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.
- 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
- LEDs: about 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.