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LMZ35003: Cin value for the LMZ35003

Izumi Maruyama
Intellectual 1350 points
Part Number: LMZ35003

LMZ35003 datasheet says below:

The LMZ35003 requires a minimum input capacitance of 4.4 μF of ceramic type. The voltage rating of input capacitors must be greater than the maximum input voltage. The ripple current rating of the capacitor must be at least 450 mArms.

 

However, reentry large size as 3216 (50V) is not easy to buy and in the future it has a possibility to be discontinue such size. So my customer is now considering to use 1uF of 1608 size (50V) or 0.47uF of 1608 size (50V). If use such small capacitor what problem could be occurred? Also would like to know what point need to check carefully during evaluation. The customer check the behavior with use 4.4uF and 1uF by the WEBENCH but the result shows no big difference.

The target circuit is:

Input voltage: 37V

Output voltage: 15V

Load current: 2A

 

Please let me know the expert’s comment.

 

Best regards,

Izumi Maruyama

  • 0
    TI__Guru 50120 points

    Hi Izumi Maruyama,

    The reason why a size of 3216 is recommended for the input capacitor is because of the effect of capacitance derating under high DC bias. If you are planning on using the maximum voltage range of the LMZ35003, which is 50V, the voltage rating for the input capacitors are generally twice that (100V rated) by design.

    The smaller the case size, the more likely the capacitors will derate under high input voltage and the effective capacitance will not match the typical expected capacitance. If using 3216 sized capacitors isn't an option, I'd recommend having multiple smaller sized capacitors in parallel that will add up to the effective 4.4uF after derating. For more information on how the capacitors derate with input voltage, you can check out the Simsurfing simulation tool.

    Some things to check for is that the input ceramic capacitors are placed close to the input of regulator to maximize the reduction of ripple voltage amplitude. A generally rule of thumb is to keep the peak to peak ripple amplitude below 75mV. 

    Regards,

    Jimmy Hua

  • 0 in reply to Jimmy Hua
    TI__Intellectual 1350 points
    Jimmy-san,

    Thanks for your feedback.

    Based on your feedback, I have talked with the customer then they understood all.

    Best regards,
    Izumi Maruyama
  • 0
    TI__Mastermind 28020 points

    Hi Izumi,

    The input capacitors recommended in table 3 of the datasheet are 4.7uF, 50V, 1206 package. I would recommend using these capacitors or 2x 2.2uF, 50V in parallel.