What is it? This is a small (6.5 mm x 15.4 mm x 0.8 mm) add-on that allows 1S LiPo battery voltage monitoring on development boards using one analog input (ADC) and one digital control pin. The bat...Read More…
This is a small (6.5 mm x 15.4 mm x 0.8 mm) add-on that allows 1S LiPo battery voltage monitoring on development boards using one analog input (ADC) and one digital control pin. The battery voltage divider resistors (27 K/100 K) are sized for a maximum 4.2 V LiPo battery such that the maximum 12-bit ADC count at 4.2 V will be 4095 (4.2 / 1.27 = 3.3 V). The ADC input (SNS) can be disabled by setting the control pin (CTL) LOW, shutting off all current (<< 1 uA) to the voltage monitor circuit. Thus, the battery voltage can be sampled at a duty cycle chosen by the user without wasting a lot of power by having current unnecessarily flow between the battery anode and ground through the voltage divider resistors. This current is (~4 V / 127 K Ohm) ~ 32 uA which would otherwise be a continuous drain without the n-type and p-type MOSFET enable control designed into this circuit.
The board is designed to be as small as possible and will mount onto the six bottom left pins of many popular development boards; those boards with the GPIO pin order VIN, GND, X, X, analog pin, digital pin. This includes,among others:
The add-on is generally usable especially if you are willing to simply wire the voltage monitor into your project.
In many applications, development boards serve as prototype platforms for portable and wearable devices, usually powered by small 1S LiPo batteries. It is useful to know what the state of charge of the battery is while the device is in use, especially for critical applications where unexpected power loss might cause embarrassment or worse. This add-on was designed as a standard voltage divider to drop the maximum 4.2 V LiPo battery voltage to the 3.3 V maximum analog voltage range of most popular development boards.
In the simplest voltage monitoring circuit, the divided voltage is simply sampled by the ADC. However, in this case the battery voltage is always shunted to ground by the combined resistance, here 127 KOhm, meaning there is a constant current drain of ~32 uA; the sleep current of any device employing such a scheme will be >32 uA. Not very practical in a world where many MCUs (Grasshopper, Butterfly, Ladybug, etc) have sleep currents < 2 uA.
Why not just use higher resistances, like 270 K/1000 K or more? Well, ADCs are charge accumulators so if the charge flow rate (current = V/R) is lowered it simply takes more sample time to estimate the voltage accurately. So while we have employed this method with an increased sampling time with success, even in this case we are still continuously burning ~3 uA, more than doubling the device sleep current. Pushing the resistances up even further pushes the limits of what most MCU ADCs can do, even with the maximum sampling time. There has to be a better way.
The solution is to use a p-type MOSFET to allow the battery voltage to be connected or disconnected from the voltage divider circuit to limit the current usage to only those times when the battery voltage is going to be measured, maybe once an hour or so, etc.
In this add-on we use a slightly more complicated circuit. We use a DMC2990UDJ dual n/p-type MOSFET. The p-type MOSFET controls when the battery voltage is available on the voltage divider and the n-type MOSFET controls the gate on the p-type MOSFET. This means that one digital GPIO from the MCU controls whether the battery voltage measurement circuit is enabled and the battery voltage available for sampling by the analog input GPIO of the MCU or whether the battery voltage is isolated from the resistor network and MCU ADC. Thus, the voltage divider can be chosen at a reasonable value (27K/100K) to allow the ADC to accurately sample in a small time interval (~10 us) the stepped down battery voltage. The result is an ultra-low-power battery voltage monitor that doesn't clobber the ultra-low sleep currents of modern MCUs.
This is a super small add-on that can be used with almost any project to allow 1S LiPo battery voltage monitoring with very little power usage. It is perfect for all wearable and portable devices powered by small 1S LiPo batteries.
Order the pcbs from OSH Park and assemble some of your own or buy the fully assembled and tested boards from me and see how lowest-power battery voltage monitoring can improve your project!
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