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TPS53355: Requirement for Rmode and Rrf

Part Number: TPS53355

Hello team,

I have a question about TPS53355's setting resistor requirement.

Could you let me know Min/Max requirement for resistor value of Rmode and Rrf?

Datasheet table 7-1 and table 7-3 show only one resistor value for each setting and there is no information of its range.

My customer is currently using multiple resistors combination for making the table value but they would like to configure it with more simple.  

Best regards,

  • Hello,

    I would highly recommend the resistor selections remain within 1% of the values specified in Table 7-1 and Table 7-3. I have summarized the minimum, maximum, and nominal resistances below to be consistent with that recommendation:

    Regards,

    Alec Biesterfeld

  • Hello Alec-san,

    Thank you for your comment.

    Is the +/-1% requirement come from design point of view? or just come from fact characterization is done with the resistor with +/-1% accuracy?

     

    I would like to know what will happen if the resistance is out from the requirement.

    For example, what will happen if we put a 470kohm(1%) resistor as Rmode to configure 5.6ms soft-start?

    It is out of the range so TPS53355 may fail to configure the setting?

    Best regards,

  • The +/- 1% requirement comes from a part characterization point of view where we characterize/validate the device with the +/- 1% requirement on the TPS53355.

    If the resistance is outside of this requirement, TI will typically not guarantee the device will behave as detailed in the datasheet. That said, I would expect a 470kohm resistor as Rmode to configure the device to have a 5.6ms soft-start and operate in FCCM. However, guaranteeing this information would require extensive characterization across a large number of samples (ideally from multiple manufacturing lots). Therefore, I would continue to recommend the +/- 1% requirement if possible.

    Regards,

    Alec Biesterfeld

  • Hello Alec-san,

    Thank you for your comment. I understand the recommended range is based on characterization.

    Is it possible to comment for resistance range requirement in design point of view?

    Best regards,

  • Hello Alec-san,

    If setting value changes analogically along with resistor value, for example, Rmode is 330kohm then soft-start time will be between 2.8ms and 5.6ms, such information is also appreciated.

    Best regards,

  • Taketo-San,

    The mode and frequency are discrete selections that are detected and latched in the part upon startup. This means that the mode/switching frequency selections do not have any linear/analog dependence on the resistor selections. As the part starts up, an internal current source will apply a current on each of these pins and the subsequent voltage is read by the part to program its switching frequency and mode selections. The the threshold voltages at which the part differentiates these settings may differ as a result of process, variation, and temperature. During final test and part characterization, TI will ensure that the +/- 1% specification will program the settings listed in tables 7-1 and 7-3. If you choose a resistor outside that range, it can work, but the part may also select an unintended setting.

    For example, if a 100kohm resistor Rrf resistor is connected to ground, the part will likely determine that the switching frequency should be set to 300kHz. However, depending on variation between parts, manufacturing lots, and such, it is also possible that the part registers this setting as 250kHz if the threshold has shifted or outside the tolerance of an internal comparator. This is why I highly recommend abiding by the +/- 1% specification as this has been carefully characterized and is consistent with the recommendation from an IC designer's point of view as well.

    Regards,

    Alec Biesterfeld