28248 Rev. 1.0

www.MonolithicPower.com

13

1/5/2012

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© 2012 MPS. All Rights Reserved.

Ramp with small ESR Capacitor

When using ceramic output capacitors, the ESR

is insufficient to stabilize the system and requires

external ramp compensation. The application

section discusses this in further depth.

.

Figure 8: Simplified Circuit in PWM Mode with

External Ramp Compensation

Figure 8 shows a simplified external ramp

compensation circuit (R4 and C4) for PWM mode,

with the HS-FET off. Chose R1, R2, and C4 of

the external ramp to meet the following condition:

12

9

SW

4

1

2

RR

11

<+R

2π fC

5

R +R











(5)

Where:

R4

C4

FB

C4

II

I

I





(6)









IN

OUT

12

RAMP

ON

44

1

2

9

VV

R//R

Vt

RC

R //R

R

(7)

follows:









OUT

RAMP

SLOPE1

off

4

4

V

V

V

tR

C

(8)

As shown in equation 8, either reduce R4 or C4 if

there is instability in PWM mode. If C4 can not be

reduced further due to limitation from equation 5,

then reduce R4. For stable PWM operation,







SW

ON

ESR

OUT

slope1

OUT

OUT

SW

on

tt

+-R

C

Io(mA)

0.7 π

2

-V

V

+

2L C

t

-t

(9)

ripple is the same whether the external ramp is

used or not. Figure 9 shows the simplified circuit

of the skip mode when both the HS-FET and LS-

FET are off.

Figure 9: Simplified Circuit in Skip Mode

mode can be determined as follow:



REF

SLOPE2

12

OUT

V

V

(R

R // Ro) C







(10)

lower than that is in the PWM mode, so generally

the jitter in skip mode is larger. For a system with

less jitter in light-load condition, select smaller

the light-load efficiency.

When using a large-ESR capacitor on the output,

add a 10µF or smaller ceramic capacitor in

parallel to minimize ESL effects.

Soft-Start/Stop

The MP28248 employs a soft start/stop (SS)

mechanism to ensure smooth output during

power up and power shut-down. When the EN

pin goes high, an internal current source (14μA)

charges up the external SS cap. The SS cap

voltage takes over the REF voltage to the PWM

comparator. The output voltage smoothly ramps

up with the SS voltage. Once the SS voltage

enters into steady state operation.

When the EN pin goes low, an internal 4.5μA

current source discharges the external SS cap

voltage. Once the SS voltage falls below the

decrease smoothly with the SS voltage until the

voltage level zeros out at high load. The SS cap

value can be determined as follows: