MP1540 – 1.3MHz, 18V STEP-UP CONVERTER

MP1540 Rev. 1.0

www.MonolithicPower.com

5

8/15/2005

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TM

APPLICATIONS INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

Set the output voltage by selecting the resistive

voltage divider ratio. Use 11.8k Ω f or the lo w-

side resist or R2 of

the voltage divider.

Determine the high-side resist or R1

by the

equation:

()

FB

FB

OUT

V

V

-

V

2

R

1

R

=

Where VOUT is the outp ut voltage and V FB is the

feedback voltage.

For R2 = 11.8kΩ and VFB = 1.25V, then

R1 (kΩ) = 9.44kΩ (VOUT – 1.25V).

Selecting the Input Capacitor

An input ca pacitor is re quired to su pply 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 in put capacit or value of 4.7

µF or

greater. This capacit

or must be placed

physically close to the I N pin . Since it redu ces

the voltage ripple seen at IN, it also reduces the

amount of EMI pa ssed back alo ng that line to

the other circuitry.

Selecting the Output Capacitor

A single 4 .7µF to 10 µF ceramic capacitor

usually provides sufficient output capacitan

ce

for most application s. If larger

amounts of

capacitance are desired for improved line

support an d transient response, tantalum

capacitors can be used in parallel with the

ceramic. The impedance of the ceramic capacitor

at the switching frequency is domin ated by the

capacitance, and so the output voltage ripple is

mostly inde pendent of the ESR. T he outpu t

voltage ripple VRIPPLE is calculated as:

(

)

SW

UT

O

IN

UT

O

LOAD

RIPPLE

f

2

C

V

V

V

I

V

×

×

−

=

Where VIN is the input voltage, ILOAD is the loa d

current, C2 is the ca

pacitance of the output

capacitor, a nd f SW is the 1.3MHz switch

ing

frequency.

Selecting the Inductor

The inducto r is re quired to force

the output

voltage higher while being driven by the lower

input voltage. Choose a n inductor that does not

saturate at the SW current limit. A good rule for

determining the inductance is to allow the peak-

to-peak ripple current to be appro ximately 30%-

50% of the maximum in put current. Make sure

that the peak inductor current is below 75% of

the typical current limit at the duty cycle used to

prevent loss of regulat ion due to

the current

limit variation.

Calculate the required inductance value L using

the equations:

I

f

V

)

V

-

(V

V

L

SW

OUT

IN

OUT

IN

∆

×

×

=

η

×

×

=

IN

)

MAX

(

LOAD

OUT

)

MAX

(

IN

V

I

V

I

(

)

)

MAX

(

IN

I

%

50

%

30

I

−

=

∆

Where ILOAD(MAX) is the maximum load current, ∆I

is the peak-to-peak inductor ripple current and η

is ef ficiency. For t he MP15 40, 4. 7µH is

recommended fo r in put vo ltages less than 3. 3V

and 10µH for inputs greater than 3.3V.

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. It is

recommended to choose the MBR0520 for most

applications. This diode is used for load currents

less than 500mA. If the average current is more

than 500mA the Microsemi UPS5817 is a good

choice.