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1.2A White LED Regulating Charge Pump for
Camera Flashes and Movie Lights
C1
2.2 μ F*
CP
CN
2.7V TO 5.5V
C IN
4.7 μ F
IN
GND
OUT
5.3V OVP UP TO 1.2A
C IN
10 μ F
R CS
5.1V, 4.6V, OR 3.4V
MAX1577Y
V OUT - (300 OR
5.1V OR 100% CURRENT,
4.6V OR 33% CURRENT,
3.4V OR 20% CURRENT,
AND SHUTDOWN
EN1
EN2
CS
100 OR 60mV)
MAIN DISPLAY
BACKLIGHT
SUBDISPLAY
BACKLIGHT
RGB DRIVER
ASIC
CAMERA-FLASH
MOVIE LIGHT
*1 μ F FOR LESS THAN 600mA OUTPUT.
Figure 4. Flash and Backlighting with the MAX1577Y
Detailed Description
1x LDO to 2x Mode Switchover
When V IN is greater than V OUT , the MAX1577Y/
MAX1577Z are in 1x LDO mode and regulate the cur-
rent through R CS . As V IN decreases and the LDO
approaches dropout, the 2x charge pump starts boosting
the output to maintain the regulation current. When V IN
rises enough above V OUT to maintain LDO regulation, the
charge pump stops and the MAX1577Y/MAX1577Z
switch back to the more efficient 1x LDO mode.
Soft-Start
The MAX1577Y/MAX1577Z include soft-start circuitry to
limit inrush current at turn-on. When starting up with an
output voltage that is not near the input voltage, the out-
put capacitor is charged directly from the input with a
ramped current source (with no charge-pump action)
until the output voltage approaches the input voltage. If
current-mode or voltage-mode regulation is met, soft-
start is terminated and normal operation begins in 1x
mode. Otherwise, after 500μs, the 2x charge-pump soft-
start operates until the voltage across the sense resistor
reaches the current regulation threshold, the output volt-
age is in regulation, or 500μs has elapsed, whichever
occurs first. If the output is shorted to ground, soft-start
is repeated until the output exceeds 2.3V.
Dimming and Shutdown
The MAX1577Y/MAX1577Z use two inputs (EN1 and
EN2) to select between three brightness levels and low-
power shutdown mode (see Table 1). By regulating the
voltage across the current-sense resistor (V OUT - V CS ) to
one of three levels, the MAX1577Y/MAX1577Z control the
output current, thus providing the three-level dimming. In
shutdown mode, the IC is turned off, reducing the supply
current to 0.01μA (typ). When the MAX1577Y/MAX1577Z
are shut down, C1 is discharged and the output is pulled
to GND through an internal 5k ? resistor.
8
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