MP3217 –36V, 670kHz STEP-UP CONVERTER WITH A 0.5A SWITCH

MP3217 Rev. 0.91

www.MonolithicPower.com

8

10/15/2009

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APPLICATION INFORMATION

Components referenced below apply to

Typical Application Circuit refer to Figure 2.

Setting the Output Voltage

Set the output voltage by selecting the resistive

voltage divider ratio. Use 5.1kΩ for the low-side

resistor R2 of the voltage divider. Determine the

high-side resistor R1 by the equation:

OUT

FB

FB

R2 (V

V

)

R1

V

×−

=

R1 = 130.6kΩ. There choose a 133kΩ standard

1% value.

Selecting the Input Capacitor

An input capacitor is required to supply the AC

ripple current to the inductor, while limiting noise

at the input source. This capacitor must have low

ESR, so ceramic is the best choice. Use an input

capacitor value of 4.7µF or greater. This

capacitor must be placed physically close to the

IN pin. Since it reduces the voltage ripple seen at

IN, it also reduces the amount of EMI passed

back along that line to the other circuitry.

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 estimated to be:

IN

LOAD

OUT

RIPPLE

SW

V

1 -

I

V

V

C2

f

⎛⎞

×

⎜⎟

⎝⎠

≈

×

switching frequency, and C2 is the capacitance

of the output capacitor.

Selecting the Inductor

The inductor is required to force the higher output

voltage while being driven by the lower input

voltage. A larger value inductor results in less

ripple current that results in lower peak inductor

current, reducing stress on the internal power

switch. However, the larger value inductor has a

larger physical size, higher series resistance,

and/or lower saturation current.

Inductance from 2.2µH to 33µH is a good choice

for high efficiency and small size. To prevent

saturation, use an inductor with a saturation

current rating that is higher than the device

current limit.

Selecting the Diode

The output rectifier diode supplies current to the

inductor when the internal power MOSFET is off.

To reduce losses due to diode forward voltage

and recovery time, use a Schottky diode with the

MP3217. The diode should be rated for a reverse

voltage equal to or greater than the output

voltage used. The average current rating must be

greater

than

the

maximum

load

current

expected, and the peak current rating must be

greater than the peak inductor current.

Layout Consideration

High frequency switching regulators require very

careful layout for stable operation and low noise. All

components must be placed as close to the IC as

possible. Keep the path between the SW pin,

output diode, output capacitor and GND pin

extremely short for minimal noise and ringing. The

input capacitor must be placed close to the IN pin

for best decoupling. All feedback components must

be kept close to the FB pin to prevent noise

injection on the FB pin trace. The ground return of

the input and output capacitors should be tied close

to the GND pin. See the MP3217 demo board

layout for reference.