0
\$\begingroup\$

I'm using a step-up converter (MT3608) to convert 3.3V to 5V Output. However, because I don't need the 5V all the time and my device is battery powered, I want to turn off the MT3608 IC through its EN pin (LOW = Off, HIGH = On).

Two questions:

  1. Do I need a pull-down resistor from EN to GND to ensure that the converter doesn't "accidentally" turn on? Here are some characteristics for the MT3608's EN pin: enter image description here

  2. What value should the pull-down resistor have to ensure reliability and minimum current draw while the converter is powered off?

I'm also using a logic level converter consisting out of a transistor (BSS138) to convert a 5V logic signal to 3.3V. Here I'm using 3 pull-ups (2 to 3.3V, one to 5V) for which I have the same question as above (best resistor value for minimum current consumption whilst maintaining reliability)

Thanks!

\$\endgroup\$
  • \$\begingroup\$ Please supply schematics for what you are intending to do. \$\endgroup\$ – Spehro Pefhany Dec 9 '17 at 22:23
1
\$\begingroup\$

The MT3608 needs input < 400mV to be turned off but there is no leakage spec. You can assume 1uA to be (fairly) safe. But if you are driving it from a 3.3V push-pull output of some kind you don't need a resistor unless the output can go tristate for some reason (and I don't know why you would do that). If you are concerned about the brief period when your micro (making presumptions) is turning on, a 360K resistor to ground will nullify that issue and will only draw another 10uA when the converter is on.

The pullup resistors used in the type of level converter I think you have in mind are mostly governed by the required rise times and not by leakage currents. So that's more for you to figure out from the specs of the parts, your circuit function and the capacitive loading of the shifted voltages.

Note: I picked 1uA as a safe value because the entire current the device draws when in shutdown is 1uA maximum (100nA typical) and therefore that's the worst case leakage of the chip enable input. If you want to be really, really safe you have to add the leakage of the driving circuit and adjust for high temperature so you might go lower by a factor of 10:1 or 20:1, but if you were designing an aerospace qualified product you'd probably not be using a chip made by some little-known company in the far West of China.

\$\endgroup\$
  • \$\begingroup\$ Thanks for your answer. I made a little error, the MT3608 is fed directly through a lipo battery (2.5-4.2V input). I will switch the EN pin from my arduino pro mini 3.3V microcontrollerboard. If I understand correctly, I should be good with a value of 360k; but I don't reallys understand why the resistor is only good while my arduino is starting up. After the arduino started up, there is no possibility for the EN pin to go above 0.4V unless I explicitly tell the arduino to switch the respective digital pin to high? Why is that so, can you explain? I'm a hobbyist and still learning. \$\endgroup\$ – Henry Dec 9 '17 at 22:48
  • \$\begingroup\$ Often when a micro is being held in reset, for example while crystal oscillator is starting up, the I/O pins will float. Sometimes we really care about what happens in that couple hundred milliseconds because it could cause something undesirable to occur. Also in the pathological case where the power browns out at, say, 4.2V rather than 5V, the pins might float but there might be enough voltage to cause bad things to happen for an indefinite period. \$\endgroup\$ – Spehro Pefhany Dec 10 '17 at 1:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.