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TPS40054 not functioning at -25 degree C

Satyajit K
Prodigy 80 points
Other Parts Discussed in Thread: TPS40054

Hi Peter

We are using TPS40054 in our design. Our input supply voltage range is 10V to 28V and output voltage is 5.2V with 2A maximum current.

We have designed a circuit and it is working fine at normal room temperature (+25C). As we reduce temperature to -25 degree C, SMPS stops functioning and both low and high gate drives are 0V. We observed that BP5 and BP10 voltage are proper. What could be the possible reason for such behaviour? Is TPS40054 tested for -45C as per specs?

In our design, we have disabled the voltage feedforward and UVLO mechanism by connecting 20K resistor between BP5 and KFF pin as per application note SLUA310. 

To replicate the same low temperature behaviour at  normal room temperature, we experimented with RKFF (20k) value. We observed that by increasing the value to 22K and above, SMPS stops functioning and gate drives are at 0V. Is this the expected behaviour? We are not able to make out the reason behind such behaviour from datasheet data?

Please provide your inputs on this issue at the earliest. If you need any additional inputs/circuit details, please let me know.

Regards,

Satyajit

  • 0
    TI__Guru 56985 points

    Satyajit,

    Most likely what you are observing is a small shift in the required current into the KFF pin to achieve UVLO at low temperature verse room temperature.

    SLUA310 describes a procedure for calculating the resistor value to be placed between BP5 and KFF to ensure sufficient current flows into the KFF pin when powered y the BP5 regulator, however the procedure provided in SLUA310 has 3 short comings:

    1) It is not perfectly clear that the Vin(min) used to calculate Rkff by the datasheet equation and the VIN used to calculate Ikff from the equation in SLUA310 must be the same voltage.

    2) SLUA310 equations use the constant 3.5V instead of the variable Vkff, which is typically 3.48V on the TPS40054 controller.

    3) SLUA310 equations use the constant 5V instead of the variable Vbp5, which can be as low as 4.7V per the TPS40054 specification.

    As a result of these 3 short comings, designers may not properly account for the tolerances on Vkff, Vbp5 or properly use the same Vin(min) for both Rkff and Ikff equations, resulting in errors in their selection of Rkff(sup).  Most likely Rkff(sup) needs to be slightly smaller than what was originally calculated to ensure operation over the full temperature range.

    Since the Rkff and Ikff equations both use the same Vin(min) - Vkff values, the Ikff equation can be simplied as the inverse of the second term of the Rkff equation from the TPS40054 datasheet - 1 / (58.14*Rt + 1340) (in Amperes)

    And Rkff(sup) should be [ Vbp5(min) - Vkff(max) ] / Ikff

    So, for the TPS40054 controller Rkff(sup) = [ Vbp5(min) - Vkff(max) ] * [ 58.14 x Rt + 1340 ] (in Ohms)

     NOTE: This is the same as substituting Vbp5(min) for VIN(min) in the Rkff equation in the TPS40054 datasheet

    If you are using synchronization, make sure to calculate R(dummy) per equation 16 in the TPS40054 datasheet

  • 0 in reply to Peter James Miller
    Prodigy 80 points

    Hi Peter,

    Thank you for thius detailed analysis and reply.

    After your suggestions, we identified that Rkff of 20K between BP5 and KFF pin was on the boundry line for our requirements. When we reduced this value to 15K, our board is working fine even at -34 degree C.

    Once again thank you for your inputs and support.

    Thanks and regards,

    Satyajit