MP1517 – 3A, 25V, 1.1MHz STEP-UP CONVERTER

MP1517 Rev. 1.4

www.MonolithicPower.com

7

4/28/2006

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Use an input capacitor value greater than 10µF.

The capacitor can be electrolytic, tantalum or

ceramic. However since it absorbs the input

switching current it requires an adequate ripple

current rating. Use a capacitor with RMS current

rating greater than the inductor ripple current.

To insure stable operation place the input

capacitor as close to the IC as possible.

Alternately a smaller high quality ceramic 0.1µF

capacitor may be placed closer to the IC with the

larger capacitor placed further away. If using this

technique, it is recommended that the larger

capacitor be a tantalum or electrolytic type. All

ceramic capacitors should be placed close to the

MP1517.

Selecting the Output Capacitor

The output capacitor is required to maintain the

DC output voltage. Low ESR capacitors are

preferred to keep the output voltage ripple to a

minimum. The characteristic of the output

capacitor also affects the stability of the regulation

control system. Ceramic, tantalum, or low ESR

electrolytic capacitors are recommended. In the

case of ceramic capacitors, the impedance of the

capacitor at the switching frequency is dominated

by the capacitance, and so the output voltage

ripple is mostly independent of the ESR. The

output voltage ripple is calculated as:

(

)

SW

UT

O

IN

UT

O

LOAD

RIPPLE

f

2

C

V

V

V

I

V

×

×

−

×

=

switching frequency, and C2 is the capacitance of

the output capacitor.

In the case of tantalum or low-ESR electrolytic

capacitors, the ESR dominates the impedance at

the switching frequency, and so the output ripple

is calculated as:

⎥

⎦

⎤

⎢

⎣

⎡

×

+

×

×

−

×

=

IN

OUT

ESR

SW

OUT

IN

OUT

LOAD

RIPPLE

V

V

R

2

C

f

V

)

V

V

(

I

V

the output capacitors.

Choose an output capacitor to satisfy the output

ripple and load transient requirements of the

design. Place the output capacitor close to SW

to minimize the AC loop and switching noise.

Selecting the Diode

The output rectifier diode supplies current to the

inductor when the internal MOSFET is off. To

reduce losses due to diode forward voltage and

recovery time, use a Schottky diode. Choose a

diode whose maximum reverse voltage rating is

greater than the maximum output voltage. The

rated average forward current needs to be

equal to or greater than the load current.

Selecting the Soft-Start Capacitor

The soft-start period is determined by the equation:

C4

0.275

×

=

soft-start period.

Determine the capacitor required for a given

soft-start period by the equation:

SS

t

3.64

C4

×

=

It is recommended that values between 10nF and

Compensation

The output of the transconductance error

amplifier (COMP) is used to compensate the

regulation control system. The system uses two

poles and one zero to stabilize the control loop.

by the compensation capacitor C3 and the

compensation resistor R3.

These are determined by the equations:

LOAD

R

C2

1

×

×

π

VEA

EA

A

C3

2

G

×

×

π

×

R3

C3

2

1

×

×

π

×

the error amplifier voltage gain.