Do I need to care about this "power good output" p

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Looking at this part: http://www.ti.com/lit/ds/symlink/lm25011.pdf

I understand it I think except for the power good output pin. I'm just using this to convert 24VDC to 5VDC for an AVR. Any reason why I need to use it? Do I leave it floating if not?

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it is an output, so could be left un-connected.
when used it needs an external pull-up.
you could use it and connect to the AVR reset pin. then it gets released only when the power-supply is OK....
You only then need to check what happens if you switch a large load, if the pin shortly drops this could cause un wanted chip resets....

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Hi fizgig

A switching regulator needs a little time to become stable. In some applications, you may not want the AVR to run until it is stable. The power good signal could be used for a reset circuit. But the AVR may run fine without, or you could use the internal brownout detector in the AVR instead.

Peter

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Ah. Well, I do use the 4.3V brownout fuse along with the maximum startup time setting linked to the external crystal... I'm going to see if I can go without using the pin and see what happens.

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meslomp wrote:

when used it needs an external pull-up.
you could use it and connect to the AVR reset pin. then it gets released only when the power-supply is OK....
You only then need to check what happens if you switch a large load, if the pin shortly drops this could cause un wanted chip resets....

Few years ago I designed a power block diagram to power a big hefty board. I needed clean noise free supplies but a lot of current, so I used LDO regulators after buck converters. Turned out controlling the LDO enable signals with SMPS powergood signals was a bad idea, when enabling LDOs the SMPS output dropped too much and powergood disabled the LDOs and so on, so I really had a power oscillator. Try to work around that.

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Quote:
Few years ago I designed a power block diagram to power a big hefty board. I needed clean noise free supplies but a lot of current, so I used LDO regulators after buck converters. Turned out controlling the LDO enable signals with SMPS powergood signals was a bad idea, when enabling LDOs the SMPS output dropped too much and powergood disabled the LDOs and so on, so I really had a power oscillator. Try to work around that.

I'm a bit unclear with what you're saying there but it sounds important. Can you dumb it down a bit?

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SMPS regulator had a powergood output signal, which goes active after output has stabilized within certain tolerance. The powergood signal was connected to LDO regulator enable input, and the LDO gulped so much current at powerup that SMPS output voltage dipped out of tolerance window and powergood signal deactivated, turning off the LDO regulator, and again the SMPS output voltage started to rise within regulation turning the LDO on again which gulped too much current.

As the sequence repeated a few times per second, there was a slight ticking noise coming from the board, most likely from the SMPS regulator coils. When powering up the board for the first time, I thought the board had a short circuit somewhere and went kaboom already, but there was no heat or smoke so this required further exploring before trying to power up another one of the five prototype boards, costing ~500 euros per pop.

The LDO regulator had no soft-start feature, it was turned hard on and there was largish amount of bulk and bypass caps on the LDO output, largish amount at least compared to bulk and bypass caps what was on LDO input side (same as SMPS output side). So when the LDO was turned on hard, it just took so heavy current surge to charge LDO output caps.

The portion that had soft-starting LDOs had better success, but it might have had less bulk/bypass as well.

So just remember, building the block that "just" gives power to components on board may sometimes be the most complex thing to achieve on board. Chips may have two or three different supply voltages like 3V3 for IO, 1V8 for memory and 1V2 for core, and powers must be turned on with correct sequence or within limits between each voltage so the chip does not fry during powerup. If you are lucky, there are only two supply voltages required on a board, but one chip requires voltages to be turned on and off in different order than the rest of the board. Like for one chip 3V3 must be on before turning on 1V8, and the other chip needs 1V8 before turning on 3V3.

Oh and board powerup is still easier to handle than board powerdown.

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Interesting. Thanks for the tip. Will keep it in mind.