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TPS63031: Power supply circuit review

Christian Steiniche
Intellectual 565 points
Part Number: TPS63031

Hi

We have for some time been using the LM53601 step down converter (5V out) with a TPS73633 drop-down regulator (3.3V). In between the 5V and the 3.3V supplies we charge a super capacitor which can provide backup supply to safely shut down the load circuitry when the external supply is disconnected. The disadvantage of this method is that the 3.3V output can only be maintained as long as the super capacitor voltage is above 3.6V (the minimum input voltage to the LDO).

We would like to upgrade the circuit to use a buck-boost-converter as illustrated below. We are currently looking the TPS63031 as a potential candidate. The advantage of this is (hopefully) that more of the capacitor energy can be utilized during shutdown.

A simple schematic of the setup is given below:

(We do not want to charge the super cap if only the +5V supply is active. If the +12V is connected, the super capacitor shall be charged to secure some backup-energy.)


In normal operation, the load is supplied directly from either the +5V and/or +12V external supplies. We want the normal operation scenario to provide a stable 3.3V output to the load. In this scenario, the 3.3V converter will operate with 5V in and 3.3V out.

When the external supplies are disconnected, the C1 super cap will deliver energy to the 3.3 V converter. Initially, the input voltage will be above 3.3 V and after some time below. According to the datasheet, TPS63031 will be able to deliver a 3.3V output until the input voltage has dropped to approx. 1.8V.


R1 is used to limit the inrush current into C1 (0.3-1.0 ohm ESR).

Questions

  1. Do you see any obvious problems with suggested circuit?
  2. Do we need D3 to prevent C1 to discharged backwards through the circuit when the LM53601L input drops to zero?
  3. The LM53601L is currently driven by the internal clock. Do you recommend synchronizing the clocks of the two converters to reduce EMI/EMC?
    1. If so, can you recommend a way of doing this?
  4. In your opinion, will the additional switching power IC drastically worsen the EMI / EMC emissions compared to using LM53601L with the current LDO?
  5. The +5V line is USB VBUS. Would you be worried about EMI from the buck-boost conterver back into the USB supply?
    1. If so, can you recommend one of the TI chips for reducing EMI for this purpose?

We highly appriciate any help you can provide.

Thanks.

Br

Christian

  • 0
    Intellectual 565 points

    Hi all


    I continue to be very interested in any help on above.


    Thanks.

    Br
    Christian

  • 0
    TI__Expert 5425 points
    I think the block diagram basically looks ok.

    As I understand your description you just replaced the LDO with the TPS63031, so regarding D3 and the operation of LM53601 there should not be a difference. I would recommend to use a standard silicon rectifier diode instead of the schottky diode shown in the block diagram if reverse leakage current is critical.

    I do not see a need for synchronizing the converters because of EMI. The filter you most likely use at the 12V input should be good enough for both converters. you may need more filtering at the 5V input since you changed the circuit supplied from the 5V rail from a linear regulator to a switchmode converter. What exactly is required depends on your circuit implementation and the EMI compliance requirements for your circuit.
  • 0 in reply to Juergen
    Intellectual 565 points

    Hi Juergen,

    thanks for the response!

    Input filter:

    The input filter is designed to attenuate the switch frequency of the converters (inspired by ti webench). Some material (on google, forums etc) mention that the "beating phenomenon" between multiple asynchronous power converters can create low frequency noise which is difficult to filter.

    In case it turns out to be a problem, I have added to possibility to synchronize the converters using an output from a MCU on the board. When the system boots, the converters will use their internal clocks. When the MCU is powered, it will enable the sync signal (2.1 MHz) to the two converters. The signals are phase shifted 180deg.

    5V EMI:

    It seems that TI offers a range of filters to be used specifically for USB applications. Can you recommend a partNo. which could be used as filter in this specific application.

    I need the EMI levels to be CE / FCC compliant.


    I appreciate the help!

    Thanks
    Christian