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TPS62873: SK-AM68 design decision

d_zero
Genius 4865 points
Part Number: TPS62873
Other Parts Discussed in Thread: SK-AM68,

Dear Team,

1)

we would like to know why in SK-AM68 design for the input capacitance 4x NFM18HC106D0G was chosen?

And why is better to use 4X NFM18HC106D0G ( c_eff at 3.3V -> 6X4 = 24uF) VS the used capacitors in the TPS62873 EVM and in DS

2x 0.47uF 10V X7S 0402

2x 2.2uF 10V X7S 0603

2x 10uF 10V X7R 0805

Price VS Performance  ( ESL, ESR, Effective capacitance)

EVM has additional 2x22uF but then the EVM has mush more eff capacitance

2) Also the output capacitance on SK-AM68 is only 47uF and 2x47uF , if compared to DS it is small, can you please explain how did you do this decision on the SK-AM68?

if we use EQ 13 from DS end use the RZ = 1.8k and L=100nH as from SK-AM68 schematic we get the t the Cout(min)(Reg) = 180uF ???

Best Regards,

d.

  • 0
    TI__Guru 53295 points

    The capacitor type (2-term vs 3-term) is optional.  The 3-term have much lower inductance package and performs better.  Note there is significant amount of capacitance on the processor power pages (~400uF for CORE/CPU) rails.  I'm checking on input capacitance differences.

  • 0 in reply to Robert Eschler
    TI__Expert 5425 points

    There is no device specific guidance on which input capacitors must be used for any DC-DC converter. It typically depends on the design implementation. The impedance of the power delivery network powering the DC-DC converter just must be low enough to reliably power the converter. All capacitors on the rail powering the converter contribute to this PDN impedance.
    In case of the SK-AM68 design you find many capacitors on the VSYS_3V3 net which is powering the TPS62873 converters. This also includes the output capacitors of the pre-regulator which is part of the same design and generating the 3.3V from a 5V input. On the TPS62873EVM it is expected that the converter is powered from a lab power supply using cable connections. This usually requires more capacitance at the input of the converter on the EVM.