Lx1995 Led Driver: Camera Flash Application

 
APPLICATION INFORMATION
Many high end PDAs and smart phones have adopted
color LCD screens.  These LCDs are usually illuminated
using white LEDs as a backlight.  Since white LEDs
require a higher voltage than the typical battery voltage, a
boost circuit is required to boost the battery voltage.  Many
smart phones and PDAs also include a digital camera. 
Many of these cameras utilize an LED Flash that helps
illuminate the subject in a dark environment.  The
requirements for the camera Flash are usually different than
the requirements for the LCD backlight which necessitates
two separate LED Driver circuits; however, if the LCD
illumination CAN vary during the period of the Flash it is
quite easy to provide both LED Driver functions using one
boost converter.
Boost circuits typically come in one of two forms; the
classical inductor based boost converter or the capacitor
based Charge Pump converter.  The Charge Pump converter
is efficient if the output voltage is a multiple (whole or
fractional) of the input  voltage which places some
restrictions on how it CAN be applied.  The inductor based
boost converter has no such output voltage restrictions and
is highly efficient over a wide range of output voltages.
This application circuit requires an inductor based boost
converter.
The schematic below shows the application circuit.  In this
circuit an SC-70 Dual NMOS (Fairchild FDG6303N
device is used to Switch on the Flash LEDs   The Flash
LEDs are set for a current of 83mA (as determined by the
value of R6).  The backlight LEDs are dimmable by either
a Logic PWM signal applied to the PWM input or a DC
adjust voltage applied to the DC_ADJ input.  When the
flash LEDs are on, the LED backlight will dim.  Other than
this, the LED backlight is not affected by the addition of
the Flash feature.  The Flash control circuit is designed to
over-ride the backlight dimming control so this has no
affect on the brightness of the Flash LEDs   The circuit
relies on the LED Flash voltage being less than the
backlight voltage(which is usually the case), however, if
this is not the case an additional Transistor CAN be placed in
series with the backlight LED string to open circuit the
string during the Flash period.
The LX1995-2 is well suited for  this application, because
it is capable of driving loads up to 1.25 watts from a
lithium ion battery.  The fixed feedback voltage node of
the LX1995 is suitable for switching between feedback
from two different Current Sense resistors.  If more power
is necessary, consult Microsemi for higher power LX1995
versions.
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LX1995 Application Schematic for Backlight & Flash Function
LX1995 Application Schematic for Backlight & Flash Function



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