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TPS61183 Min Boost Voltage

Anthony Pampena
Prodigy 20 points
Other Parts Discussed in Thread: TPS61183EVM-528, TPS61180, TPS61183

Hello,

I am currently evaluating the TPS61183EVM-528 module and have found that the controller will not regulate to a voltage below 13.5V on the output when adjusting down the string voltages. If say, I light 6 strings of 3 series LEDs (Vf = ~9V), the controller will regulate to 13.5V on the output (across C3) and there will be ~4.5V on each of the IFB pins. If I light 6 strings of 5 series LEDs (Vf = 15V), the controller will function as normal, regulating to the largest string Vf + ~600mV accounting for the headroom on the drain of the current source.
I can not find in any of the documentation a reference to a minimum boost voltage, though I can see some logic in maintaining the 13.5V regulation point as this is also the threshold for determining a shorted string. Other similar TI controllers (TPS61180/1/2) explicitly state in the datasheet that the boost can not regulate below 15V, however I can not find the same declaration for the TPS61183.

Some clarity here would be greatly appreciated as it would be hard to design in this part knowing that with low output voltage applications we would be burning excessive power in the controller.

Thank you,

Anthony Pampena

  • 0
    TI__Genius 11020 points

    Hello Anthony,

    Does the all the LED strings light up normally in this condition (3 LEDs in series)? Does it trigger any fault?

    It might need adjustment of the resistor divider in the OVP pin.

    Thanks.

  • 0 in reply to Tomi Koskela
    Prodigy 20 points
    Tomi,

    Adjusting the OVP divider did solve the issue and shed some light on the root cause. During a normal startup, the controller appears to regulate to 700mV on the OVP pin before ramping up to the appropriate output voltage for a given string of LEDs. If this initial 700mV corresponds to a high enough output voltage where the LEDs can light at the full driving current, the controller will "stick" in this state and not attempt to fold back the output voltage. All of the strings light and no fault is thrown. Would you be able to confirm what I am experiencing here as typical for this part?

    Thank you,
    Anthony
  • 0 in reply to Anthony Pampena
    TI__Genius 11020 points
    Hi Anthony,
    In the startup phase device goes through the LED detection etc. and if the resistors are not set correctly what you have seen can happen.
    The formula for determining the minimum output voltage is: Vmin = 0.72 * (1 + Rupper/Rdown)
    Default components on EVM should give about 14V minimum voltage, so that would also explain what you have seen.
    Thanks.-Tomi