MP1529DR-Z データシートの表示（PDF） - Monolithic Power Systems
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MP1529DR-Z Datasheet PDF : 11 Pages
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MP1529 – FIXED FREQUENCY STEP-UP CONVERTER LCD BACKLIGHT PLUS FLASH
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.
MP1529 Rev. 1.0
www.MonolithicPower.com
7
11/23/2005
MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2005 MPS. All Rights Reserved.
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