Post Go back to editing

ADP2120 max Cout

Hello,

What is the max recommended output capacitance allowed on the ADP2120?  The datasheet specifies a min of 22uF, but can another 100uF or 200uF cap be added to this Cout distributed elsewhere on a board?  At what output capacitance woudld the device enter hiccop mode?

Thanks,

Ali

Parents
  • To prevent the device to enter hiccup mode you need to guaratee that your start-up current requirement does not force the device to hits its current limit.

    Current limit is set to 2A

    Current at start-up < Ilimit =  0.7 * Cout* Vout / Tss

    Tss = 850us at 1.2Mhz or 1.7ms at 600Khz, this is the preset soft start timer

    I use 0.7 coefficient to reflext a 30% increase in current due to ripple current in the inductor

    So for a 1.8V output

    Coutmax < 2A * 850us / 1.8V / 0.7 = 1300uF

    If in adition to the cap requiring current your load is also asking the full 1A then

    Coutmax < 1A * 850us / 1.8V / 0.7 = 670uF

    All this assumes the loop is stable with the large output cap. You may want to consider entering your requirements into the ADP2120 ADIsimPower tool and set it for a stringent ripple and transient requirement, the tool will double check stability and start-up and suggest the size capacitor that meets your needs.

    You can also slow down the start-up timer by using an external ramp on the TRK pin. You can do that with a RC network connected to Vin that guarantees the TRK pin ramps to 0.6V slower than the preset timer.

Reply
  • To prevent the device to enter hiccup mode you need to guaratee that your start-up current requirement does not force the device to hits its current limit.

    Current limit is set to 2A

    Current at start-up < Ilimit =  0.7 * Cout* Vout / Tss

    Tss = 850us at 1.2Mhz or 1.7ms at 600Khz, this is the preset soft start timer

    I use 0.7 coefficient to reflext a 30% increase in current due to ripple current in the inductor

    So for a 1.8V output

    Coutmax < 2A * 850us / 1.8V / 0.7 = 1300uF

    If in adition to the cap requiring current your load is also asking the full 1A then

    Coutmax < 1A * 850us / 1.8V / 0.7 = 670uF

    All this assumes the loop is stable with the large output cap. You may want to consider entering your requirements into the ADP2120 ADIsimPower tool and set it for a stringent ripple and transient requirement, the tool will double check stability and start-up and suggest the size capacitor that meets your needs.

    You can also slow down the start-up timer by using an external ramp on the TRK pin. You can do that with a RC network connected to Vin that guarantees the TRK pin ramps to 0.6V slower than the preset timer.

Children
No Data