23 / 84
Figure 1: ADP5301 functional block diagram
data to be transferred as much as
possible. To demonstrate how much
energy can be lost, if four pins have
a capacitance of 5pF in a system
running at 20MHz from a 3.3V
supply, 660μA will be drawn from
just pin capacitance. This figure can
be determined from the equation
I = 0.5CVf. The current drawn will
be the total of both the data sent
and received, which can mount
up. This figure can be cut by using
a highly integrated chip. Internal
communications don’t suffer from
pin capacitance, therefore having
more peripherals on-board is better
from a power consumption point-of-
view. On-chip RAM and flash memory
offer the same power savings.
An Efficient Power
Supply
Choosing a switching regulator for a
switched mode power supply is a key
factor in maximizing efficiency. This
is particularly true for synchronous
regulators where efficiencies of
over 95% are possible. However,
it is not just headline efficiency,
or even standby efficiency, that is
necessarily the most critical factor. It
is necessary to look at the current
in different modes for the device
and determine the contribution to
overall power consumption from
each mode after taking into account
the switching regulator efficiency
at each current level. There are
some quite impressive regulators
around though, such as the new
Analog Devices ADP5301 Step-down
Regulators.
The quiescent current of these
devices is down as low as 180nA
when not-switching, but still
operating in hysteresis mode. It
will switch for a short burst to add
charge to the output capacitor using
the inductor at very light loads, then
return to just the quiescent current.
The low quiescent current can give
efficiencies as high as 80% at 1μA
depending on the input and output
voltages. It is more likely that there
will be lower figures in practice than
this optimum value, but they should
still be above 40%. The devices also
deliver up to 0.5A and have a single
pin programmable output with a
fixed resistor. These figures are
very impressive compared to older
regulators, which would take a few
milliamps with no load.
If you are using a switching regulator
with an external MOSFET, bear in
mind that the MOSFET switching
time can result in significant losses.
The transition from non-conducting
to conducting is the time when a
switching MOSFET dissipates the
most power. When it is turned fully
on, the voltage drop will usually
be very small and hence power
dissipation will be low. However,
partly turned on there will be a
significant voltage drop across the
MOSFET accompanied by significant
current. You therefore want to
minimize the time that the transistor
spends in that state by choosing a
fast switching device and low gate
capacitance. Low ON resistance is a
must.
Power Supply Shutdown
You can keep power supply
capacitors small if power supplies
are shut down in sleep mode. It
takes energy to charge them and if
the power supply is shut down when
in a sleep mode then the energy in
the capacitors is normally wasted.
For example, a 1μF capacitor on
the power supply of circuitry which
is shut down 100 times per second
will consume 165μA at 3.3V (same
calculation as before). Many ICs
will take less than that in shutdown
or sleep mode, so it is often better
to keep circuitry powered but in
a sleep state than to actually do
power switching to save power. The
exception to this advice would be if
the device used didn't have a sleep
mode or if its sleep mode was not
New-Tech Magazine Europe l 23