Hello all,
I have an Adafruit Powerboost 1000 basic buck booster. The EN pin on this booster (based on the TPS61030) is pulled Hi by default, which keeps the booster on. If the EN pin is pulled Lo by connecting it to GND, the booster gets turned off. I want to do the opposite of this; I'd like to connect the booster to a capacitor and have the booster to be off by default until the capacitor charges to 5V, at which point it pulls the EN pin Hi and turns the booster on. Is there any way I can accomplish this? I don't want to use Arduino I/O pins for this since the booster is what would turn the Arduino on (i.e. the booster powers the Arduino, so I can't have the Arduino signal the booster because it wouldn't be powered without the booster being on...you get the idea). Thank you for your help!
There is a pull-up resistor on the EN pin. The Adafruit schematic is a little "fuzzy" but it looks like R13 and I can't clearly read the value. You can "ohm it out" find the right resistor then remove it. Or you can download the Eagle files which should be clear.
I'd like to connect the booster to a capacitor and have the booster to be off by default until the capacitor charges to 5V, at which point it pulls the EN pin Hi and turns the booster on.
Yeah... A basic [u]RC low-pass filter[/u] should work. As a ballpark starting-point you can calculate the [u]RC time constant[/u] (i.e. 1uF and 1M is 1 second).
![[u]RC low-pass filter[/u]](https://en.wikipedia.org/wiki/Low-pass_filter#/media/File:RC_Divider.svg){kind=link}
You should probably add a "high-value" pull-down resistor in parallel with the capacitor (at least 10 times the resistance in the "filter").
There are a couple of unknowns - I don't see an input-current rating for EN so I don't know the impedance of that input. I assume it's a high-impedance MOS input but I'm not sure. (The resistance of the RC circuit should be low relative to the input impedance.)
And we don't know the exact voltage at which it will be enabled. The data sheet says EN is high at 0.8V x VBATT and low at 0. x VBATT. And, it's a digital input so we can't be sure that "funny things" won't happen in-between.
Alternatively, you could build an op-amp comparator (to use with the RC circuit) or maybe an NE555 could be used as a timing circuit. Either of those would give you a good-solid digital output and you wouldn't need to remove the existing pull-up.
Hello Doug, thank you so much for the tip. I realized that I may not actually need to disable the booster for my application but hopefully someone else can find this information helpful.
On a different note, do you know if this circuit has some sort of protection against backwards current flow? I want to use the booster to basically take a lower voltage and boost it to 5V and use that to charge a 5.5V 5F supercapacitor, but at some point the supercap's voltage becomes higher than the input voltage and it would want to dump current in the opposite direction. Would this be an issue?