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WEBENCH® Tools/LM43602: LM43602

varaprasad rayudu
Prodigy 90 points
Part Number: LM43602
Other Parts Discussed in Thread: LMX2572

Tool/software: WEBENCH® Design Tools

Hello,

Iam new to Step down DC converters and can anybody can support me to solve the the spurious(refer below snap) generate in the LMX2572 RF output spectrum due to the LM43602 Step-down DC converter.

Iam using two stage step down for generate the required voltage for the LMX2572 PLL chip and initially the input +30V step down to +3.6V by using LM43602 and then it step-down to +3.3V by using TPS7A9101 LDO(refer below schematic).

Below are the requirement.

Input Voltage --> +30V

Required output voltage -->+3.3V/+1.8V

Output current -->0.2A@30V

Iam operating LM43602 part with default switching frequency of 500kHz(RT resistor open) below are the BOM which i had used.

Cin --> 4.7uF X 4 --> 18.8uF(total)

Lin --> 33uH(power inductor)

CBOOT --> 470nF

CFF --> 6.8pF(also tried with higher value of 36pF)

Cout-->22uF+47uF+10uF --> 79uF(total)

Lout --> 33uH(part# IHLP-3232DZ-11)

Css --> open

We had observed the spurious around the 500kHz offset which is same as the switching frequency of LM43602 and if we change the switching frequency by changing the RT res value the the spurious offset also moving as per that.

Could you please suggest optimum BOM values to eliminate the spurious issue and the requirement will be -75dBc with 50MHz span from the carrier of 1550MHz.

Can you suggest any ferried bead need to be used in between LM43602 & TPS7A910 to isolate the signal path and if so please suggest suitable part?

Looking forward your suggestions.

Regards,

RVP

  • 0
    TI__Guru 72465 points

    We have several app notes tthat will be of some help.

    Please see the following:

    AN-643, AN-2162, AN-2155, and SNVA721A

    If you have trouble finding these, let me know and I can get them for you.

  • 0 in reply to Frank De Stasi
    Prodigy 90 points

    Dear Frank,

    Thanks for your suggestions and inputs.

    Could you please suggest optimum switching frequency to be set for LM43602 to get best spurious performance and if possible please share the web-bench file for the reference.

    Is lowest switching frequency is best for low noise levels?

    For designing the input filter, how can we distinguish between Cin and Cd capacitance?

    What will be be least value of Cin cap can be used to calculate the Cd cap?

    Regards,

    RVP

  • 0 in reply to varaprasad rayudu
    TI__Guru 72465 points

    Regarding the best frequency, I think that depends on what band your are interseted in.

    You would set the frequency to be out of you band.

    Regarding the difference between Cf and Cd, Cd will be a bulk cap such as an Al. electro.

    The app note gives formulas for calculating all of the values.