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Load change rate and frequency of load change in SLVA381B

Aneesh TS
Expert 1735 points
Other Parts Discussed in Thread: TPS5430

Dear friends,

I have just read SLVA381B document from TI. This document explains the effect of Phase margin in Load transient. Finally in the document Load-Step Response of TPS5430 is given in Figure 8.

I have some doubts regarding this

1.  What is the Load change rate(in A/uS) for the TPS5430 example?

2.  What is the frequency of Load change in TPS5430 example?

3. What  percentage of switching frequency need to keep as  the frequency frequency of Load change, when we are co-relating the load transient waveform with phase margin?

for example if my converter switching frequencies are 100KHz, 500KHz and 1MHz what should be the respective frequency of Load change while doing the load transient test?

Regards

Aneesh

  • 0
    TI__Guru**** 191445 points

    Aneesh,

    That application report is 10 years old now, so the details may be from memory:

    1.  Figure 8 states 50% load step at 1 A / 10 usec.  TPS5430 is a 3 A device so the load step was 1.5 A.  1A/ 10 usec translates to100 mA / usec.

    2.  The frequency of the load change is not recorded.  During that time frame we typically set the dynamic load to 50 % duty cycle and 300 Hz or so.  The frequency is not particularly important for this measurement.  In fact, you could measure it as a single pulse.  The important part is to keep the load step time longer than the recovery time, which is around 100 usec in the figure 8 example.  You just need to see the load step voltage response completely dampen out.

    3.  See 2 above.

  • 0 in reply to JohnTucker
    Expert 1735 points
    Dear John Tucker,

    Thanks for sharing the information. I have some more query regarding this

    Currently the recovery time is 100uS in TPS5430 fig-8 example.

    1. what will be the maximum load switching frequency we can go without affecting the measurements?

    2. Will i get the same recovery time with 300Hz, 600Hz and 1200Hz?

    3. switching frequency of TPS5430 is 500KHz. Switching period is 2uS. Recovery time is 100uS . Which is 50 times the switching period.

    What will be the typical Recovery time(How much times of a Switching period) for the Power supply ?

    Regards
    Aneesh
  • 0 in reply to Aneesh TS
    TI__Guru**** 191445 points
    Aneesh,
    1. Assuming 50% duty cycle and 100 uS settling time, the maximum load switching frequency, f = ( 2 * 100E-6) = 5 kHz. You can add some margin if you like and use 1 / (2 * 150E-6) =3.33 kHz.
    2. You should see the same reovery time It is independent of the load switching frequency as long as the period is sufficient for the supply to recover from both the low to high and high to low transition. It is very likely that you will see different recovery profiles for load step and load dump. The recovery times should be of similar but not identical duration. You will have to adjust the load switch frequency accordingly. For the application circuit of the app note 300 Hz, 600 Hz and 1200 Hz are all less than the maximum frequency so you should see no difference.
    3. Depends on lots of factors, control topology, output filter components and loop compensation.
  • 0 in reply to JohnTucker
    Expert 1735 points
    Dear John Tucker,

    Thanks for the feedback.

    Regards
    Aneesh