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.

UCC28811: What is UCC28811 maximum switching frequency?

Part Number: UCC28811
Other Parts Discussed in Thread: UCC28050

 Hello guys,

 One of our customers is using UCC28811 for many their products. The customer wants to know what the maximum switching frequency of the device. I think UCC28811 switching frequency gets higher when the output current is very low and VINS voltage is low.

 Could you please tell me what UCC28811 maximum frequency capability is?

 Your reply would be much appreciated.

 Best regards,

 Kazuya Nakai.

  • Hi Kazuya

    Thanks for reaching out. The maximum switching frequency is 250kHz.

    Best Regards,
    Ben Lough
  • Hi Ben,

    Thank you for the prompt reply.

    Could I ask you a few additional questions?
    The customer uses UCC28811 as a constant current voltage power supply. Their application is LED ceiling light and downlight. They connected buck type DC/DC switching LED driver to the output of power supply which uses UCC28811.

    Q1. When LED current is very low, GDRV pulse width of UCC28811 becomes very narrow (The pulse period is about 4us). Also The pulse width is about 50ns~100ns.
    Can UCC28811 generate such narrow pulse?
    (I understand that GDRV rising is generated by TZE falling and GDRV falling is generated by ISENSE terminal voltage rising.)
    Q2. We can see R-S flipflop in block diagram. What is the output Q level when R=S= both 1(high level)? R input is higher priority?

    Thank you very much again and best regards,
    Kazuya Nakai.
  • Hi Kazuya,

    In normal operation, the output of the voltage error amplifier is multiplied with the sensed line voltage and the resulting signal is compared to a level shifted ISNS to determine GDRV falling. At very light load, I would expect the controller to be operating in zero power mode where the gate drive is off the majority of the time and I would expect the pulse width to be narrow.

    I would expect the output to be 0 in the theoretical case where R and S are both 1.

    Best Regards,
    Ben Lough
  • Hi Ben,
    I confirmed that EAOUT voltage was higher than 2.3V. So UCC28811 is not in the zero power mode because Zero Energy Detect comparator output is . I want to know why such very narrow pulse width (50ns~100ns) is output with each 3us~4us priod?

    Could you give me any advice?

    Thank you and best regards,
    Kazuya Nakai.
  • Hi Kazuya,

    Take a look at VINS, EAOUT and ISNS on a scope. You can calculate the output of the current reference generator to be =0.67×VINS×(VEAOUT–2.5V). Compare this to VISENSE+75mV. This will tell you if the gate drive is being terminated by ISENSE.

    Best Regards,
    Ben Lough
  • Hi Ben,

    Thank you very much for your advice. Each signal voltages on the customer board were the follows.
    EAOUT=2.7V. VINS=0.47V.
    So compared VISENSE is 0.67*0.47V*(2.75V-2.5V)-75mV=3mV. Is it too small?

    Can I ask you a few questions?
    Q1. You said the maximum frequency is 250kHz. If GDRV switching frequency on the customer board is more than 250kHz, what is happen?
    Q2. If GDRV switching frequency on the customer board is more than 250kHz, must the frequency be reduced? In that case, could you tell me how to reduce it? Is it by inductor value increasing? (The customer inductor value is about 0.4uH.)
    Q3. Is Figure 7, VINS – Current Reference Generator Input Voltage – V graph correct? I think it has some error compare with the equation you told. Also I think UCC28811 is a family device with UCC28050 series. I found Figure7 in UCC28811 datasheet is same as graph, Figure 5 in UCC28050 datasheet though UCC28811 has 75mV offset on VISENSE and UCC28050 has no offset.

    Thank you again and best regards,
    Kazuya Nakai.
  • Kazuya,

    This would make the on time of the FET very narrow. Whether this is a problem or not will depend on your power stage. I wouldn't expect any damage to the converter in the theoretical case where the switching frequency is higher than 250kHz but your efficiency will suffer due to higher switching losses. You can increase the magnetizing inductance to lower the switching frequency.

    The equation I provided is an approximation for the output of the current reference generator (you can find this expression on page 8 of the UCC28811 datasheet). Figure 7 is measured data.

    UCC28811 and UCC28050 are similar devices that use a similar IC architecture to perform power factor correction.

    Best Regards,
    Ben Lough
  • Hello Ben,
    Thank you very much for your kind reply. I undersstood what you said.
    Can I ask you one additional question?
    About 250kHz maximum frequency, I think that this is not the absolute maximum value and it is a recommended condition.
    Is my understanding correct?

    Thank you agai and best regards,
    Kazuya Nakai.
  • Hi Kazuya,

    Your understanding is correct.

    Best Regards,
    Ben Lough
  • Hi Ben,

    Thank you for your reply!

    Best regards,
    Kazuya Nakai.