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LP8863-Q1: R_BST_FSET Selection

PY (Pengyu) Song
Expert 5860 points
Part Number: LP8863-Q1

Hi team,

For table 2, to select a specific boost frequency, what is the acceptable range of the R_BST_FSET? Say if the recommenced resistor value is not available, how much deviation is acceptable, in % or in absolute resistance value? 

  • 0
    TI__Genius 11836 points

    Hi Pengyu,

    Please ensure the 1% accuracy of resistance.

  • 0 in reply to Charles-Chen
    TI__Expert 5860 points

    Hi Charles,

    What if the tolerance of the resistor is >1%? What if the resistance itself is not listed in the table. For example, what will be the boost frequency if R = 4kohm or 5kohm? 

  • 0 in reply to PY (Pengyu) Song
    TI__Genius 11836 points

    Hi Pengyu,

    For R = 4kohm, the boost frequency may not ensured as listed in table 2. It could be 303kHz or 400kHz, which means the frequency is not changing continuously with the value of resistor. The same reason for 5kohm.

  • 0 in reply to Charles-Chen
    TI__Expert 5860 points

    Hi Charles,

    So are you suggesting that the boost frequency can only be guaranteed for resistance values listed in Table 2 with tolerance of 1% or less. For resistors with tolerance higher than 1% or resistance values deviate from that listed in Table 2, the switching frequency will not be guaranteed - the resulting frequency will be one of the two nearby frequency values. Please confirm if the understanding is correct. 

    In fact, I would expect less stringent requirement resistor values as this may limit the resistor selection to the customer. For example, customer will have to use exact 5.76kohm for 606kHz switching frequency. Customer can't even use 5.7kohm or 5.9kohm? 

  • 0 in reply to PY (Pengyu) Song
    TI__Genius 11836 points

    Hi Pengyu,

    Please refer to my comments in red text.

    So are you suggesting that the boost frequency can only be guaranteed for resistance values listed in Table 2 with tolerance of 1% or less. For resistors with tolerance higher than 1% or resistance values deviate from that listed in Table 2, the switching frequency will not be guaranteed - the resulting frequency will be one of the two nearby frequency values. Please confirm if the understanding is correct. 

    Correct.

    In fact, I would expect less stringent requirement resistor values as this may limit the resistor selection to the customer. For example, customer will have to use exact 5.76kohm for 606kHz switching frequency. Customer can't even use 5.7kohm or 5.9kohm? 

    For resistors with tolerance higher than 1% or resistance values deviate from that listed in Table 2, the switching frequency will not be guaranteed under all conditions in enormous quantities.

  • 0 in reply to Charles-Chen
    TI__Genius 11836 points

    Hi Pengyu,

    Do you have any more questions?