Hello,
I'm planning on using an Arduino on battery (4.5V from 3AA), and also power a RFM69HW wireless module using a 3.3V regulator (MCP1702).
The RFM69HW has a "sleep mode" where it only consumes 5uA. When transmitting, it uses up to 150mA @ 3.3V.
The MCP1702 voltage regulator is rated for up to 250mA output, and has a low 2uA "quiescent current".
For my application, the RFM69HW is NOT transmitting 99.8% of the time. So it just sits idle most of the time, consuming that 5uA @ 3.3V from the regulator. How much overhead does the voltage regulator produce? When the regulator is supplying the 2uA to the wireless module, how much current does the regulator consume from the battery? Is this the 2uA "quiescent current"? Or some other current value?
I had assume that the regulator's "overhead" current would be dependent upon the load current. The more current you ask of the regulator, the more overhead it creates, so the more waste.
I'm trying to figure out if I need to switch power off from the regulator-wireless portion of the circuit But if the idle current for the whole thing is very very low, it's not worth adding more components.
Here are the datasheets if you would like to read it for yourself:
Read page 13 sect 5.0 - 5.3...
There are 2 places that both clearly state the minimums and recommendations if your load exceeds certain conditions
1 uF up to 100 mA
5.0 FUNCTIONAL DESCRIPTION
The MCP1702 CMOS LDO linear regulator is intended
for applications that need the lowest current
consumption while maintaining output voltage
regulation. The operating continuous load range of the
MCP1702 is from 0 mA to 250 mA (VR ? 2.5V). The
input operating voltage range is from 2.7V to 13.2V,
making it capable of operating from two or more
alkaline cells or single and multiple Li-Ion cell batteries.
5.1 Input
The input of the MCP1702 is connected to the source
of the P-Channel PMOS pass transistor. As with all
LDO circuits, a relatively low source impedance (10?) ]<---- This is pointing the requirement for a good quality capacitor when using batteries
is needed to prevent the input impedance from causing
the LDO to become unstable. The size and type of the
capacitor needed depends heavily on the input source
type (battery, power supply) and the output current
range of the application. For most applications (up to
100 mA), a 1 ?F ceramic capacitor will be sufficient to
ensure circuit stability. Larger values can be used to
improve circuit AC performance.
5.2 Output
The maximum rated continuous output current for the
MCP1702 is 250 mA (VR ? 2.5V). For applications
where VR < 2.5V, the maximum output current is
200 mA.
A minimum output capacitance of 1.0 ?F is required for
small signal stability in applications that have up to
250 mA output current capability. The capacitor type
can be ceramic, tantalum or aluminum electrolytic. The
esr range on the output capacitor can range from 0? to
2.0?.
The output capacitor range for ceramic capacitors is
1 ?F to 22 ?F. Higher output capacitance values may <--- and are recommended for high current loads
be used for tantalum and electrolytic capacitors. Higher
output capacitor values pull the pole of the LDO
transfer function inward that results in higher phase
shifts which in turn cause a lower crossover frequency.
The circuit designer should verify the stability by <----- And this section is pointing at the obvious... Test it using the recommended test
applying line step and load step testing to their system
when using capacitance values greater than 22 ?F.
5.3 Output Rise Time
When powering up the internal reference output, the
typical output rise time of 500 ?s is controlled to
prevent overshoot of the output voltage. There is also a
start-up delay time that ranges from 300 ?s to 800 ?s
based on loading. The start-up time is separate from
and precedes the Output Rise Time. The total output
delay is the Start-up Delay plus the Output Rise time.
When I read a data sheet these are the notes that I look for..
When using a battery there is an absolute requirement for a much larger input capacitor due to the increase in the battery ESR when the battery is in the discharged state. I typically will use a 1 uF ceramic or Tant cap with a 150 - 220 uF electrolytic and a 100 nF cap for HF noise. For currents up to .1 A (100 mA) and a 470 to 1000uF cap up to about 1/4A (250 mA).. My thoughts on this subject are very conservative...
That is I will always use more capacitance than what is recommended because in my experience batteries are generally going to fail at the worst possible time... and capacitors do change over time and temperature.... Not good changes either..
The "quiescent current" is the power drawn by the regulator just to keep the regulator itself running. In other words it will always be consuming that 2uA in addition to whatever current it is supplying to your transmitter (5uA + 2uA = 7uA in sleep mode).
It's a puny amount of power; 35 years from 2200mAH, AA batteries -- assuming the batteries had that long of a life.