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TPS22918: Should CIN be more than 10 times of CL?

Kazuya Nakai59
Guru 21515 points
Part Number: TPS22918

 Hello guys,

 One of our customers is considering using TPS22918 for their new products. In their application, TPS22918 VOUT is connected to 10uF as CL. The device datasheet (SLVSD76C) says that "becuase of the integrated body diode in the MOSFET, a CIN greater than CL is highly recommended. A CL greater than CIN can cause VOUT to exceed VIN when the system supply is removed." in 9.2.2.2 Output Capacitor (CL) (Optional) section.

 According to the description, the customer needs to use 100uF for VIN-GND capacitance (CIN). But the customer doesn't want to use 100uF ceramic capacitor becasuse the capacitor will be high pric and the capacitor area is big.

 Can CIN value be reduced? If the answer is Yes, could you give me your advice or comment that how much the CIN can be reduced?

 Your reply would be much appreciated.

 Best regards,

 Kazuya Nakai.

    

  • 0
    TI__Genius 10855 points

    Hi Nakai-san, 

    Thanks for reaching out on E2E!

    We recommend maintaining a 10:1 ratio between input capacitance and output/load capacitance. While this is inherently not a problem, there are two potential concerns to think about when COUT > CIN:

    1. Normally, the CIN capacitor is used to help the power supply handle large amounts of inrush current. During startup, if CIN isn’t large enough, the sudden change in current can cause the input voltage to sag.

     

    2:    If VIN suddenly drops while the device is enabled, there could be a brief moment where VOUT > VIN. If this happens, this could potentially bias the body diode inside the device and cause reverse current to flow upstream. Will there be a condition where VIN could suddenly be removed while the device remains enabled?

    If you don't see either case being an issue, then you could reduce CIN. Keeping it at the same value as COUT should be acceptable, so 10uF on the input should work.

    Thanks,

    Arthur Huang

    To find the latest information on Power Switches, visit www.ti.com/powerswitch

    Please click "This resolved my issue" button if this post answers your question.

  • 0 in reply to Alpha H
    Guru 21515 points
    Hi Arthur,

    Thank you very much for your reply.

    Could I ask you an additional question?
    In the TPS22918 case, how much is the allowable maximum current value and current flowing time from VOUT to VIN through the body diode?

    Thank you and best regards,
    Kazuya Nakai.
  • 0 in reply to Kazuya Nakai59
    Guru 21515 points
    Hi Arthur,
    Could I ask you one more question?
    X5R/X7R capacitors are recommennded for the bypass capatitor between VIN and GND on 11.1 Layout guidelines section. Is the bypass capacitor needed separate from CIN?

    Thank you and best regard,
    Kazuya Nakai.
  • 0 in reply to Kazuya Nakai59
    Guru 21515 points
    Hi Arthur,
    I'm sorry to ask you many questions. But please let me ask you the following question. DIN is very close.
    My customer is going to use 10uF for both CIN and CL. VIN=1.2V in their system and their load circuit connected to VOUT of TPS22918 is in sleep mode (Icc=0uA) when VOUT exceeds about 1.2V*97% after TPS22918 is turned on. Then they want to know how much rising time of VOUT is in some CT value case. Table 2 in the device datasheet shows rise time in CIN=1uF, CL=0.1uF and RL=10ohm conditions. Is the rising time not big difference from Table 2 even if VIN=1.2V, CIN=10uF, CL=10uF and RL=no load?

    Your reply would be helpful.

    Thank you and best regards,
    Kazuya Nakai.
  • 0 in reply to Kazuya Nakai59
    TI__Genius 10855 points

    Hi Nakai-san, 

    Let me try to answer your questions one at a time. Please find the answers to your questions below:

    1:   In the TPS22918 case, how much is the allowable maximum current value and current flowing time from VOUT to VIN through the body diode?

    In our recommended operating conditions, we specify that VOUT should not be greater than VIN. If the body diode was forward biased, then the amount of current through the diode would not be controlled. Since this cannot be controlled, we do not test or characterize the body diode to guarantee a forward voltage or specific current amount through the diode. 

    2: X5R/X7R capacitors are recommennded for the bypass capatitor between VIN and GND on 11.1 Layout guidelines section. Is the bypass capacitor needed separate from CIN?

    No, the bypass capacitor is the same recommendation as CIN. Please refer to the image 11.2, Layout Example, for additional details. 

    3: My customer is going to use 10uF for both CIN and CL. VIN=1.2V in their system and their load circuit connected to VOUT of TPS22918 is in sleep mode (Icc=0uA) when VOUT exceeds about 1.2V*97% after TPS22918 is turned on. Then they want to know how much rising time of VOUT is in some CT value case. Table 2 in the device datasheet shows rise time in CIN=1uF, CL=0.1uF and RL=10ohm conditions. Is the rising time not big difference from Table 2 even if VIN=1.2V, CIN=10uF, CL=10uF and RL=no load?

    The rise time of the load switch is ultimately dependent on a few factors, including slew rate, load resistance, and input voltage. CIN/COUT doesn't impact rise time as drastically, it is more important in the fall time. You can use section 8.3.3 in the datasheet to estimate the rise time of the device across common CT values. 

    Thanks,

    Thanks,

    Arthur Huang

    To find the latest information on Power Switches, visit www.ti.com/powerswitch

    Please click "This resolved my issue" button if this post answers your question.