I am developing a tool that will allow direct connection of our automotive modules to a laptop USB port for diagnostics, re-flashing, etc. Our module requires only about 20mA @ 13.5V - so its power requirements are well within the abilities of a USB port. We are going to require only 500mA capable ports be used.
My design is based around an FT232R USB chip, with some custom circuitry for our communications interface. I am intending to power the FTDI chip directly from the USB bus. For automotive module power, I have designed a circuit that uses a TPS2552 power switch and an LMR62014 SIMPLE SWITCHER to supply 13.5V to our module. The TPS2552 is between the USB VCC and switcher input. For now, I have a 66.5k resistor between ILIM and GND to limit current to 400mA +/- about 50mA. The TPS2552 can be enabled/disabled via the USB chip to maintain USB power consumption compliance. I have used the WEBBENCH tools to build a BOM around the switcher, and it has selected a 4.7uF CIN and COUT, a 10nF inductor, along with 133.0k and 13.3k resistors on the feedback (with a 220pF capacitor parallel to the 133.0k). This all seems reasonable and the numbers match my requirements.
Here is where my questions start: Will I have issues with the TPS2552 supplying to the LMR62014? I am well aware of the soft-start functionality of the TPS2552. Maintaining USB compliance - and protecting the host's USB port at all costs - is of top priority. I want to protect against short-circuit and over-current conditions, as field wiring errors are possible. I am placing extra ESD capacitors and TVS diodes on all pins to protect against ESD.
From what I can tell, a short circuit on the LMR62014 output would simply result in the TPS2552 limiting its input current, resulting in a voltage drop on the output. That is not a problem. I plan to utilize a comparator with a set threshold on the output to inform the software host of an under-voltage condition. In a short circuit condition,I believe the LMR62014 should still remain well below its operating current/temperature limits. I calculate roughly 150mA at the output given 450mA limiting by the TPS2552. I would likely shut down the TPS2552 after a brief short (under-voltage), with user-prompted software retry. Does this seem like an appropriate protection strategy?
I will also be counting on the MBR0520 on the LMR62014 output to act as reverse-voltage protection. In the rare event that I do connect to power (by mistake), it would never be more than 16V, and I believe the switcher should survive this condition.
Will there be any issue starting up the LM62014 if a large capacitive load is present on its output - and a soft-start is present on its input (again, due to the TPS2552)? Some of our modules have > 700uF of capacitance on their input circuits. They are tolerant to a slow supply ramp-up by design. I am not concerned about this from their POV; my software will wait until the supply voltage stabilizes. My concern is whether the LM62014 will have any issues starting, or if this scenario presents any risks to reliability.
Thanks for any help you can provide. I have searched the forums, but so far could not find any scenarios that closely match mine.
Matt
