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

MP1529 Rev. 1.0

www.MonolithicPower.com

7

11/23/2005

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© 2005 MPS. All Rights Reserved.

TM

OPERATION

The MP1529 white LED driver performs both

dual string backlight and single string flash

functions for portable applications. The features

include pulse width modulated (PWM) dimming

of the backlight LEDs to 30mA, preview with the

same dimming capability on the backlight

strings and flash only mode to 150mA. Preview

allows the flash LEDs to illuminate the camera

scene such that the correct exposure setting

can be set. The 4mm x 4mm package fits in

most applications.

Startup and Shutdown

EN1 and EN2 are the enables for the backlight

strings and the flash string, respectively. When

the inputs are low, the part is either in shutdown

mode or on the way to shutdown. We will

describe the shutdown behavior later.

Upon enabling the MP1529, the SS pin ramps

its voltage to provide soft-start protection

against supply inrush current. Typically the

ramps last 1ms to 10ms depending on

capacitor selection. EN1 lights up the LED1 and

LED2 strings for the backlight application. The

current setting is 950 times the current in ISET1

for each of the strings. EN1 and EN2 together

turn on the backlight strings and the preview

mode in the flash string. The preview mode

current is 1100 times the current in ISET2. The

final setting depends on the PWM timer, which

is the soft-start timer after the soft-start has

completed and has the same timing.

PWM Brightness Adjust

The PWM timer provides continuous operation

of the flash string and other internal functions

when the PWM low interval of EN1 is shorter

than the PWM timing. Should the EN1 low

interval be longer than the timer interval, the

EN1 pin will assume a steady state low value.

One example: when the PWM is too slow on

EN1, it will result in the part turning off or

entering flash mode depending on the state of

EN2. EN2 uniquely affects this mode by

resetting the PWM timer each time EN2

changes state. This allows a user to make a

transition from preview to flash faster by

passing through the off state. The change in

EN2 resets the PWM and causes the flash to

react immediately. The slower path is to stop

the PWM and make a low transition on EN1 in

the preview mode and to wait for the PWM

timer to clear for flash mode. The time for this

method is that of the PWM timer, which is the

same as the SS timer.

Other features of the 1529 are constant

frequency operation with cycle-by-cycle current

limit as well as over voltage and over

temperature protection.

Open String Protection

There is also open string protection through the

over voltage protection that prevents the 1529

from trying to start an open string indefinitely.

The over voltage signal forces the part to forget

about open LED strings and to resume

regulation on the remaining strings. Initially, any

open string will force the part to pump to its

maximum voltage. At this point, the part will

mark off strings that it simply cannot light

properly.

Once

the

mark-off

operation

completes, the remaining LED strings force the

output voltage back into tight regulation. The

string with the highest voltage drop is the ruling

string during output regulation. Once a string is

marked off, its current regulation cannot force

the loop regulation to set the output voltage.

Marked-off LED strings will continue to seek

current regulation and may come into current

regulation if their connections return or improve.

However, the ability to light the string properly

will vary due to the lack of output voltage

control.

The

part

will

maintain

mark-off

information until the part shuts down or another

LED string opens to force an over voltage

condition. Each time the over voltage condition

is triggered, the part marks off strings according

to their ability to reach current regulation. The

MP1529 will always try to light at least one

string and will ignore the case wherein all the

strings are marked off.