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Original question:

TPS63020: Prevent 3.8V GSM Voltage dropping

TPS63020: Compensation loop for improved load transient performance

yancula overlord
Prodigy 50 points
Part Number: TPS63020

Hi .

According to the datasheet for TPS63020 I place R-C compensation loop to improve load transient performance (R115-C121 shown on schematic GSM-Power_Supply.jpg). Is there any possibility to get voltage spikes on Vout greater than defined Vout=3.8V, because modem fail after normal operation and it has been established that pin-32 and pin-33 (baseband domain) are shorted to GND on BG96 module. Absolute maximum voltage ratings for BB supply and RF Supply of the module are 4.7V. VGSM-Power Supply is switched ON/OFF via Load Switch U107 shown on schematic Dev-Power_Supply.

Thanks in advance.Dev-Power_Supply.pdf

  • 0
    TI__Guru 65340 points

    Hello Yancula,

    TPS63020 in your system is creating a nominal output voltage of 3.839V. If you are using output resistors with 1% accuracy, this average value is changing around 1.5%. The feedback voltage is varying by 1%, so there will be an inaccuracy of roughly 2% on your output voltage (for details on accuracy statistics, please have a look at this app note). So the highest DC value you might see in your application is around 3.93V.

    During a load transient (load reduces suddenly) the output voltage of a DC/DC converter is overshooting somewhat because there is energy stored in the inductor and even if the converter would stop switching immediately the remaining energy in the inductor will charge the output cap. This means that the voltage will go higher than the nominal 3.84V you set.

    As this is a dynamic process, it is heavily dependent on the specific load transient. If you know the worst case transient (maximum load to minimum load and transient slope), we can check in the lab on an EVM how the transient looks like. Maybe best would be if you have a look yourself in your system for this worst case transient. Transients on the output of the Load Switch U107 will in addition look different than on the output of the TPS63020. The load switch has impedance and therefore a load step on your module will get to the TPS63020 with some time delay. What exactly is happening on the output of the U107 during such a step needs to be measured with an oscilloscope.

  • 0 in reply to Brigitte
    Prodigy 50 points

    Thanks Brigitte.

    What about the output overvoltage protection, is this refers to the defined output voltage (in this case Vout=3.8V) or output overvoltage protection is implemented to avoid exceeding critical output values  (5.5V-min and 7V-max, as shown in table for Electrical characteristics for TPS63020)?

    Respectfully,

    Yancula.

  • 0 in reply to yancula overlord
    TI__Guru 65340 points

    Hello Yancula,

    The output voltage protection is making sure that the device is not destroyed, if the feedback divider is not properly connected to the FB pin. E.g. if the top resistor is open, the converter always sees GND connected to FB and therefore is increasing the duty cycle. At maximum 7V (min. 5.5V), the device stops switching and makes sure that the voltage does not go higher.

  • 0 in reply to Brigitte
    Prodigy 50 points

    Hi Brigitte.

    Can you check in the lab how the transient will look like if the worst case transient is from 1.6A load to 100mA load? If the spikes are larger than 4.7V should I use additional protection component (for example, a zener diode) оr protection circuit to avoid damaging of the modem?

    Thanks in Advance,

    Yancula.

  • 0 in reply to yancula overlord
    TI__Guru 65340 points
    Hello Yancula,

    1. I think it is important that you check the input voltage directly at the device that is damaged during a load transient. The additional isolation switch at the output will change the behavior in your system.

    2. The rise and fall time of this transient influences the transient response, as well the specific capacitors (especially DC bias) you are using in your system, so it would be best to do these tests on your system.

    3. I am sorry, but we are approaching season holidays and there will be no one available in the lab until Jan. 7th.
  • 0 in reply to Brigitte
    Prodigy 50 points

    Hi Brigitte.

    Are the values of R-C compensation loop correct for configuration on schematics? If I change value for L102 to 1.5uH  should I change values of R-C compensation loop?

    Thanks,

    Yancula.

  • 0 in reply to yancula overlord
    TI__Guru 65340 points

    Hello Yancula,

    For every circuit change, stability has to be tested again. Please have a look at this app note for a fast stability check:

    Simplifying Stability Checks

    For these feedforward components, I wonder if you need them at all. In general I recommend to use them only if the transient response is not fast enough without them.