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Original question:

LM25118: LM25118 min VIN

LM25118: LM25118 Wide input range issue

Kami Huang60
Genius 3830 points
Part Number: LM25118

Dear Sir/Ms.

Product specifications INPUT = 20 ~ 28VDC / VAC, OUTPUT = 24V / 1.7A (3.6A peak)
1. Add a bridge diode at the input terminal for VAC input. According to TI WEBENCH design recommendations (Vin = 20 ~ 40Vdc), the input capacitance value is <100uF!
But when it is applied to VAC input, because the Vin input ripple is too large, the load cannot be started! Is it related to EN pin or Vcc UVLO?
2. If the input capacitor value is increased to 1000uF + 220uF, it can improve the start-up problem (Io = 1.7A is okay)!
However, when it is applied to VDC input, it cannot start with heavy load (Io> 500mA). It must be started after light load!
3. Can TI / LM25118 be applied to AC / DC input (AC input via bridge diode)? How to evaluate Cin capacitance?
4. Any other suggestions for buck / boost IC?

Attached file is customer's sch.

SA-1625A2 power.pdf

Best Regards,

Kami Huang

  • 0
    TI__Guru* 78430 points

    Hi Kami,

    Thank you for considering the LM25118.  Can you show us some key waveforms of the following pins:  VIN , SS , VCC, and HS, before we can answer your questions?  Please show us both the waveforms with VAC input and DC input, with large or small input capacitors. 

    Thanks,

    Youhao Xi, Applications Engineering