How to use the power good signal properly?

I'm powering a circuit from a buck converter. I've seen that many buck/boost converters (for example, TPS62177) have a PG or power good output signal which indicates when the device has managed to regulate its output to the desired voltage.

I am not a power management expert but I'm curious, what could go wrong if I simply disregard the power good output? When do I need to use it and how should I do it properly?

• I usually tie that PG output to the reset input of my microcontroller. – brhans Jan 26 '16 at 20:47

There are many, many uses for a PG signal. One of the most common is to control power-sequencing of circuits with many different rails. For instance, you may have a Supply B that is not supposed to turn on until Supply A is on, and stable. By feeding the PG signal from Supply A to Supply B's enable input (either directly, or through some simple logic gates or a CPLD), they will sequence themselves in the proper order.

Another common usage is for reset supervisor ICs. If such a device has an active-low manual-reset input, OR'ing together all the PGs from supplies (assuming each is open-drain), if any of them fail / go out of regulation, the reset supervisor will toggle a reset of the system.

So it's really up to you, but there's two common uses. You should check the datasheet to see when PG is asserted/de-asserted; for some regulators, if they detect you're out of regulation by 7.5% - 10%, the signal will assert.

In your case, here's the relevant section:

If useful for you as the designer to know if regulation falls out of those ranges, then you should wire it up to whatever you need. Perhaps you want your microcontroller to take an action, or go into reset. Maybe you just want to turn on a LED when PG is not OK. At a minimum, I would bring it out to a test point with a pull-up resistor populated (or at least the footprint) so it's present and accessible if you end up needing it. :)

• If my whole circuit runs from this single buck converter, then what purpose can I use the power good signal for? – Venemo Jan 27 '16 at 16:00
• What's your circuit? – Krunal Desai Jan 27 '16 at 16:28
• It contains a microcontroller and two RF chips and some other minor stuff. – Venemo Jan 27 '16 at 16:31
• I'd run it to a test point, perhaps route it to a transistor that controls a LED so you can tell at a glance if you're power OK or not. Alternately, you could add a reset supervisor, but those generally have a built-in VDD monitor. – Krunal Desai Jan 27 '16 at 18:15

There's nothing wrong with ignoring the "power good" signal if you don't need or want it. If you do want to use it the majority are open drain outputs so that you can OR them together if desired to have a signal that is high only when all converters are "good". It's generally a logic level signal and what you do with it is up to you. For example you can use it for power sequencing, you can feed it into a GPIO on a processor for monitoring or hook it up to a red LED that will light up if power isn't good.

• I think what I don't understand here is what exactly it means when power isn't “good”. Should I worry about my ICs getting overvoltage, or something similar? – Venemo Jan 26 '16 at 20:55
• @Venemo, typically the chip (your TPS62177 or whichever) monitors the output voltage of the converter. If the voltage is stable within a preprogrammed band, the PG pin will activate. If the voltage goes too high or too low, the PG pin will deactivate. The datasheet of each chip will give specific details of what is considered within the acceptable voltage range. – Dan Laks Jan 26 '16 at 21:01

The datasheet of TPS62177 states "power good (PG) function indicates that the output reached regulation".

The buck converter is in fact a regulator that operates in feedback loop: when the output voltage is above threshold it turns off the switch that feeds the inductor. Then, the voltage on the output will start failing until lower threshold is reached, which is when the switch is turned on again to feed the inductor. Voltage on the output rises until threshold is reached and the story repeats.. The cycle timing is variable depending on load.

It takes a (short) while to charge up the coil during startup. in that time voltage on output rises slowly, PG will not be active until the voltage meets the specifications. That is the main reason for the PG pin to be there (that and indication of overload).