MP8763D ― 12A, 18V, SYNCHRONOUS STEP-DOWN CONVERTER

MP8763D Rev. 1.1

www.MonolithicPower.com

15

1/15/2015

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© 2015 MPS. All Rights Reserved.

R1

R2

ESR

POSCAP

SW

L

FB

Figure 6—Simplified Circuit in PWM Mode

without External Ramp Compensation

To realize the stability without an external ramp,

usually select the ESR value as follows:

OUT

ON

SW

ESR

C

2

T

7

.

0

T

R

+

π

×

≥

(4)

Ramp with a Small ESR Capacitor

Use an external ramp when using ceramic output

capacitors, because the ESR ripple is not high

enough to stabilize the system.

R1

R2

Ceramic

SW

FB

L

R4

C4

R9

Figure 7—Simplified Circuit in PWM Mode

with External Ramp Compensation

Figure 7 shows the simplified circuit in PWM

mode with the HS-FET OFF and an external

ramp compensation circuit (R4, C4). Design the

external ramp based on the inductor ripple

current. Select C4, R9, R1 and R2 to meet the

following condition:

⎟

⎠

⎞

⎜

⎝

⎛

+

+

×

×

<

×

×

π

9

R

2

R

1

R

2

R

1

R

5

1

4

C

F

2

1

SW

(5)

Where:

4

C

FB

4

C

4

R

I

I

I

I

≈

+

=

(6)

⎟

⎠

⎞

⎜

⎝

⎛

+

×

×

×

−

=

9

R

2

R

//

1

R

2

R

//

1

R

T

4

C

4

R

V

V

V

ON

OUT

IN

RAMP

(7)

4

C

4

R

V

T

V

V

OUT

OFF

RAMP

1

SLOPE

×

−

=

=

(8)

Equation 8 shows that if there is instability in

PWM mode, reduce either R4 or C4. If C4 is

irreducible due to equation 5 limitations, then

reduce R4. For a stable PWM operation, design

SW

ON

3

ESR

OUT

OUT

SLOPE1

OUT

OUT

SW

ON

TT

RC

I10

0.7

2

VV

2L C

T

T

−

+−

×

×

×π

−≥

×

+

××

−

(9)

is almost same whether the external ramp is

used or not. Figure 8 shows the simplified circuit

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

are off.

R1

R2

FB

Figure 8—Simplified Circuit in skip Mode

skip mode as follows:

OUT

OUT

REF

2

SLOPE

C

]

R

//

)

2

R

1

R

[(

V

V

×

+

−

=

(10)

than

that

is

in

PWM

mode,

so

it

is