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 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:

• Input voltage range.
• The dropout voltage.
• Output current capacity.
• Quiescent current.
Looking at the MCP1700T used in the project, the specs are:
• Input Operating Voltage Range: 2.3V to 6.0V
• Low dropout voltage: 178 mV typical @ 250 mA for V OUT >= 2.8V
• 250 mA Output Current for output voltages >= 2.5V
• 1.6 µA Typical Quiescent Current

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

• Instrumentation amplifier: uA range.
• Operation amplifiers: uA range.
• Microcontroller: 5mA max
• Micro SD card: 100mA max
• Voltage dividers, switch: less than 1mA

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 Farnell.com’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.