6

Feb 9-07 Rev E

Constant Current LED Driver

© 2007 Sipex Corporation

APPLICATION INFORMATION

Shutdown

The SP769 has a low current shutdown

function. In shutdown mode the part draws

lessthanµAcurrentmaximum.Thepartcan

be placed into shutdown using the EN pin.

ENABLE and CTRLpins cannot be tri-stated

or left floating. There are no predefined in-

ternal states, so leaving these pins open will

cause the part to operate incorrectly.

Timeout

A Time Duration Before Shutdown feature

actsasafailsafetoprotecttheLEDfromlong

ON times. The nominal timeout is defined as

4 seconds at 500 (700)mA output current.

Power-on-Reset

This function is used to set Time Duration

counters into their initial state after power up.

No signal applied to the CTRL pin can be

accepted during this time, even if the input

voltage is applied simultaneously to the Vcc

and ENABLE pins.

To obtain the best performance from the

SP769,aprintedcircuitboardwithground

plane is required. High quality, low series

resistance ceramic µF bypass capaci-

tors should be used at the Vcc pin (pin 3).

These capacitors must be located as close

to the pins 3 – 6 as possible. The traces

connecting the pins and the bypassing

capacitor must be kept short and should

be made as wide as possible. Pins  – 2

are connected together internally but we

recommend connecting the LED cathode

to both pins to avoid additional power losses

at high current. Pins 6, 7 and 8 should be

connectedtothegroundplane.Boardlayout

shouldpreventhighLEDgroundcurrentfrom

flowing through signal ground connected to

pin 6. This pin is a separate ground for the

reference and logic sections of the SP769.

The quiescent current for the part flows into

the VCC pin and out of the GND pin. This

divided by approximately 500.

BOARD LAYOUT AND GROUNDING