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TPS785-Q1: ESD/TVS protection for input coming from USB VBUS

Matthias Kunnen
Prodigy 10 points
Part Number: TPS785-Q1
Other Parts Discussed in Thread: TVS0500

Hi

I selected the TPS78533QKVURQ1 (LDO TPS785-Q1 fixed 3.3V out) for a design for the purpose of converting the 5V coming from a USB connector's VBUS to 3.3V.
Now, I need to add ESD/Transient voltage protection to the VBUS. The datasheet of the LDO states in the absolute maximum section that the maximum input is 6.5V. From this, I believe that I need to find a TVS with a working voltage of 5V and a clamping voltage between 5.5V and 6.5V. Finding one is proving difficult.

The TVS0500 is recommended in ESD and Surge Protection for USB Interfaces section 8.2: VBUS protection. However, its clamping voltage is higher than the absolute maximum input voltage of the LDO.

As to my question: would the TVS0500 be a suitable pairing for this LDO? If not, does TI offer a solution for ESD/transient voltage protection that would fit this LDO. A requirement is minimum (not maximum) pitch of of 0.5mm to make hand soldering feasible for prototyping purposes.

Best regards,
Matthias

  • 0
    TI__Mastermind 34885 points

    Hi Matthias,

    In general we don't have a specific TVS part recommendation for your application.  You need to ensure that the abs max conditions listed in the TPS785-Q1 datasheet are met under all times to prevent damage to the device.  I understand the concern regarding the working vs clamping voltage of TVS diodes.  Perhaps review your ESD / Transient requirements, determine the source impedance of the ESD / transient generator, and find out if you absolutely need a TVS diode in the first place.  Sometimes the source impedance of these generators are sufficiently high such that a simple ceramic capacitor is enough to dampen the pulse to within acceptable levels.

    Thanks,

    Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 10 points

    - Would this input capacitor be in addition to the recommended one (1 µF)?
    - Are there any recommendations regarding the capacitance/voltage of such an input capacitor?

    The reason for concern regarding the transients would be voltage spikes or ringing when hot plugging the connector.

  • 0 in reply to Matthias Kunnen
    TI__Mastermind 34885 points

    Hi Matthias,

    It may or may not be in addition to the 1uF input capacitor.

    I would look at parallel RC damping networks in parallel with your 1uF capacitor.  Your ringing is a result of the interaction between the undamped harness inductance and low ESR Cin.  Estimate the harness inductance from the oscilloscope measurement that gives you the frequency of ringing.  Using the LC equation and 1uF capacitance as the "C" in the LC equation, you can estimate the harness inductance.  Then calculate the characteristic impedance of the undamped LC network (harness inductance = L, Cin = C).  This then becomes the approximate damping resistance you need.  Select the damping capacitance to be in the range of 4-10x Cin.  Often a dry tantalum capacitor is sufficient to meet both requirements. 

    We add unpopulated damping networks on the input rails of all of our performance evaluation modules now, to help with any undamped ringing which may exist on Vin.  I believe we use a case size C dry tantalum (unpopulated), but you may be able to get away with a smaller case size depending on your measurements and needs.

    Thanks,

    Stephen