# How to turn off DC-DC step-up converter to run on 3 AA batteries

simulate this circuit – Schematic created using CircuitLab

I am using this step-up converter : DC-DC step-up converter with 3xAA batteries (input voltage for the circuit is ranging from 4.65v down to approx. 3v). The main chip is labeled F193. I would like to take care about current consumption. The first thing I did is to unsolder USB and the LED (so going down from 10mA to 0.8mA). Now, I try to shut down the step-up when not needed. This step-up is used for a stepper motor (28BYJ48 - requiring 5v) that is ON 100ms every minutes.

I am working with an AVR microcontroller that goes to deep sleep when not used consuming about 20uA. I know there is room from improvement on this side but I am trying to fix the problem in order of the more consumption first. This microcontroller runs as low as 1.8v input (far lower than the 3xAA battery depleted around 3v).

I tried with a 2N2222 transitor : base linked to the AVR (with 1K resistor) and in common emitter mode (emitter to ground and the input of the step-up converter from Vcc to collector). This doesn't work as I mesure always a 0.8 mA current even when OFF. I think this is due to the Vce ~ 0.7v.

I also tried with a TIP122 in the same configuration but with the same result (0.7mA still there).

Is there a way to completly shut-down the DC-DC step-up converter ?

• Which IC is the boost converter module based on? Most of them have a shutdown pin, which may not be wired to anything at the moment but you could break it out. – pericynthion Dec 3 '16 at 19:56
• Instead of describing your schematic, you can use the embedded schematic drawing tool. Edit your question and press Ctrl + M to use it. – Bence Kaulics Dec 3 '16 at 20:18
• Does your (unidentified) stepper motor REALLY require 5V? It does not operate at 4.5V? You are probably wasting more power by doing that questionable conversion from 4.5V up to 5V than you would save trying to power down the converter. If you really want to turn off the converter, get one that has a shutdown input. Most chips support that even if it is not brought out to a pin on the converter board. – Richard Crowley Dec 3 '16 at 21:40
• Because I want to run on AA batteries, I expect the voltage to be 4.65 (when full) to 3v (when empty). The stepper (I updated the question to provide info), seems to not be able to run on low voltage from my experiments. This is the reason of the addition of the converter. – TrapII Dec 3 '16 at 21:48
• You have selected an unworkable combination of power source (AA batteries), stepper motor that won't operate at your voltage available, and converter without a shutdown function. You have painted yourself into a corner. Suggest going back to the drawing board and re-evaluating your design choices. – Richard Crowley Dec 3 '16 at 21:57

Working at such low voltages is a bit of a challenge, I'd suggest first of all that the ULN 2003 is replaced with a TPL7407l (http://www.ti.com/lit/ds/symlink/tpl7407l.pdf) this will get your saturation voltage down to a more acceptable level for driving the stepper motor.

To switch the boost convertor off without having to figure out adding wires to the module you could add a high side switch using a P-Channel switch such as the MIC94052 (http://www.ti.com/lit/ds/symlink/tpl7407l.pdf), this is capable of 2 A with reasonably low RDS(on). If it still proved too high an RDS(on), you could just parallel 2 of them. When your ATTiny goes to sleep, all the outputs will float and leakage current for the device is under 5 uA.

• Thanks for the reply. I don't see why to change the ULN for the TPL. ULN start at 3v which is why I expect as lowest input voltage. The P-Channel solution seems good, I have seen some post on the construction in stackexchange. Does the NDP6020P is a good one (MIC is a small packaging) ? For ATTiny I will work on it after I fixed the 0.8mA problem :) – TrapII Dec 4 '16 at 10:52
• @Trapll. Your drive voltage to the stepper mote seems critically low. The ULN3002 has a VCE(sat) of about 1.2-1.6 Volts. If you drive a stepper motor with a supply of 5 V then you are left with only 3.4 - 3.8 volts across each coil. The FET replacement has a max 600 mV VOL(VDS) and you could parallel two to get it even lower. – Jack Creasey Dec 4 '16 at 17:01
• Many thanks! So I tried with a P-channel MOSFET (I chose a NDP6020P) driven by a 2N2222A PNP transistor and it works very well. No current when low on my AVR and the motor just runs from 2xAA cells (3v) to 3xAA cells (4.65v max). I found a schematic here. Thanks a lot again !!! – TrapII Dec 9 '16 at 18:16