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Topic: 5V regulator is not USB spec - need boost (Read 3117 times) previous topic - next topic

5V regulator is not USB spec - need boost

The 5V regulator on Bus Pirate will not meet the minimum 4.75V requirement of many 5V chips, at least not under all normal conditions.  As delivered, the Bus Pirate would fail USB testing, at least in any situation where the 5V regulator is needed.

1) An unpowered USB hub can deliver as little as 4.01V under transient conditions or 4.35V average to the connected USB device such as the BP.  The MIC5205-5.0 requires a minimum of 5.01V to start, and as much as 5.35V when supplying a load.  Thus, the provided 5V regulator will not work completely.

2) Many 5V chips have a +/-5% requirement on their supply.  This translates to a working range of 4.75V to 5.25V, but the provided regulator cannot guarantee the minimum voltage under all conditions.

The MIC5205-5.0 chip should be replaced by a boost regulator.  Suggestions for a replacement include the Maxim MAX1595, MAX682 or MAX1797.  There should be plenty of room for the SMD capacitor or inductor needed by these parts to boost the USB power to a full 5.0V

Linear also makes similar chips which might not be as hard to obtain as Maxim parts sometimes can be.

Re: 5V regulator is not USB spec - need boost

Reply #1 –
I really appreciate your valuable experience being applied to the Bus Pirate design. I'm totally open to the possibility of adding a boost regulator instead of an LDO, but here's my primary thoughts.

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the Bus Pirate would fail USB testing

I don't see that as a big problem as we're kind of the opposite of the crowd that would seek USB certification.

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unpowered USB hub can deliver as little as 4.01V under transient conditions

I'd prefer to deliver an affordable piece of hobby hardware than raise the component count (price) to meet all corner cases. When using an external hub and trying to interface a 5volt chip, it's probably best to use an external supply. The on-board voltage monitors provide the user with notification of the low voltage condition so they can make an informed decision. Also, the new short circuit detector will disable the supplies if the voltage is too low.

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provided regulator cannot guarantee the minimum voltage under all conditions.

Nope, not at all. But sometimes, you just might find, you get what you need :)

I always ask for the voltage report during debugging, we could do a quick survey of the 5v rail values from posts in the forum. I usually see 5.01 to 4.85 in reports, personally I sometimes get down to ~4.7 under a heavy load that way exceeds the 150ma limit (LCD with backlight, LED matrix boards).

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The MIC5205-5.0 chip should be replaced by a boost regulator...replacement include the Maxim

If there were lots of incidents and user complaints about the 5volt rail I could instantly justify the 10x part cost increase, supply chain difficulties, etc, to use something like that. However, out of the Bus Pirates Seeed has sold (and probably 1000s at SparkFun that I also support), there has never been a user report of an underpowered 5volt regulator being an issue.
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Re: 5V regulator is not USB spec - need boost

Reply #2 –
Regarding USB test failure, that was just my overly terse way of saying that it won't work in all valid USB systems.  In other words, an unpowered USB hub which does pass USB testing would cause the BP to fail.  I'm not even sure whether USB testing includes verification that all device features remain functional under corner cases, i.e. the Bus Pirate might actually pass an official USB test even though the 5V output fails to work.

I replied with the following in the original thread, but should have just waited to post it here where it's more appropriate.

[quote author="ian"]The MAX1595 looks really interesting, and I'd be willing to try something like that. The MAX1595 is (according to shifty Maxim) $1.43 @ 1000. I could only find the msop8 version at Digikey for $4.05. Even at the 1000 price (remember, we only make 100s of Bus Pirates), it's 10x more expensive than the 5volt regulator.[/quote]Point taken.  I have also suffered from the difficulty in obtaining Maxim parts, such as an RS-485 receiver that I ended up getting 9 free samples to meet my total needs.  But despite how cheap the Micrel part may be, it just won't work in all USB setups.

You might want to look into Linear, or maybe Texas Instruments.  I'm sure lots of companies make boost regulators for the USB world.  I recall considering Linear for my commercial design which used the MAX1595.  I looked at the LT1268 and LTC1157, but forget the prices.  It seems like Texas Instruments should have something to offer, but I do not recall any part numbers.  There has to be something that will work, even if you roll your own with the PWM on the PIC.