MP1529 View Datasheet(PDF) - Monolithic Power Systems

Part Name

Description

Manufacturer

MP1529

Fixed Frequency Step-Up Converter LCD Backlight Plus Flash

Monolithic Power Systems 

TM

MP1529 – FIXED FREQUENCY STEP-UP CONVERTER LCD BACKLIGHT PLUS FLASH

Selecting the Inductor

The inductor is required to force the output

voltage higher while being driven by the lower

input voltage. Choose an 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

approximately 30%-50% of the maximum input

current. The maximum input current usually

occurs in flash mode. Make sure that the peak

inductor current is below 75% of the typical

current limit to prevent loss of regulation due to

the current limit variation.

Calculate the required inductance value L using

the equations:

L = VIN (VOUT - VIN )

VOUT × fSW × ∆I

IIN(MAX)

=

VOUT ×ILED _ FL

VIN × η

∆I = (30% − ) 50% IIN(MAX)

Where ∆I is the peak-to-peak inductor ripple

current and η is the efficiency at flash mode. A

4.7µH or 10µH inductor is recommended for

most cell phone applications.

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. For output voltage less than

20V, it is recommended to choose the

MBR0520 for most applications.

Selecting the Compensation Capacitor

The compensation capacitor needs to be

determined so that the control loop is slow

enough to allow for stable and smooth mode

transition. A 1µF ceramic capacitor is

recommended for most cell phone applications.

Selecting the Soft-Start Capacitor

The soft-start capacitor sets both the soft-start

timer and the PWM timer. A 10nF to 22nF

capacitor is recommended for most

applications.

Layout Considerations

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

current setting resistors must be kept close to

the corresponding ISET pin to prevent noise

injection for accurate LED current setting. The

ground return of the input and output capacitors

should be tied close to the GND pin. See the

MP1529 demo board layout for reference.

MP1529 Rev. 1.0

www.MonolithicPower.com

9

11/23/2005

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© 2005 MPS. All Rights Reserved.

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