Power sources for embedded hardware

1. Designed and Presented by
Anand K

2.  The AC voltage from wall outlets must be converted
to DC voltage of significantly lower magnitude.
 We can use DC “lab” power supply, standard PC
supplies, or AC adapters.
 AC adapters a.k.a plug packs.
 Typically they will provide output voltage somewhere
in the range +5VDC to +12VDC and can supply a
current of few mA.
 One warning with plug packs is the polarity of the
connector.
 Always check the technical data sheet for polarity.

3.  A better way is to incorporate a bridge
rectifier as part of the design.
 That way, polarity of the power source makes
no difference.
 The input power is DC, but the polarity of the
connection makes no difference.
 The ES uses the output of the rectifier as its
power source and has a internal voltage
regulation.

4.  An embedded computer may need only an
average supply of 20mA but may require as
much as 100mA at peak loads.
 This is especially true of systems using flash
memory, which may require high currents
during write operations.
 Thus the battery for such a system must be
able to supply not just the continuous load,
but also the peak load when required.

5.  RISC processors often have lower power consumption than CISC
processors, hence used in low power applications.
 The PIC and AVR microcontrollers can have current draws of less
than 5mA.This is considerably less than the 35mA used by the
68HC11 microcontroller.
 Many memory chips and peripherals will enter low power mode
when they are not in use.
 Others may be placed in low power mode by toggling a digital
input or by an appropriate software command.
 The power consumption of some devices can be reduced even
further by turning them off when not in use.
 If the processor is executing code from RAM and outputting data
to a serial port , then the power to the ROMs and any other I/O
devices may be turned off since they are not in use.

6.  Implementing this requires separate power
sources for the chips that are to be disabled,
switched via software control.
 Some voltage regulators have shutdown
inputs , allowing the subsystem they are
powering to be turned off.
 Further, some low power devices(sensors)
may need little current, so little that they can
be powered from the I/O line of the
microcontroller.

7.  A voltage regulator is a semiconductor device that
converts an DC voltage to a fixed DC voltage.
 Provide constant supply voltage within a system.
 A fixed operating voltage is necessary for devices such as
ADCs, since they use the internal power supply as
reference.
 The output of the sensor is sampled as a percentage of the
the voltage supply of the ADC, if the supply is not a known
voltage, then sampling performed by ADC is meaningless.
 Voltage regulator provides constant voltage source.
 Voltage regulator can also help in removing power-supply
noise.
 Can also degree of protection and isolation for the
embedded system from the external power supply.

8.  If the system is operating from a battery, the
varying current draw of the system can
combine with battery’s internal resistance to
create a varying supply voltage.

9.  Regulators are normally termed DC-DC
converters.
 Three types of DC-DC converters:
 Linear regulators, provide lower voltage than
supply voltage
 switching regulators, step up or step down or
invert the supply voltage.
 Charge pumps, which can also step up, step down
or invert the supply voltage, but with limited
current drive capability.

10.  Select a regulator that can supply appropriate
output voltage and current needed by your
embedded system, yet has the lowest quiescent
current.
 Linear regulators – small, low-noise, easy to use,
and cheap.The inputs and outputs are filtered
using decoupling capacitor, but beyond that, no
external components are required.
 The capacitors also help to remove momentary
glitches in the power source known as
Brownout.

11.  They switch a power transistor at their output.
 More efficient – waste less power during the conversion process.
 Drawback – require more external components(like inductors and
diodes), and therefore take up more space.
 Typically cost more and generate more noise than linear
regulators.
 More versatile than linear regulators.
 Eg. A switching regulator can take a supply voltage of 3.6V from a
battery and provide you with a regulated 5V supply. Alternatively,
a switching regulator may take unregulated 8V and convert this to
a regulated -12V supply.
 Careful understanding about switching regulator and their
characteristics is necessary, and must be comprehended from the
datasheet.

12.  Performs just like switching regulators but
they require no external inductor.
 They are commonly used due to their limited
capacity to supply current
 MAX3222 use internal charge pumps to
generate +12V and -12V required for RS232
level shifting.

13.  Commonly used linear regulators.
 Come inTO-220 package
 LM7805 and LM7812 gives a regulated 5V and
12V output.
 They provide output current upto 1A with a
quiescent current of between 5mA and 8mA.
 They also feature overload and short circuit
protection.
 Simple to use.
 Decoupling capacitors are required on the input
(pin 1) and output(pin 3), pin 2 is connected to
ground.

14.  Far less quiescent current than LM78xx regulators and
are ideal for lower power systems.
 They are available in tiny SO-8 or in standard DIP
packages and require only two external components.
 can provide up to 500mA and can operate from an
input voltage between +2.7V and +11.5V.
 Have built in protection in case you inadvertently
switch power and ground and consume as little as
15µA of current for their own use.
 Ideal for low power embedded applications.