Hardware -- Voltage regulator

A project log for Heartbeat Logger

A portable device that logs a snippet of your heart at the push of a button.

Ole Andreas UtstumoOle Andreas Utstumo 02/15/2016 at 22:180 Comments

The regulator must regulate from a 3.7V nominal battery voltage down to 3.3V. Since this system is sensitive for noise, a linear regulator is preferred over a switch mode. Regulating from 3.7V to 3.3V yields a efficiency of eff. = 3.3/3.7 = 89 percent, which is close to what a switch mode regulator can offer anyways. There are three important parameters for choosing the right regulator for this system:

Looking at the MCP1700T used in the project, the specs are:

First off, let's do a quick calculation of the required current load. You'll find these values in the datasheets and by calculation:

250mA is more than twice that's nedded. Neat!

The dropout voltage is the smallest difference between Vin and Vout where the regulator will still regulate properly. Looking at a typical discharge curve of a single cell li-ion battery, we see why this is of importance. The battery discharge will approximately follow the red line:To make use of as much battery capacity as possible, the dropout voltage should be minimal.
There comes a point when the battery voltage is so low that the regulator will stop outputting 3.3V, which is Vout + Vdrop. The MCP1700T had one of the lowest dropout voltages in’s catalogue. A dropout voltage of 0.178V means that it will stop regulating at 3.3V + 0.178V = 3.478V. At the specified current of 250mA, that is. This dropout decreases pretty linearly with the current load, as can be seen in the figure below, which, with our ~110mA current load, will halve the dropout voltage and even more of the battery can be used!

In the case with the battery in the discharge curve, somewhere between 75-95 percent of the battery’s capacity can be within reach.

The quiescent current is also of importance, as the regulator will be active as long as there is
a battery connected. The chosen regulator's 1.6µA and microcontroller's typ 2.46 uA in Standby mode will enable the device to be "off" for years.