LTC3452 datasheet(14/16 Pages) LINER

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LTC3452

3452f

APPLICATIO S I FOR ATIO

Unused Outputs

If fewer than 7 LED pins are to be used, unused LEDx pins

should be connected to VOUT. 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 (10

µA: 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 VOUT). 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 VOUT higher. As the open circuited

LED will never accept its programmed current, VOUT must

be voltage-limited by means of a secondary control loop.

The LTC3452 limits VOUT 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.

LTC3452 bảng dữ liệu(PDF) 14 Page - Linear Technology