LTC3452_15 Datasheet, PDF (14/16 Pages) Linear Technology – Synchronous Buck-Boost MAIN/CAMERA White LED Driver

LTC3452
APPLICATIO S I FOR ATIO
Unused Outputs
If fewer than 7 LED pins are to be used, unused LEDx pins
should be connected to V_OUT. The LTC3452 senses which
current source outputs are not being used and shuts off
the corresponding output currents to save power. A small
trickle current (10µA: low power bank, 30µA: high power
bank) is still applied to unused outputs to detect if a white
LED is later switched in and also to distinguish unused
outputs from used outputs during start-up.
LED Failure Modes
If an individual LED fails as a short circuit, the current
source biasing it is shut off to save power. This is the same
operation as described previously (if the output were
initially designated unused at power-up by connecting its
LEDx pin to V_OUT). Efficiency is not materially affected.
If an individual LED fails as an open circuit, the control loop
will initially attempt to regulate off of its current source
feedback signal, since it will appear to be the one requiring
the largest forward voltage drop to run at its programmed
current. This will drive V_OUT higher. As the open circuited
LED will never accept its programmed current, V_OUT must
be voltage-limited by means of a secondary control loop.
The LTC3452 limits V_OUT to 4.5V in this failure mode. The
other LEDs will still remain biased at the correct pro-
grammed current but the overall circuit efficiency will
decrease.
3452f
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