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Thread: Teensy 3.2/4.0 inrush current?

  1. #1
    Junior Member
    Join Date
    Oct 2019

    Teensy 3.2/4.0 inrush current?

    What's the inrush current on the teensy 3.2 and 4.0? I'm going to use a teensy together with a bunch of other stuff powered by USB and it would be nice to know much more inrush current I can add.

  2. #2
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Usually USB inrush is described as the equivalent capacitor + resistor load. The actual peak current and actual behavior depends heavily on the specifics of the cable used, so the specs are in terms of equivalent load. This is discussed in section 7.2.4 of the USB 2.0 spec, starting on page 177.

    Teensy 3.2 has 4.4uF capacitance directly across VBUS and ~6.9uF downstream of the 3.3V regulator. The regulator may limit the current into those other capacitors somewhat, so an accurate estimate is difficult. But if the regulator is considered a short circuit, the total is already at or maybe slightly over the 10uF limit. There's very little margin to add more capacitive load, if you care about staying under the USB limit.

    Teensy 4.0 is much more complicated. It has a lot of capacitance, but it also features a staged startup. It has 2.6uF across VBUS. At powerup, a 10uF capacitor on VDD_USB_CAP is charged, but the chip limits that current to 25 mA. Later the 3.3V regulator is enabled. It has about 30uF of capacitance (considering the ferrite beads to be wires), but the TLV757P 3.3V regulator has a very nice soft start feature built in. The big inrush with Teensy 4.0 happens several milliseconds later, when the DCDC converter turns on. It very quickly charges about 15uF up to 1.15V. I've tried measuring that current drawn from the 3.3V side. The peak can be around 100 mA, which is right at or maybe even slightly over the USB limit. But that peak happens long after the other capacitors are all charged, so if you connect more capacitance directly to VBUS, you might still be ok.

    With either board, the ideal way would be to turn on the power to exta loads at runtime, either with a transistor or an enable signal (if the device has a low power disable mode). Ideally that should be done after USB enumeration is complete.

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