MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 31

• Manufacturer Part Number: MCP6271R
• Description: 170 ?a, 2 Mhz Rail-to-rail Op Amp
• Manufacturer: Microchip Technology Inc.

Arming Yourself With nanoWatt Technology Techniques
By Bonnie C. Baker, Microchip Technology Inc.
We have come to expect and require more from our battery-
powered equipment. My first Personal Digital Assistant (PDA)
would only retain its battery power for a day. If the calendar alarm
was enabled, the battery power would quickly go to nothing.
Today, my PDA holds its battery charge for an entire week under
the same conditions. Both PDAs had the same battery chemistry,
Lithium Ion (Li Ion), with the same power density. So what has
changed? Simple. The hardware was improved and the power-
management techniques were refined. Battery improvements
were secondary.
PDAs are a great example of where system sophistication is
improving rapidly, but as fate would have it, the battery-powered
equipment requirements are increasing while battery chemistries
remain the same. You can accomplish this increase of
functionality only if the firmware/software engineers understand
the tools available in the microcontroller-based processing unit
and if the hardware designers understand the efficiency of the
solutions available on the market today.
Making Analog and Digital Play Together
You can accomplish an improvement in power consumption, and
consequently an increase in functionality, if you understand the
hardware options and the tools available in microcontrollers. One
dimension of power conservation is controlling the magnitude
of the power-supply voltage in your application. You are probably
interfacing to the real world at some point during your code
operation. If you are, you will have analog content in your
circuit. Analog power-supply requirements are higher than the
requirements for digital. Don’t forget that analog-noise margins
are much smaller than digital-noise margins. The analog noise
floor does not reduce with lower power-supply voltages. It stays
the same over power-supply-voltage changes. For instance, a
12-bit Analog-to-Digital Converter (ADC) can produce good, solid
conversions with a 5V supply. However, that same 12-bit ADC will
produce a smaller number of noise free-bits when you use a 2V
supply. This is because the least significant bites (LSb) size has
become smaller, but the magnitude of the noise is consistent.
The solution to this problem is to use higher supply voltages
when running analog and lower voltages during the digital-only
operation.
The diagram in Figure 1 shows a simple, microcontroller-
based, battery-operated system using in the PIC18F1320 Flash
microcontroller from Microchip Technology Inc. The PIC18F1320
has features, such as a variety of idle modes and a two-clock
start-up capability, that can enhance your low-power strategy.
On the hardware side, the industry is continuing to develop
classes of external peripherals as well as the internal
microcontroller peripherals, with lower power performance in
mind. In terms of the external peripherals to the microcontroller,
you can achieve lower power by reducing power-supply-voltage
requirements to the chips and optimizing topologies for the
lower-power jobs. This simple example (Figure 1) has low-power
operational amplifiers, an ADC and a regulated, adjustable
charge pump.
Miscellaneous Articles
Figure 1: The lower-power, external peripherals to the
microcontroller only provide half of this battery-powered
system implementation. The programmable capabilities of the
microcontroller allow control of the power-supply voltage in the
system (MCP1252-ADJ) and control of the microcontroller system
clock and sleep modes.
The design of the operational amplifiers in Figure 1 uses CMOS.
This type of operational amplifier is continuing to push minimum
power-supply voltage requirements down. The MCP6041 from
Microchip, in Figure 1, is a 14 kHz, 600 nA amplifier and requires
a supply voltage as low as 1.4V and up to 5.5V. The combination
of reduced supply-voltage and lower quiescent current provides
a good solution for power management concerns in battery-
operated equipment.
With internal or external integrated ADCs, the amount of power
dissipated is more dependent on the converter topology than on
IC design innovation. For instance, the ratio of conversion time-
to-current consumption in the SAR (Successive Approximation
Register) converter is considerably lower than in the delta-sigma
converter. You will probably use the SAR converter in battery-
powered applications, unless you need higher resolution and
accuracy.
The power supply in the circuit in Figure 1 is adjustable. A higher
voltage of 5V is best suited for analog circuitry and a lower
voltage of 2V is best suited for digital activities. The adjustable
power-converter in Figure 1 has high efficiency with low-output
currents and Li-Ion battery input voltages (4.2V down to 2.8V).
For these reasons, this circuit uses a regulated, adjustable
charge pump, DC/DC converter (MCP1252-ADJ).
Controlling the power-supply voltage for various operations is only
half of the story. If you really have a lower power “state of mind”,
you will want to power down some parts of the microcontroller
while letting other sections continue, to operate. As an example,
you can independently run an A/D or D/A conversion or the
USART communication interface, from the microcontroller. These
device functions may only need power locally.
Analog and Interface Guide – Volume 2
29