• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 23 subscribers
  • Views 476 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

Inquiry regarding ECO-mode on TPS54540-Q1

Atsushi Okui
Genius 4015 points
Other Parts Discussed in Thread: TPS54540-Q1

Hello, all

Now we are considering to use TPS54540-Q1 on emergency vehicle application, then would like to know the method not to enter ECO-mode when the load is light.

The actual usage for this device is as following;

Input Voltage: 9.5 to 30V

Output Voltage: 8V@ 3A(max)

Switching frequency: more than 1.8MHz

Since this deice is intended to supply into LED which has 50msec ON/OFF duty, the output current from this device is vary from less than 100mA to 3A.

Meanwhile, we also need to take in consideration to fix the switching frequency of this device on the term when LED is OFF, on which supplied current is less than 100mA.

We understand that output inductance could be increased in order not to ECO-mode, which causes variation on the switching frequency.

Please let us know whether our understandings are correct.

We thank you in advance for your information. 

Best regards,

  • 0
    TI__Guru**** 191445 points

    It looks like the TPS54540-Q1 uses the same scheme as other devices in that family.  Eco-mode is entered when the COMP pin voltage falls to teh minimum clamp voltage, about 600 mV.  The output peak switch current is proportional to teh COMP pin voltage, but it is not a well controlled relationship.  In a closed loop system, the absolute value of the intermediate control voltage (COMP) is not critical so long as it remains within the upper and lower clamp voltages for the required operating current range.  Also there is quit a bit of tolerance in the other intermediate transconductance amplifiers between COMP and the actual current comparator at the output.  So it is generally not possible to predict the eco-mode threshold wit respect to output load  current or peak switch current.  It could be a 10 mA or 300 mA.  During eco-mode operation, the TPS54540-Q1 will decrease it's effective switching frequency.  If constant switching frequency is required, then you probably want to use a device that does not have pulse skipping eco-mode.

  • 0 in reply to JohnTucker
    Genius 4015 points

    Hello, thank you for your reply.

    Please let me clarify whether we could use enclosed external switching circuit on TPS54540-Q1, since we would like to have 2 type of RT value which is selectable from CPU.

    0143.frequency switching circuit.pdf

    Our concerning is whether output voltage may be effected when switching frequency by transistor during operation.

    We thank you once again for your information.

    Best regards,

  • 0 in reply to Atsushi Okui
    TI__Guru**** 191445 points

    In general that may work.  Here are the potential issues:

    1.  The IC sets the internal frequency by means of a PLL.  A voltage is st at the RT/CLK pin and the current sourced out of the pin sets the frequency.  Any leakage in one RT circuit may affect the frequency of the other.

    2. Any on resistance of the transistor switch will affect the frequency.  There will only be a few uA in the RT resistor.

    3.  The RT/CLK pin is noise sensitive.  It works best when RT resistor is connected directly between RT/CLK and GND with very short traces.

    As to your main question, there will be voltage change during the transition time.  It will be similar to a load step transient.  The output voltage will settle back to the regulated value as the control loop adjusts.

    Also you should note that the inductor ripple current will change proportional to the frequency change.