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TPS54201: Output only active for a short time

LucEE
Prodigy 80 points
Part Number: TPS54201
Other Parts Discussed in Thread: TPS92200, TPS54200,

I used this a example https://www.ti.com/lit/an/slua979a/slua979a.pdf to control a TEC module. Please note that I took the wrong value for Rsens and later changed this to 0.250R(measured). I don't have the TEC module connected yet, so only probing the output

Should I keep lowering the Rsens, it is already making a buzzing sound with the value. Or is there something else going on?

I connected the PWM pin to both 3V3 continuous and a PWM 50kHz, 50% (2V22)

125R Rsens

Rsens change to 0.250R

  • 0
    TI__Genius 15335 points

    Hi Luc,

    It looks like you are running into LED open protection as is shown in Figure 37 from the datasheet.

    Please connect the load and retry.

    BR,

    Steven

  • 0 in reply to Steven Li
    Prodigy 80 points

    There was indeed nothing connected to the output yet for test purposes.

    I connected both a power resistor and a TEC module

    Resistor:

    TEC:

    These look like the Sense resistor short protection figure 39. However, the FB resistance to GND is 0.250R - 0.300R, the slua979a suggests a even lower resistance value.
    Any ideas?

  • 0 in reply to LucEE
    TI__Genius 15335 points

    Hi Luc,

    It looks like the sense resistor short protection is trigger due to overcurrent (2.7A in your scope capture). Please refer to the current limit shown in 7.5 Electrical Characteristics from the datasheet. Please check your sense resistor value and also check if the FB pin is connected correctly.

    BR,

    Steven

  • 0 in reply to Steven Li
    Prodigy 80 points

    He Steven,

    The sentence "The magnitude of the PWM signal is used to determine which dimming mode the device enters" is not really clear to me. 
    I expect this: The magnitude is measured by the RMS value of the PWM signal (this makes sense because we can control that value with the duty cycle)

    However, is this really the case? Or does the peak detector only checks the peak of the signal, 300[us] after VIN exceeds UVLO? My PWM is a 3.3[V] output, so that would mean it will always be in Analog dimming mode when directly connected to the MCU.

  • 0 in reply to LucEE
    TI__Genius 15335 points

    Hi Luc,

    No, not the RMS value. The peak detector only checks the peak value. So if you always apply a 3.3V 50-kHz PWM signal to PWM pin, you will always be in the analog dimming mode.

    I would suggest you switch to next generation of TPS54200 / TPS54201, which is TPS92200. TPS92200 can directly goes into different dimming mode based on which version of chip you select - TPS92200D1 or TPS92200D2.

    BR,

    Steven