Battery Operated Latching Power Switch

Sometimes you need a high side latching power switch that will work well with low voltage, such as that produced by batteries. Two AAA or AA battery cells in series produce, depending on the state of charge and battery chemistry, between 1.8 and 3.5 volts. The MOSFET switch circuits shown on the following pages operate well with voltages from 5V to 18V, but will fail to switch on reliably at lower voltages:

• Latch and Toggle Power Circuits;
• Limiting In-rush Current in MOSFET Power Switches; and,
• Controlled Turn-ON and Shutdown of Microcontroller Products.

Their voltage limitation is entirely the result of their gate threshold voltage, V_GS-TH. These circuits use the IRF7319 MOSFET pair, whose transistors start turning on at V_GS-TH=1V (for only 250 uA current) and are strongly ON at V_GS=3V. You'll notice from their datasheet that at V_GS=3V they can pass more than an amp with an ON resistance of less than 0.1 ohm. So, they are strongly ON at 3V. However, you'll notice that the MOSFETs datasheet doesn't specify where in that 1V to 3V gap they start turning ON strongly. The datasheets show graphs with VGS starting only at 3V. At that voltage, the R_DS-ON starts climbing very quickly as V_GS decreases, for V_GS < 3V.

You can extend the range of those circuits to lower input voltages by choosing appropriate MOSFET transistors, for example the Si3588DV MOSFET pair. It turns on fully at V_GS ≥ 1.5V (and the datasheet graphs show the curves at VGS as low as 1.5V) but can't pass as much current as the IRF7319. Further, it's maximum V_GS is 8V, so the battery voltage must be less than that.

The circuit of Figure 1, using the Si3588DV MOSFET pair, works well with input voltages from 1.5V to 7.5V, making it suitable for use with two, three or four alkaline or NiMH cells in series, one nominal 3.7V cobalt-based lithium-ion cell (with charge voltage up to 4.2V), or two nominal 3.2V LiFePO₄/LFP cells in series with charge voltage up to 7.2V.

This circuit exhibits a Press-ON-Hold-OFF action, where a short press turns power to the electronic load ON, but a longer button hold is needed to toggle it back OFF.

You can use the Si3588DV MOSFET pair in the other high-side switch circuits with limited in-rush current, microcontroller turn-OFF, or toggle behavior shown on the pages cited above, provided you keep in mind the MOSFET's maximum safe V_GS of 8V.

Note that as of 2014 the Si3588DV appears to be a discontinued part. The IRF7317PbF MOSFET pair may also be used for low-voltage applications.