MP8758H–22V, HIGH-CURRENT SYNCHRONOUS BUCK CONVERTER

MP8758H Rev.1.0

www.MonolithicPower.com

14

10/13/2015

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© 2015 MPS. All Rights Reserved.

Operating with External Ramp Compensation

Usually, the MP8758H is able to support ceramic

output capacitors without an external ramp. In

some cases, the internal ramp may not be

enough to stabilize the system, and external

ramp compensation is needed. Please refer to

the Application Information section on page 17

for

design

steps

with

external

ramp

compensation.

R1

R2

Ceramic

SW

FB

Vo

L

R4

C4

R8

Figure 7: Simplified Circuit in PWM Mode with

External Ramp Compensation

Figure 7 shows a simplified external ramp

compensation (R4 and C4) for PWM mode.

Chose R1, R2, R8, and C4 of the external ramp

to meet the condition shown in Equation (3) and

Equation (4):

12

8

SW

4

1

2

RR

11

R

2F

C

5

R

R

⎛⎞

×

<×

+

⎜⎟

π×

×

+

⎝⎠

(3)

R4

C4

FB

C4

II

I

I

=+

≈

(4)

Equation (5):

IN

OUT

12

RAMP

ON

44

1

2

8

VV

R//R

VT

RC

R // R

R

−

=×

×

×+

(5)

estimated with Equation (6):

−

−

==

×

OUT

RAMP

SLOPE1

off

4

4

V

V

V

TR

C

(6)

If there is instability in PWM mode, either R4 or

C4 can be reduced. If C4 cannot be reduced

further due to limitation from Equation (3), then

only R4 can be reduced. For stable PWM

SW

ON

-3

ESR

OUT

slope1

OUT

OUT

SW

on

TT

+-R

C

Io 10

0.7 π

2

-V

V

+

2L C

T -T

×

×

≥

××

(7)

Where Io is the load current.

ripple is the same whether the external ramp is

used or not. Figure 8 shows the simplified circuit

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

FET are off.

R1

R2

ESR

Cout

FB

Vo

Ro

Figure 8: Simplified Circuit in Skip Mode

mode can be determined with Equation (8):

()

REF

SLOPE2

12

OUT

V

V

(R

R // Ro) C

−

=

+×

(8)

Where Ro is the equivalent load resistor.

lower than it is in PWM mode, and the jitter is

larger as well. For a system with less jitter during

resistors should not be too large. However, this

will decrease the light-load efficiency.

EN Control

The regulator turns on when EN is high and turns

off when EN is low.

For automatic start-up, pull EN up to the input

voltage through a resistive voltage divider.

from VIN to EN) and the pull-down resistor

automatic start-up voltage using Equation (9):

UP

DOWN

IN START

DOWN

RR

V1.25

(V)

R

−

+

=×

(9)

then

−

IN START

V

is 4.93V.

A 10nF ceramic capacitor from EN to GND is

recommended to avoid noise.

There is an internal Zener diode on EN, which

clamps the EN voltage to prevent runaway. EN is

clamped internally using a 12V Zener diode,

which limits the pull-up current to a maximum of

1mA.