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LP2985: check the Cin , Cout value for LP2985IM5X-5.0/NO with specific condition

Masaharu Takahashi
Expert 4170 points
Part Number: LP2985

Hi, Team.

this is MASA.

could you check below 2 Caps (Cin, Cout) for LP2985IM5X-5.0/NO with below condition whether it is ok or not?

Cin : GRM155R61E105KA12 (murata 1005 size 25V 1.0uF)

Cout: GRM155R61A225KE01 (murata 1005 size 10V 2.2uF)

Vin:6～(10V)

Vout:5V typ

*I've asked same question as offline though, if you could answer here.

I'll share it to customer by showing this page.

  • 0
    TI__Guru 55940 points

    Hi Masa-san,

    LP2985-N requires 1 uF or greater capacitance on the input and 2.2 uF capacitance on the output. Your selected capacitors do not account for any derating. The following Blog goes into more detail on the difference between capacitor and capacitance:

    e2e.ti.com/.../ldo-basics-capacitor-vs-capacitance

    We generally recommend applying 50% derating to the nominal capacitance in the design. As such we would recommend a 2.2 uF input capacitor and a 4.7 uF output capacitor. Murata also has many tools on their website to help customers better understand how their specific part number will derate.

    Very Respectfully,
    Ryan

  • 0 in reply to Ryan Eslinger
    TI__Expert 4170 points

    hi, Ryan-san.

    Thanks your answer.

    what about below?

    <Cin>

    what if customer uses Electrolytic capacitor 100uF where close to input pin of LDO.

    is this ok?

    <cout>

    if customer uses 2.2uF not 4.7uF, how much different we see regarding output voltage regulation?

    Does it make any influence or the cap value at output makes influence only transient stability ?

    do we know its impact and which spec would be impacted ?

  • 0 in reply to Masaharu Takahashi
    TI__Guru 55940 points

    Hi Masa-san,

    The capacitor requirements as stated in the datasheet are for stability.  We always recommend that customers prototype their applications in order to ensure that they are meeting the performance that they expect; however, we also provide the minimum capacitor requirements for stability in the datasheet.  The following application report discusses how to quickly check stability by performing load transients:

    Very Respectfully,

    Ryan