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WEBENCH® Tools/LM5122ZA: LM5122ZA frequency setting

Nick Hsu1
Prodigy 20 points
Part Number: LM5122ZA
Other Parts Discussed in Thread: LM5122

Tool/software: WEBENCH® Design Tools

Hi,

LM5122ZA Feature shows frequency up to 1 MHz, why I cant simulate over 352.94 1KHz?

So I setting Rt as 18 KOhm, the frequency is 500 KHz, but it shows an alert

M1 Tj:341.27 °C Specification: <150.00 °C

M1 MOSFET junction temperature is too high

  • Suggested:<150.00 °C

  • This may lead to a device malfunction or breakdown.

  • Some possible solutions are below:

  • Decrease Switching Frequency to decrease MOSFET or Diode switching losses

  • Decrease ambient temperature

  • Decrease thermal resistance by adding a heat sink if feasible

  • Increase the copper area and thickness of the board

I'd change many MOSFET with low Ciss/Coss, low Qrr.

It still shows this alert. I have size issue, can't design the frequency under 500 KHz.

Here is my setting specification as below:

Vin=19V, Vout=60V, Iout=1.6A (Pin = 100W)

Frequency = 500 to 800 KHz

One more question about current limit

I want set Ipeak=7A, according to section 7.3.10, Rs=10 mOhm

 According to section 8.1.4, if I want to use Rdcr of inductor, how to calculating 7A limit?

  • 0
    TI__Mastermind 31015 points

    Hi Nick,

    Sorry to hear you are having troubles. I can get back to you by next week since Friday is US holiday.

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    TI__Mastermind 31015 points

    Hi Nick,

    To answer each of your questions:

    1) I'm able to build the circuit using the LM5122 model. What you should do is follow the attachment below:

    I believe there were some initial WeBench issues that may have prevented you from accessing this, but it seems to work for 500KHz. Additionally, if you are having more trouble getting WeBench to work, I would recommend using the LM5122 quick-start calculator here: 

    http://www.ti.com/product/LM5122ZA/toolssoftware

    2) If you want to implement DCR sensing, your peak current limit is ideally based on the inductor's dc resistance. The resistor and capacitor that you see in that scheme is to make sure that the time constants of the LR network and the RC network match. 

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 20 points

    Hi Richard,

    Thanks for reply !

    For the (2) question, for example

    According to section 8.1.4 of datasheet, I can calculate a value for LR and RC matching.

    But how could I know about my current limiting value?

    If my Iin_max = 5A, I want to set the current limiting at 7A. 

    The only is changing my inductor's DCR to 10 mOhm? 

    If my inductor's DCR is 20 mOhm, is there anyway to set the current limiting to 7A without a external sense resistor ?

    Thank you. 

  • 0 in reply to Nick Hsu1
    TI__Mastermind 31015 points

    Hi Nick,

    The peak inductor current is determined as the sum of your average input current (worse-case) and half of your inductor peak-peak ripple. Your current limit value is determined by the DCR resistance, so if you wanted to set an exact current limit, you would need to change the inductor's resistance to match.

    In fact, a better solution is to select an inductor with a DCR such that your IL_pk*DCR does not trip current limit. Additionally, you could add a resistor in parallel to C_dcr to increase your current limit, but this may require more calculations and an extra component. This second method can only increase your current limit. 

    Thanks,

    Richard