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

MP1529 Rev. 1.0

www.MonolithicPower.com

8

11/23/2005

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© 2005 MPS. All Rights Reserved.

TM

Table 1—Enable Operation Table

(5, 6, 7)

Mode

EN1

EN2

Strings 1 & 2: LCD Backlight LEDs

String 3: Flash LEDs

Off

L

L

Off

Off

Backlight

H (PWM)

L

On (PWM)

Off

Backlight plus

Preview

H (PWM)

H

On (PWM)

Preview current

Flash

L

H

Off

Flash current

Notes:

5) Backlight and Backlight plus Preview may use PWM for backlight brightness control. Only strings 1 & 2 will respond to high speed PWM.

String 3 will remain continuously on or off depending on the mode.

6) Any combination of backlight string may be used. 1 or 2 or 1 & 2 will regulate properly. The part uses over voltage detection to determine

which strings are available to light. The remaining strings are then used for regulation. The part will not remain in an open loop mode if

there is at least one string of LEDs to light.

7) Changing from backlight plus preview to flash will have a delay equal to the soft-start timer due to the PWM filtering necessary for preview.

Switching the part to off mode then to flash will cancel the timer. This will take a few microseconds instead of the 10 milliseconds typically

associated with the timer being active.

APPLICATION INFORMATION

Setting the LED currents

Resistors RS1, RS2 and RS3 set the backlight

LED string (LED1 and LED2) current, the flash

LED string (LED3) preview current, and the

flash

LED

string

current,

respectively.

Determine each current setting resistor in kΩ by

the equations:

BL

_

LED

SET

I

V

950

1

RS

×

=

PV

_

LED

SET

I

V

1100

2

RS

×

=

FL

_

LED

SET

I

V

1000

3

RS

×

=

the backlight LED string current, the flash LED

string preview current, and the flash LED string

flash current in mA, respectively.

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

A single 1µF to 4.7µF ceramic capacitor usually

provides sufficient output capacitance for most

applications. The impedance of the ceramic

capacitor

at

the

switching

frequency

is

dominated by the capacitance, and so the

output voltage ripple is mostly independent of

calculated as:

(

)

SW

UT

O

IN

UT

O

LED

RIPPLE

f

2

C

V

V

V

I

V

×

×

−

=

is the 1.2MHz switching frequency.