MP1529 – FIXED FREQUENCY STEP-UP CONVERTER LCD BACKLIGHT PLUS FLASH
MP1529 Rev. 1.2 www.MonolithicPower.com 7
1/22/2010 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2010 MPS. All Rights Reserved.
OPERATION
The MP1529 white LED driver performs both
dual string backlight and single string flash
functions for portable applications. Its features
include pulse width modulated (PWM) dimming
of the backlight LEDs to 30mA, preview with the
same dimming capability on the backlight
strings and a flash only mode to 150mA.
Preview allows the flash LEDs to illuminate the
camera scene such that the correct exposure
setting can be set.
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. The
shutdown behavior will be discussed 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 high 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 two backlight
strings. Both EN1 and EN2 high 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 soft-startup.
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.
If the low interval is longer than the timer
interval, the EN1 pin will assume a steady state
low value. For example: when the PWM timing
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 by passing through the off state. The
change in EN2 resets the PWM and causes the
flash to react immediately. Another method
(though slower) to accomplish this is to stop the
PWM timer 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.
