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TPS62136EVM-698: Purpose and Necessity of Input Tantalum Capacitor

Deniel Go
Intellectual 395 points
Part Number: TPS62136EVM-698
Other Parts Discussed in Thread: TPS62136, BQ24133EVM-715-15V, BQ24133

Hello TI Engineers,

I would like to know what is the purpose of the 68uF Tantalum Capacitor in the input side of the TPS62136 Buck Converter Circuit? What factors determine the capacitance value? When TPS62136 circuit is designed using the TI Web bench, it only includes the 10uF input capacitor.

Can the tantalum capacitor be changed to a regular ceramic capacitor? Also, the description of the tantalum capacitor in the Eval Board Document states that it is rated at 35V but the actual part number listed, TPSE686M025R0125, is rated at 25V. Which one is correct?

Our main circuit design layout consist of 2 TPS62136 circuits that are connected to the power pathing output of the BQ24133 charger IC. It follows similarly to the layout of the BQ24133EVM-715-15V where it could select either a 3S Battery or a 15V adapter. 

Thanks,

Deniel

  • 0
    TI__Mastermind 21161 points

    Hello Deniel,

    The purpose of adding 68uF Tantalum Capacitor is to dampen parasitic inductance especially when there is long input trace.

    Actually if the input trace is short , only a 10uF ceramic capacitor is enough to acting as CIN.

    TPSE686M025R0125 is rated at 25V not 35V, and I want to make a clarify that this is a error in Eval Board Document.

    Thanks,

    Colin

  • 0 in reply to Colin Chen
    Intellectual 395 points

    Hello Colin,

    Thank you for the explanation for the Tantalum Capacitor and the capacitor voltage rating in the Eval Board Document. The 2 Buck converter circuits we have do not connect directly to our battery and input power adapter. Rather, the input trace are connected to the Power Pathing MOSFETs. (See the circuit diagram below, buck circuits are connected to System) The battery has input ceramic capacitors connected to the charging IC. The Input Jack for the Power Adapter has a RC snubbing circuit. 

    What is considered a long vs short input trace? The Trace length from the MOSFETs is about 50mm to the first buck converter circuit and 130mm to the second buck converter. If we include the lengths up to the battery terminals and Input DC Jack, the trace lengths going to the 1st and 2nd buck converter circuit would be around 26mm more (76mm/156mm) from the battery and 35 mm more (85mm/165mm) from the Input DC Jack.

    Given the lengths of our traces, up to the MOSFET and up to the actual voltage sources, do we need to add tantalum capacitors? If yes, how many do we need and where is/are the optimal location/s to add it? Our maximum input voltage to the TPS62136 is 15V, can we go with the 25V rated tantalum capacitor if we need to add one or should we go with 35V to have a 50% derating value of 17.5V?

    Thanks,

    Deniel

  • 0 in reply to Deniel Go
    TI__Mastermind 21161 points

    Hello Sir,

    It is suggested to adding a tantalum capacitor for more stable VIN, considering there is room to put it in your board.

    25V DC rating can be enough.

    Thanks

    Colin