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TPS54160: Switching frequency decreases in some operating conditions

Shawn_Wang
Expert 3811 points
Part Number: TPS54160
Other Parts Discussed in Thread: PMLKBUCKEVM

Hi, team

When I measured the PMLKBUCKEVM (based on TPS54160), the switching frequency decreases in some operating conditions, which prevent me from collecting the experimental data at certain operating conditions. 

To be more specific, when the input voltage is fixed, the switching frequency decreases if the load current is higher than certain values. For example, reduced from 500kHz to 240kHz, and then decrease to 120kHz. Noted that if the load current is fixed, this phenomenon will also occur when the input voltage increases.

Detailed information is described below.

The schematic given by TI Power Management Lab Kit Buck Experiment Book is shown as below.

Input voltage=6V

Load current=1A

J17 shorted → C16 (220uF) output capacitor connected.

J13-J15 shorted → L2 (18uH, ferrite) inductor connected

J22 shorted→ switching frequency fs = 500kHz

Purple curve: Inductor current ripple.

Increase the input voltage to 10V, the switching frequency is still 500kHz.

Continue to increase the input voltage to 14V. At this time, the switching frequency is not PWM control.

Continue to increase the input voltage to 18V. At this time, the switching frequency decreased to 240kHz.

Finally, when the input voltage is 30V, the switching frequency decreased to 120kHz.

Noted that if the output capacitor is switched to the ceramic capacitor, this still occurred again. I don't know whether this is a kind of normal behavior or just something went wrong.

Some experiments need the measurement data, which is similar to the experimental data collection table below. This made me feel troubled because it seems that when the input voltage is fixed, the switching frequency is always a certain value if the load current increases.

---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Besides, In experiment 4 - test#2, according to the description, I need to measure the inductor current ripple when the load current is 2.0A. However, according to the TPS54160 datasheet, the maximum output current is 1.5A. I don't understand why.

Thanks for your patience and look forward to your reply.

  • 0
    TI__Mastermind 34790 points

    If you would like the device to operate across fixed frequency, you can sync to external clock on RT/CLK pin.

    The device will foldback in frequency at light load to boost efficiency, at heavy load (above 1.5A) to avoid current runany, and possibliy at high VIN to avoid min ton violation.

  • 0 in reply to Marshall_Beck
    TI__Expert 3811 points

    Marshall

    Thanks for your reply.

    When the load is low or the input voltage is high, the switching frequency set by the external resistor is reduced. This solved my question.

    In this case, I don't know why need to measure the data when the switching frequency has changed.  You can see the penultimate figure (Table1) in my post. When the input voltage is 36V and the load current is 0.15A and J22 is shorted(switching frequency is set as 250kHz), in this condition, the switching frequency is still labeled as 250kHz. Actually, at this point, the switching frequency has fallen to half of the original.

    Besides, In experiment 4 - test#2, according to the description, one needs to measure the inductor current ripple when the load current is 2.0A. However, according to the TPS54160 datasheet, the maximum output current is 1.5A. You can see the last figure (Table2 and Table3) in my post.

    It seems that the experimental conditions required in the manual are inconsistent with the characteristics of the actual device.

  • 0 in reply to Shawn_Wang
    TI__Mastermind 34790 points

    It sounds like you found some possible errors in the learning manual.

    The datasheet should be number 1 resource for understanding device functionality.