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UC2845: Missing data in the datasheet

Zhonghui Xu
Expert 5445 points
Part Number: UC2845

Hi team,

I got a question from customer.

"

In the Application Report Loop Response Considerations in Peak Current Mode Buck Converter Design SLVAE09B – JULY 2018 – REVISED AUGUST 2021 and in other publicly available established engineering texts the following is needed to do a full analysis:

1 - Ri; the current sensing resistor or its equivalent value
2 - The transfer function of the error amplifier
3 - The compensation current ramp slope (change in current over time)
Also, in the modulator section of a controller there is at the input to the PWM comparator the valley and the peak of the ramp. The values of this are necessary to properly simulate the circuit. In the UCx84x variants Current-Mode PWM Controllers datasheet (www.ti.com/.../uc2845.pdf in Figure 11 and Figure 12 the almost universal circuit between the E/A and the PWM comparator is used. I suspect the same exist in this part. Can I use the same values for the valley and peak as is used in this diagram? I have figured out the resistor values and diode drops for this circuit by analysis.

Please provide this information. It should be in the datasheet.

"

Thank you very much for your help.

Best regards,

  • 0
    TI__Guru 73915 points

    Hi,

    1. If you mean external current resistor value, then it is designed by you.

    2. The error opamp gain 90dB and bandwidth at unit gain 1MHz are listed in the electrical table on page 7 of the datasheet.

    3. Slope compensation is designed by you to get how much you need. On the implementation, you can refer to section 8.3.9 of the datasheet.

    UCx842/3 is based on Fig 11. UCx844/5 is based on Fig 12. Basically 42/3 is with Dmax 100%, and 44/5 50%. 

  • 0 in reply to Hong Huang
    TI__Guru 73915 points

    Here some additional comment:

    2. Fig 3 shows error opamp gain and phase which provides you with transfer function in Bode Plots