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TPS1HC100EVM: Unexpected behavior when PWM'ing enable pin

Zareef Huq
Intellectual 1905 points
Part Number: TPS1HC100EVM

Tool/software:

Hi team,

I am testing using a TPS1HC100EVM as a PWM controller to drive an LED strip to avoid using a large external MOSFET. The LED strip would consume about 1A at 12V, which I believe is within the capabilities of this HSS. While I currently don't have an LED strip to act as the load yet, I am testing PWM'ing the EN pin to vary the output voltage of the EVM, but no matter what I set the duty to, it outputs 12V until I get below ~25% duty. At that point, the output voltage slightly decreases but the lowest I've seen is 10.6V. The switching frequency is 490Hz, which should be acceptable according to the datasheet. 

Is there something else that I need to do other than just PWM'ing the EN pin? I confirmed the PWM signal being sent to EN is good by measuring the frequency and voltage being output with a multimeter, which were as expected. Max voltage when I set it to 100% duty is ~4.7V, 50% duty shows ~2.4V, and so on. There is no load attached at this time while I wait for the LED strips to come in.

  • 0
    TI__Expert 7581 points

    Hi Zareef,

    PWMing the EN pin is correct, but I think there is a misunderstanding about the concept. Switching the EN pin on and off does not change the voltage on VOUT and changing the duty cycle does not change the amount of voltage being supplied to the EN pin. The duty cycle is the amount of time that a signal is on for a single cycle. For example, if I am supplying 5V to the EN pin, for a 100% duty cycle, the EN pin will always be high at 5V; for a 50% duty cycle, the EN pin will be high for 50% of the time at 5V and off for 50% of the time at 0V. Regardless of what the duty cycle is, when EN is on, VOUT will see the same voltage that is on VS; turning the channel on through the EN pin allows current to flow through to the output but the switch itself does not change the voltage. Please let me know if this makes sense.

    For more information, please refer to the following application note - Driving PWM Loads with TI High-Side Switches. 

    Thanks,

    Rishika Patel

  • 0 in reply to Rishika Patel
    TI__Intellectual 1905 points

    Hi Rishika,

    Thank you for the response. This does help clarify things for me as I am a bit rusty on these concepts since I haven't used this information in a long time. Is my use case of PWM'ing the HSS to control the brightness of an LED strip still valid?

    I am leaning on yes as your app note talks about controlling LEDs but it would be nice to confirm.

  • +1 in reply to Zareef Huq
    TI__Expert 7581 points

    Hi Zareef,

    Yes, your case of using PWMing to control the LED is valid.

    Thanks,

    Rishika 

  • 0 in reply to Rishika Patel
    TI__Intellectual 1905 points

    Hi Rishika,

    I managed to successfully control an LED and motor through PWM'ing the HSS, thank you for your help.