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Original question:

LM2576: Inductor selection

LM2576: Design Clarification & Components Ranges

M SANDEEP
Prodigy 160 points
Part Number: LM2576

Hai Sir,

Based on Customer recommendation we are using LM2576-5.5 for voltage conversion.

End application- Controlling & Analog outputs driving.

Can you please give me the clarification about my queries posted below...

1) Designing Vin= 12V to 36V and Vout= 5V, Iout= 1.5A to 2A (Step down conversion)

 -> As per the Datasheet reference 8.2.1 Fixed Output Voltage Version , page no.19. Downloaded the Pspice model from TI, Simulated and Verified the Results as per the design.

-> Here external components used Cin- 100uF, Cout- 1000uF . Can we use these capacitor values with Ceramic capacitors and is there any extra smaller capacitors required with these bulk values.

-> Inductor used L-100uH, can we use this value for Iout-1.5A to 2A ?

-> Is it required heat sink for this device with Vin-12-36V, Vout- 5V @ 1.5A, Ambient temp- 50°C

-> Please clarify above mentioned queries

  • 0
    TI__Mastermind 25200 points

    Hi M.

    Thanks for considering using the LM2576 in your design. Usually for application recommendations, we recommend input and output capacitors to be aluminum electrolytic. This is because ceramic capacitor can derate quickly depending on DC voltage.  

    Regarding the external component values, can you clarify that Cout should be 1000uF, and not 100uF as shown in the schematic? Otherwise, it looks fine. Please make sure that the input and output capacitors are rated properly for maximum voltage conditions.

    For the inductor, based off of the application section, you may need to update your inductor selection to be H150 (or 150uH) inductor instead of the 100uH you have. 

    Based off of your power dissipation calculated in section 10.4 of the datasheet, it does not seem like you will need a heatsink. 

    Pd = (Vin*Iq)+(Vo/Vin)*(Iout)*(Vsat) = (36V*5mA)+ (5V/36V)*(2A)*(2V) = 0.735W

    Ta ~= (0.735W)*(42.6C/W) + 50C = 81.3C, which is within range. 

    Regards,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 160 points

    Thank you for giving m the reply,

    Richard Fu22 said:
    Regarding the external component values, can you clarify that Cout should be 1000uF, and not 100uF as shown in the schematic?

    Yes sir...it is 1000uF. And is it single electrolytic capacitor is enough for output side ? or can we place smaller ceramic capacitor also in parallel with electrolytic? pls clarify

    Richard Fu22 said:
    For the inductor, based off of the application section, you may need to update your inductor selection to be H150 (or 150uH) inductor instead of the 100uH you have. 

    Ok sir.. And what is the output current range with this 150uH inductor?

  • 0 in reply to M SANDEEP
    TI__Mastermind 25200 points

    Hi M.

    It would be preferable to have some smaller ceramic capacitor in parallel with electrolytic capacitors. You may also consider adding some smaller ceramic capacitors in parallel with the input capacitor if operating in cold temperature.

    For the inductor, I recommend your inductor have a saturation current rated to handle your maximum peak current with additional 20% margin.  

    Regards,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 160 points

    Ok sir,

    Thank you sir....