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.

  • Resolved

UC1845: how to pick RT/CT values

Prodigy 30 points

Replies: 4

Views: 2158

Part Number: UC1845

Hi,

The UC1845 datasheet section 8.3.1.4 RT/CT provides a recommended range for timing resistor R_RT (5Kohm-100Kohm) and timing capacitor C_CT (1nF-100nF), so does it mean the recommended maximum oscillating frequency f_OSC is 1.72/(5Kohm*1nF) = 344KHz? If I want to obtain higher frequency, say 450KHz, is it recommended to keep 5Kohm RT and reduce CT, or keep 1nF CT and reduce RT? Thank you.

  • Hey Yue,

    The numbers you talk about are just recommendations, if there is a need to go to a higher switching frequency you can certainly try different values.

    My suggestion would be trying to go lower with resistance rather than capacitance.

    Thanks,

    Daniel

  • In reply to Daniel Hartung:

    Hi Daniel,

    Thanks for the reply. I was trying to lower the capacitance value because I saw that in the Pin Functions Table, it says "Never use a timing resistor less than 5Kohm". (datasheet Pin Functions Table RT/CT) So I guess that's an absolute constraint on the resistor RT. Is that true?

    Thanks,
    Yue
  • In reply to Yue Wei91:

    Hey Yue,

    I believe that the comment in the pin functions table is simply referring to the 5k recommendation, but you can always be safe and use capacitance lower for now.

    I believe the real issue lies in the fact the clock ramp signal will become somewhat non-linear due to being an RC curve with really small values used. This may cause problems the clock changing over temperature, but that is something that would need to be tested in circuit.

    Thanks,
    Daniel
  • In reply to Daniel Hartung:

    Okay, Thank you.

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.