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TPS92512 operating at 100% duty cycle

Tony Spirovski
Prodigy 10 points
Other Parts Discussed in Thread: TPS92512, LM3401, LM3409

I am using  a TPS92512 operating from a rectified DC power supply. The circuit is based on the reference design, operating at 1A and driving a 3 LED string.  

When the input voltage dips below the required operating voltage of the LED string, the device, as expected, starts to operate at 100% duty cycle and the LED current drops below the set point.

However, when the voltage then rises above the required LED string voltage and the current exceeds the set point value, the drive to the LED's remains on and the current rises well above the set point. 

The device continues to operate at 100% duty cycle even though the current limit is exceeded and does not recover for up to 4mS once the input voltage is high enough.

Any ideas as to why the drive to the LED's does not switch off? 

      

  • 0
    TI__Genius 13700 points
    Hello Tony,

    Could you post a copy of your schematic?

    Also what is the voltage range in which this transition occurs?
  • 0
    TI__Mastermind 25500 points

    The TPS92512 is not an ideal device for 100% duty cycle operation since it is an bootstrapped NFET regulator. When it goes into 100% duty cycle the bootstrap capacitor will discharge until it hits its UVLO threshold and then will not restart until the LED voltage has dropped enough to recharge it. It is basically completely out of regulation during this time and if VIN jumps up while the internal FET is still in an on state the output current can overshoot. You can get faster recovery by increasing the bandwidth (reduce the COMP capacitor to say 10nF) but it is still an undesirable operational mode. If you really need 100% duty cycle capability you may want to consider the LM3401 or LM3409, otherwise it is recommended to have a minimum of the VCC UVLO voltage of headroom (VIN above Vout).