DESIGN FEATURES White LED Driver Minimizes Space,
Maximizes Efficiency and Flexibility
by Steven Martin Introduction White LEDs are the LCD backlight of
choice for portable equipment due to
their small profile, exceptional ruggedness and high luminosity. Li-Ion
batteries, also rugged and small, are
the popular power choice for the same
portable equipment. The white LEDs
have a nominal forward voltage between 3V and 4V and a Li-Ion battery
has a nominal voltage of 3.6V, thus
requiring circuitry to step-up the Li-Ion battery’s voltage. The problem is,
stepping up tends to reduce efficiency,
which in turn reduces battery life. The
LTC3205 solves this problem by powering up in direct-connect mode rather
than step up mode. Once powered up,
it uses a dropout detector for each
white LED to determine if any are running out of drive. As the first white LED
just begins to lose current, the chip automatically engages a very powerful 2:
C1+ C1– C2+ C2– 5 4 6 3 800kHz
OSCILLATOR 25 PGND
15 CPO VIN 16
1:1 AND 2:3 CHARGE PUMP –
+ ENABLECP + …