MP8763 — 12A, 18V, SYNCHRONOUS STEP-DOWN CONVERTER

MP8763 Rev. 1.3

www.MonolithicPower.com

14

6/21/2016

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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 PWM-Mode Circuit 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

2

1

2

1

5

1

4

2

1

R

R

R

R

R

C

f

SW

(5)

Where:

4

C

FB

4

C

4

R

I

I

I

I

≈

+

=

(6)













+

×

τ

×

×

−

=

9

2

1

2

1

4

4

R

R

//

R

R

//

R

C

R

V

V

V

ON

OUT

IN

RAMP

(7)

4

4

1

C

R

V

V

V

OUT

OFF

RAMP

SLOPE

×

−

=

τ

=

(8)

Equation 8 shows that if there is instability in

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

irreducible due to limitations from equation 5,

then reduce R4. For stable PWM operation,

SW

ON

3

ESR

OUT

OUT

SLOPE1

OUT

OUT

SW

ON

T

T

R

C

I

10

0.7

2

V

V

2 L C

T

T

−

+

−

×

×

× π

−

≥

×

+

× ×

−

(9)

almost 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 LS-FET

are off.

R1

R2

FB

Figure 8: Simplified Skip-Mode Circuit

skip mode as follows:

OUT

OUT

REF

2

SLOPE

C

]

R

//

)

2

R

1

R

[(

V

V

×

+

−

=

(10)

lower than it is in PWM mode, so it is reasonable

that the jitter in skip mode is larger. To achieve

less

jitter

during

ultra-light-load

conditions,

reduce R1 and R2, though that will decrease the

light-load efficiency.

Configuring the EN Control

The regulator turns on when EN goes HIGH.

Conversely it turns OFF when EN goes LOW. Do

not float the pin.

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

input voltage through a resistive voltage divider.

from the IN pin to the EN pin) and the pull-down

determine the automatic start-up voltage:

UP

DOWN

IN START

DOWN

(R

R

)

V

1.5

(V)

R

−

+

=

×

(11)

To reduce noise, add a 10nF ceramic capacitor

from EN to GND.

An internal zener diode on the EN pin clamps the

EN

pin

voltage

to

prevent

runaway.

The

maximum pull-up current (assuming the worst

case, 6V) for the internal zener clamp should be

less than 1mA.