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TPS92512HV: 48V SUPPLY with led load of 42V

giovanni copler
Prodigy 195 points
Part Number: TPS92512HV

Dear Texas,

I'm using the TPS92512HV as a led driver. I need to work with a Vin that goes from 12V to 48V.

I need to reach an output voltage that goes from 10% Vin to at least 90% of Vin (or even higher if possible).

Supplying with 24V or 12V it can reach the 90% of the duty cycle using the bootstrap capacitor, supplying with 48V I can reach at least 87% of the duty cycle.

Since I knew I was near the border of the max duty cycle, i have provided an optional supply 8V higher than Vin.

But something odd happens connecting this voltage.

Even if the Pdim is off and the switching doesn't switch, the output voltage doesn't goes to 0V and the leds connected to the output remains on (even with a low current). It seems that a certain amount of current can pass between the BOOT pin and the output (it seems that there is a zener beteen output and vboot with a vr < 8V. Is it correct? If so, which is the correct voltage to be provided to the BOOT pin?) In the data sheet is reported that if a voltage at least 2V higher than Vin is provided it's possible to reach even 100% of duty cycle. I have seen that I can reach nearly 95% of duty cycle providing Vboot external which is ok for me, but the output doesn't turn to 0V if the switching is off.

 Thanks,

  • 0 in reply to giovanni copler
    TI__Mastermind 28655 points
    Hello Giovanni,

    You cannot provide boot voltage from Vin + 8V. First, the abs max on Vboot is 8V so the additional boot voltage should be less than that. Next, what happens when the MOSFET turns on? Now the Boot pin is getting Vin + 8V, not just the 8V. This is probably why the LEDs are staying on. The additional supply needs to have it's common tied to the switch node, PH, so Vboot is a constant voltage which is lower than 8V that is sourced to BOOT and returns through PH. It can be quite a bit lower but I would add a diode in place so when the boot tries to refresh when the MOSFET is on the boot current cannot go back to the added supply. Again, the added supply needs to only provide current to BOOT with respect to PH. Also, if it helps, C3 can be a larger value.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 195 points

    Dear Irwin,

    The problem a diode is already present (I haven't provided the complete schematic) but the problem is that there are 2 switching and the higher voltage  is provided through a third one that is used to create a 3.3V to supply the logic... I can't connect the common to PH because I have 2 PH for the 2 switching. I show below a more complete schematic so that you can understand better what I mean.

    So I think the only solution is to reach the 90% of Duty cycle using only the bootstrap capacitors. I have made a prototype and one switching is able to reach 90% with the boostrap capacitor while the other not (the led flashes). The diode D1 and D4 aren't present. But for the switching which flashes with only the bootstrap capacitor I have shorted circuit D1 and removed C3.

    This way I can reach a value even higher than 90% of duty cycle bit I have the problem that when the switching is off that a current is still going to the led.

    The 3.3V goes to supply the logic and it seems a simple and low cost way to increase the output voltage range. Is it possible to reach the 43-44V on the led with 48Vin?

  • 0 in reply to giovanni copler
    TI__Mastermind 28655 points
    Hello,

    You cannot provide the bootstrap voltage this way, when the PH voltage goes low the source voltage will be too high for BOOT (probably why the LEDs stay lit. I'm surprized it's not getting damaged. I would recommend not adding any external voltage and lower the switching frequency so off-time is long enough to refresh the BOOT capacitor. Yes the output voltage can be 43-44V as long as the switching frequency is low enough.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 195 points
    Hello,

    Ok. One of them was broken but i hadn't understood why. I replaced it, but I wasn't sure if it was the boot voltage or what else. This is why I have contact you. I will try lowering the frequency.

    Thanks,
    Cristian
  • 0 in reply to giovanni copler
    Prodigy 195 points

    Hi, 

    by reducing the frequency I was able to improve the range a bit.

    I'm going to test better the performance to see if there are any other issue.

    Thanks,

    Cristian

  • 0 in reply to giovanni copler
    TI__Mastermind 28655 points
    Hello,

    I haven’t heard back from you, I’m assuming you were able to resolve your issue.
    If not, just post a reply below (or create a new thread if the thread has locked due to time-out)

    Best Regards,