Quote:
Originally Posted by juchong
That's great and all, but I'm only testing the 5V / 2A supply rail.
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My point is: If you don't let the VRM draw max current during power-up, then observations about how long it takes the power to stabilize output voltage is not indicative of normal operation. I'm guessing there are circuits in the VRM that need to be energized in order to provide stable output power. That start-up load can easily exceed 1.5 amps.
The VRM has a minimum input voltage of 5v. So, in order to supply 12 volts, it must first step-up the power to 13 or more volts, and then it can regulate it down to 12 volts. I know you are not using the 12v circuit in your test, but the VRM doesn't know that. So, it has to energize that circuit during power on.
In your Test 1, you say that when a 1.6 amp load is attached, the VRM will not start up. It may be because as soon as it starts to energize the circuit, it is drained away by the load. There is not enough excess power in the circuit that is allowed to build up to the operating level.
In Test 3, you finally have a situation where the the amount of power draining out is slow enough that the amount of power remaining in the VRM can build up to operating levels in 5 seconds. Now, whether that is normal or not, I don't know.