MP8758–18V, HIGH CURRENT SYNCHRONOUS BUCK CONVERTER

MP8758 Rev. 1.0

www.MonolithicPower.com

13

1/13/2015

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© 2015 MPS. All Rights Reserved.

R1

R2

Ceramic

SW

FB

Vo

L

R4

C4

R9

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, R9 and C4 of the external ramp to

meet the following condition:

12

9

SW

4

1

2

RR

11

R

2F

C

5

R

R

⎛⎞

×

<×

+

⎜⎟

π×

×

+

⎝⎠

(3)

Where:

R4

C4

FB

C4

II

I

I

=+

≈

(4)

as:

IN

OUT

12

RAMP

ON

44

1

2

9

VV

R// R

VT

RC

R // R

R

−

=×

×

×+

(5)

follows

−

−

==

×

OUT

RAMP

SLOPE1

off

4

4

V

V

V

TR

C

(6)

As can be seen from equation (6), 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 (3), then we can only

reduce R4. For a stable PWM operation, the

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)

Io is the load current.

ripple is the same whether the external ramp is

used or not. Figure 8 shows the simplified circuit

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

FET are off.

R1

R2

ESR

Cout

FB

Vo

Ro

Figure 8—Simplified Circuit in skip Mode

mode can be determined as follow:

()

REF

SLOPE2

12

OUT

V

V

(R

R //Ro) C

−

=

+×

(8)

Where Ro is the equivalent load resistor.

is lower than that is in the PWM mode, so it is

reasonable that the jitter in the skip mode is

larger. If one wants a system with less jitter

resistors should not be too big, however, that will

decrease the light load efficiency.

EN Control

The regulator turns on when EN goes high.

Conversely it turns off when EN goes low.

For automatic start-up the EN pin can be pulled

up to input voltage through a resistive voltage

divider. Choose the values of the pull-up resistor

determine the automatic start-up voltage:

UP

DOWN

IN START

DOWN

RR

V1.25

(V)

R

−

+

=×

(9)

the

−

IN START

V

is set at 4.93V.

To avoid noise, a 10nF ceramic capacitor from

EN to GND is recommended.

There is an internal Zener diode on the EN pin,

which clamps the EN pin voltage to prevent it

from running away. The maximum pull up current

assuming a worst case 12V internal Zener clamp

should be less than 1mA.