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LT3965/LT3965-1

39651fa

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APPLICATIONS INFORMATION

longer exists when the write command is updating the

command register at the I2C STOP condition, the fault

status bit matching the channel will be cleared and the

ALERT pin will be deasserted. Otherwise, the fault status

bit will remain set, and the ALERT pin will remain asserted

or be asserted again if previously deasserted.

OVERHEAT FAULT DETECTION AND ALERT ASSERTION

AnoverheatfaultwillbetriggeredwhentheICtemperature

exceeds 170°C. Once an overheat fault is triggered, all

status bits in both the OLFREG register and the SLFREG

asserted (pulled down) and all eight channel switches to

be turned on (LEDs to be turned off) for cooling down

the system.

OVERHEAT STATUS BITS CLEARANCE

The fault status bits set in the OLFREG register and the

SLFREG register by an overheat fault can only be cleared

by an ACMODE write command with all 1s in its data byte.

If the IC temperature is below 160°C when the ACMODE

write command is updating the ACMREG register at the

I2C STOP condition, the fault status bits will be cleared

and the ALERT pin will be deasserted. Otherwise, the fault

status bits will remain set, and the ALERT pin will remain

asserted or be asserted again if previously deasserted.

ALERT DEASSERTION

The LT3965LT3965-1 deasserts the ALERT pin in either

of the following two situations:

1) The LT3965/LT3965-1 has successfully completed the

ARA procedure initiated by the master. Please note that

the successfully completed ARA procedure does not

clear fault status bits. It only deasserts the ALERT pin.

2) The LT3965/LT3965-1 has received an ACMODE or

SCMODE write command which cleared the fault status

bits, resulting in the ALERT pin deassertion.

PRINTED CIRCUIT BOARD LAYOUT

When laying out the printed circuit board, the following

checklist should be followed to ensure proper operation

of the LT3965/LT3965-1:

1. Connect the exposed pad of the package (Pin 29) di-

rectly to a large ground plane to minimize thermal and

electrical impedance.

2. Keep the LED connection traces as short as possible.

3. Place power supply bypass capacitors as close as pos-

sible to the supply pins.

4. Place the RTCLK resistor as close as possible to the IC

if a resistor is used to set LED dimming frequency.

Long Wires or Cables Between LT3965/LT3965-1 and

LEDs

The best practice is to place the LT3965/LT3965-1 and the

LEDs it controls on the same PCB and to keep LED con-

nection traces as short as possible. Long wires (>>10cm)

between the LT3965/LT3965-1 and the LEDs introduce

parasitic inductance that leads to an underdamped RLC

response (ringing) in the switching voltage when chan-

nel is switching on and off. A meter of 30-gage wire can

introduce about 1µH of parasitic inductance. The ringing

can trigger open LED protection due to false open LED

detection, and cause the channel to bypass good LEDs. In

extremecases,theringingmayexceedabsolutemaximum

ratings and damage the part. The parasitic inductance also

generates a step voltage waveform (relative to GND) at

the switches at the frequency of the switching regulator.

The magnitude of this step waveform depends upon the

current ripple in the source and the parasitic inductance.

The fast edges of the step waveform can cause unintended

toggling of the LT3965/LT3965-1 switches.

RC snubber circuits (shown in Figure 14) can suppress the

ringing and allow use of wires up to 1 meter with no false

fault detection. The snubber should be placed close to the

IC. Please note that an 8-LED string requires 9 snubbers:

one snubber across each of the 8 switches and a snubber

across all 8 switches (R9, C9). The 9th snubber (R9, C9)

softens the stepped waveform edges. With the snubbers,

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