| ??? 12/08/09 06:02 Read: times |
#171515 - ... glad you asked that ... Responding to: ???'s previous message |
Erik Malund said:
Have you ever observed what your switcher does when the load current goes from 20% of rated current to 80% of rated current? If so, what did you observe?
yes, I have and nothing much By that do you mean that you didn't look? Have you ever observed what your linear does when the load current goes from 20% of rated current to 80% of rated current? If so, what did you observe? I just did, since I had a broken gadget in house (diode failure), and it drops about 45 mV for about 2 microseconds, rings as much as 50 mv for 6 microseconds, and then settles back to 5.0087 volts +/- about 5 mV. It does pretty much the same thing when the load is switched off though the waveform is a mite different. Normal noise level under average load is about 15 mV when the MCU is running. That's on the 5-volt supply rated at 10 amperes, regulated with a 78P05 in TO-3. I'd expect somewhat lower performance from an LDO, as its output transistor has less gain. The 'P05 has a darlington output, as does the 7805 and the LM340. With a pass transistor, the thing is a little less simple, hence a bit slower to respond to load changes. When I last subjected a switcher rated at 10 amperes to the same load (stepping motor) driven from Vcc, it burst into flame. I did it again at the time, and got the same result. I've never bought a switcher from that supplier again. I also don't use switchers to feed steppers bigger than a small apple any longer. I suspect that sort of thing happens pretty often in your bus-display environment, so I imagine you've looked at it quite a bit. Of course, it's probably not particularly important, since your display can probably recover within a few milliseconds if the supply drops out. Nevertheless, you've probably seen the worst-case.
since a color display have one 'dose' of 1/2ms the maximum allowable 'dip below min' is in the 10us range for the LED supply. Of course, for the logic supply it is zero. Erik dose: (could not find a better name) one way to create color is to display one display for 4ms, another display for 2ms, another display for 1ms, another display for 1/2ms. This gives 15 possible intensities each for red, green and blue I wasn't so much interested in a "dip-below-min", as, generally, that's never tolerable in Vcc. When you say, "zero," what do you mean? Are you saying that your switcher has zero ripple? Are you saying that your Vcc supply always has a precisely constant load? Are you saying that there's no noise riding on Vcc? I'd imagine that there's something. |
