• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 62 subscribers
  • Views 150 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS65262-1: enable LDO with buck output

Galen Seitz
Intellectual 475 points
Part Number: TPS65262-1

Hi,

I have a TPS65262-1 regulator configured to output 3.3V on BUCK3. LVIN1 and LVIN2 are connected to the output of BUCK3 (3.3V). LDO1 is configured to output 1.2V, and LDO2 is configured to output 1.66V. I would like to enable LDO1 and LDO2 with the BUCK3 output. Can I just tie the output of BUCK3 to LEN1 and LEN2? Paragraph 2 of section 9.3.2 of the datasheet says that the enable pins should be driven using open-drain logic. Is this required? What is the downside of driving the enable pins with a push-pull output? I.e. what is the downside of tying LENx to 3.3V?

How do you suggest that I enable the LDO regulators with the output of one of the buck regulators?

Thanks.

  • 0
    TI__Genius 17390 points

    Hi Galen,

    Thanks for reaching out. It's not recommended. Please kindly follow datasheet and using open-drain logic or open-collector logic to interface with the pin to control the EN pin.

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Intellectual 475 points

    That really doesn't answer the why or how. I noticed that in the schematic (SLVUA28) for the TPS65262EVM, they use a resistor divider (R19 and R20) off of VIN as one of the potential options for driving the enable inputs. This suggests that it is possible to use something other than an open-drain type of drive.

    Regardless of the above, I have reevaluated our needs, and I have decided that since our 3.3V supply is always enabled (EN3 floating), I can have our LDO outputs always enabled as well. I'll just leave LEN1 and LEN2 floating.

  • +1 in reply to Galen Seitz
    TI__Genius 17390 points

    Hi Galen,

    Let me clarify it.

    (1) Could you please let us know do you have sequence requirement for BUCK3 and LDO1 or LDO2?

    (2) As you mentioned, LEN1 and LEN2 floating to enable. If no sequence requirement, you can float it and LDO output will be enabled.

    (3) The reason that I don't recommend to tie Vout3 directly to LEN1/LEN2 is shown below: Suppose such a situation, VIN for Buck1 to Buck3 is ready. EN3 is pulled low and disabled this time. The EN pin has a small pull-up current Ip which sets the default state of the pin to enable when no external components are connected which means that there exist EN pull up current. If you tie VOUT3 to LEN1, since VOUT3 also feedback resistor connected this time, the pull up current will charge the resistor and the output capacitor this time. It means that even though Buck3 is not enabled, LDO is enabled this time since LEN1 voltage will rise over it's threshold. If you need a sequence that LDO power on after Buck3, it can't be achieved this time. 

    Hope it will answer your question. Let us know any of your more concern. Thanks!

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Intellectual 475 points

    (1) LDO1 and LDO2 should come up when BUCK3 comes up, but that is guaranteed by the fact that BUCK3 is the input to LDO1 and LDO2.

    (2) Yes, I will just float LEN2 and LEN2.

    (3) Thanks for the explanation. Given the size of the feedback resistors, it seems unlikely to me that this would ever occur in a typical design. For example, the highest combination of suggested feedback resistors in Table 1 is 73K + 10K (5V output). It would take 14 uA of pullup current to reach the 1.2V threshold, and that's without any actual load on the output. Granted, the datasheet doesn't spec a worst case number for the pullup current, only a typical number of 3.6 uA. It seems the supply would have to be very lightly loaded, and the feedback network would have to use high resistor values in order for this to be a problem.

    Regardless, my questions are answered. Thanks for your help.