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TPS7A78: Output current limit of cap-drop type regulator

Expert 3785 points

Replies: 3

Views: 37

Part Number: TPS7A78

Dear team, 

My customer was considering using TPS7A78 for their AC to DC power supply. But the requirement of output current is changed up to 300mA. But output current limitation of TAP7A78 is up to 120mA.

Is there any solution to meet customer request? Main concern point of customer is solution size. 

The customer can accept Discrete Capacitor drop regulator. Is it available up to 0.3A output current using cap-drop regulator? 

<Customer concept> 

- input : 100 ~ 240VAC (50/60Hz)

- output : 3.3V / 300mA

- non isolated and transformer-less type

Please let me know your opinion regarding minimum solution to meet customer concept. 

Thank you.  

  • Hi Dino,

    Unfortunately, the device is not able to supply 300mA. If this is a short duration peak current, the LDO may be able to handle this, but the power stage supplying the input rail for the LDO is limited to 120mA. 

    If the application also needs a low current 3.3 or 5V rail, you can use the TPS7A78 to make the 3.3 or 5V rail and then tap off of the SCIN rail with a DC/DC converter to make a 300mA rail. See below example from the online calculator:

    Let me know what you think.

    John Cummings 蔣康明
    Texas Instruments 德洲儀器
    Johncummings@ti.com

    Please click the "Resolved" link at the bottom of this post if I have answered your question.

    Thank you,

  • In reply to JCHK:

    Hello John, 

    Thank you for your comment. 

    What voltage is on SCIN? 

    Their concept is 3.3V/30mA for MCU and sensor and 5~10V/250mA for others.  Is this concept available? 

    Thank you. 

  • In reply to Nam, Dino:

    Hi Dino,

    The lowest voltage on the SCIN pin is either 4*(VOUT+0.5) or 16V (whichever is lower). 

    While you can create a 10V rail at 250mA you would need a very large cap-drop capacitor to support this:

    It looks like the CS (cap drop) capacitor would need to be 5.6uF.

    I hope this answers your question.

    John Cummings 蔣康明
    Texas Instruments 德洲儀器
    Johncummings@ti.com

    Please click the "Resolved" link at the bottom of this post if I have answered your question.

    Thank you,

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