LM3463 thermal foldback

Hello Tom,

Figure 18 in the data sheet shows the linear current regulator.
Knowing the method of using the IOUTADJ function from the datasheet details, it is possible to vary the divider voltage based on an external temperature.
One example would be a 10K NTC thermistor in parallel with RIADJ2, as temperature increases the NTC will decrease and lower IOUTADJ.

You could even pull IOUTADJ so low it could shutoff the LED current at high temperatures.

Even if you do not use dynamic headroom, the LM3463 will function with current regulation AND INTERNAL thermal foldback of the IC junction temperature.

Without dynamic headroom control, you can not minimize the heat of the mosfet pass element, thus no thermal control in that aspect.

Thanks for the reply. Will the LM3464 also function without Dynamic Headroom Control as the LM3463?

Tom,

I need a verification from our product group that indeed the dynamic headroom does not have to be used.

                I fear that there is something not obvious in the parts' operation that requires it.

It looks fine to me, but I must make sure.

LM3463 can work as output sinks without DHC (dynamic headroom control), provided that:-

1. OutP is open so no feedback to primary supply for DHC;
2. VLedFB needs biased higher than 2.5V to enable linear regulators (refer to DS session “Testing LEDs at System Startup”);
3. Make sure VDRn never exceeds 4 x “bias voltage at DRVLIM pin”, or the current will be cut down to save the power dissipation of the powerFET (refer to DS session “MOSFET Power Dissipation Limit”)

LM3464 can work as output sinks without DHC also, similar to LM3463 provided that:-

1. OutP is open so no feedback to primary supply for DHC;
2. VLedFB needs biased higher than 2.5V to enable linear regulators;
3. Make sure VDRn < 8.35V, otherwise, the channel will be latched off due to short-fault (See DS sessions “SHORT CIRCUIT OF LED STRINGS” and “DRx PIN OVER-VOLTAGE PROTECTION”);

And, LM3464 is doing analog-to-PWM for thermal foldback, see DS “THERMAL FOLDBACK INTERFACE”, “SETTING MINIMUM THERMAL DIMMING DUTY CYCLE” and “PWM DIMMING”.

While LM3463 is doing analog-DIM through the IOUTADJ pin for thermal foldback.

Hello Pascal, all information we have would be in the product folder for the device.
http://www.ti.com/product/LM3464

You can also look at similar devices folders for other hints.

Figure 27 in the LM3464 datasheet shows a cascade connection of 2 LM3464 drivers.
The VRAIL must provide enough overhead for the LED Vf, Rsns voltage drop, and keep the Qx in the linear region.

That is what the OUTP pin does by driving the input supply feedback.
The LM3464 EVAL board users guide discusses how to use the chip with a constant voltage input.
4 Channel LED Driver Evaluation Board