LM3492: Possible configurations and DIM pin as a Switch ON/OFF

Hello Daniele,

I don't know of anyone that has made a SEPIC converter out of the LM3492.  I'm not sure if it would work, or work correct.  One thing that would be a concern is maximum input of 32V, max output of 31.8V is 63.8V plus the SEPIC diode Vf plus the voltage ripple of the SEPIC capacitor at the SW pin.  This would also eliminate Over Voltage Protection.  I would not create a design that close to absmax.

What is the range of Vled?  You have a maximum of 31.8V how low can it be?  The LM3492 can handle input voltage higher than output voltage in a boost configuration.  I believe holding the dimx pin low will turn off channelx.

Best Regards,

Hello Daniele,

Additionally, we may have parts that are better suited to your design. Can you provide more details on what your end equipment is, and what you are looking for in an IC? Thanks in advance!

Best Regards,

Caroline

Hi Irwin,

thanks for your reply.

I've just found this about LM3492 as a SEPIC: lm3492-as-sepic. Anyway, no AN or design examples are provided and, according to your write, it seems a bit risky to me using LM3492 out of spec.

As for the minimum VLED voltage, it can be as low as 22.8V. Since I can have 32V input, LM3492 needs to "boost" at least 33V, and that isn't a problem. But, for example, what about when VLED is 22.8V and Vin is higher? I guess LM3294 stop working. To avoid this, I should add a resitor on each LED string so that VLED is at least 33V when VLED is at is minimum 22.8V (R=(33V-22.8V)/50mA=204 Ohm), but this way Ill end to throw away nearly 500mW per string.......unless LM3294 raises VLED over Vin in any conditions, in order to maintain 50mA current (But I don't think it works this way). Even if it does, we have the same amount of "power loss", this way dissipated directly on the chip.

The "Power loss" mentioned above is the reason that led me to SEPIC (or Boost-To-Vin) configuration.

Again, many thanks  for your support and time.

Regards,

Daniele

Hi Caroline,

thanks for your reply.

We are working on a LED LAMPS (with Stop, Dir, and Position features) for automotive.

I'm looking for an IC which is capable of:

- adapt the out voltage to VLED while regulating 50mA currents into each of the two LED strings. This have to be done considering 9-32V input voltage.

- dim and switch ON/OFF each LED string independently by means DIM pins or EN pins

-  support at least 40V input voltage or higher to face with Load dump

The IC also has to be qualified AEC-Qxxx.

Since there is a uC on board, it's also possible to use a programmable IC. In this case, I think, we also have to take into account the price, which will probably be higher than the LM3294 one.

Hope my write answer your questions.

 Many thanks for your support and time.

Regars,

Daniele

Hi Daniele,

40V puts the SW node above 65V for a SEPIC.  So if you want to use a SEPIC I would look at the TPS92691 and TPS92692.  Another option is the TPS92682, it is a dual channel, as an example you can look at TIDA-050029.  It's much more complicated than you are looking for but it shows two SEPICs from one TPS92682 capable of up to 40V input and 40V output.

Best Regards,

Hi Daniele,

What you are running into is that your output power is quite low for a switching solution.  We don't have many parts that are optimum for low power.

Another thing you can consider using the LM3492, since this is automotive and assuming a battery input and a 12V battery, is how often will the input be above 16V?  Seems this could be set up as a boost and the design can afford some loss during the load dump situation provided nothing gets damaged.

Best Regards,

Hi Irwin,

thanks for your reply.

I need 40V input capable IC only to face with Load dump. In normal conditions the input voltage will be 9V -32V, which means nearly 32V +32V output voltage....very close to the SW abs. value, as you already said.

I've had a look at TPS92691/2 and TPS92682, but they really are too much for my target: they need external MOS/  BJT plus shunt resistor...to much for 50mA current in my opinion.

Thanks for you support and time.

Best regards,

Daniele

Hi Irwin,

thanks again for your reply.

Maybe you are right when you say  "...your output power is quite low for a switching solution.", but I can't do it without Switching, since to do it without DC/DC I'd need to make 2x(4 strings x 3LEDs) out of 2string x 12LEDs currently planned. This is because LEDS must be ON even at the lowest input voltage 9V, and that's why only 3 series LEDs per string can be put on it. But what if when Vin = 32V?  As you can imagine, the lamp become a heater!

Probably, as you suggested, I'll end it with LM3294 as a boost, consequently accepting some loss power.

As for your last question, consider that our product must suit both 12V and 24V battery, so it could even ever work above 16V i.e. beetwen 18V and 32V.

 Many thanks for your support and time.

Regars,

Daniele