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Elm 327 Manual

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Modifications for Low Power Standby Operation
If you wish to semi-permanently install your
ELM327 circuit in a vehicle, current consumption
quickly becomes an issue. A typical ELM327 circuit
draws about as much current as a dome lamp, so can
not be left on for long without the vehicle’s engine
running.
When it is not needed, the ELM327 may be placed
in a low power ‘standby’ mode in which it consumes
very little current. Reducing the integrated circuit
current itself is generally not enough, however, as you
must also consider the current drawn by the other
components as well.
The following discussion takes the circuit of Figure
9, and modifies it slightly in order to highlight our
suggestions for reducing current consumption. The
resulting circuit (Figure 15) is shown following. Note
that portions of the circuit that are the same as Figure
9 are shown in grey, while the changes that we are
making are shown in black.
Consider the circuit of Figure 9 in the Example
Applications section. With 12V to 15V applied to the
‘Battery Positive’ input, the total current used by this
circuit is typically:
base current = 29.8 mA
Without making any wiring changes, you can
reduce this current by placing the ELM327 into the
low power standby mode. This only requires sending
the Low Power command (AT LP), after which the
current would typically drop to:
current after AT LP = 17.6 mA
This reduction is due entirely to the change in the
ELM327’s operating current (it only needs a very small
current to stay in standby mode). But where is the
other current coming from? One obvious load is the
LED that shows that the power is on. The other is the
CAN transceiver IC, U2. By disconnecting the common
connections from R24 and R31, and then returning
both to pin 16 of the ELM327, we can switch the
current that these two use. With this change (which is
shown as modification #1 in Figure 15), the current
after AT LP drops further:
current after mod #1 = 8.2 mA
There is a considerable current still flowing in the
circuit at this point, but it should mainly be the voltage
regulators that are using it. In the next step, we will
change U3 (a 7805) to an LP2950ACZ-5.0G, and see
how effective that is. While the LP2950 is a good
choice for its lower quiescent current, it does suffer
from stability problems if you do not provide capacitive
loading as shown. Note that the 4.7uF capacitor is
tantalum, while the 2.2µF one is aluminum. At this
point it may also be useful to review our Application
Note ‘AN03 - ELM327 Low Voltage Resets’, as you
may want to use an even larger capacitor on the 5V
side. After changing U3 for an LP2950, the current is
typically:
current after mod #2 = 3.5 mA
If we continue to reduce the load currents beyond
this point, we will quickly get to a point where any
currents injected from external sources (ie. through the
protection diodes at inputs such as pins 2 or 12) will
become significant compared to the load currents. If
these currents should exceed the load current, the Vdd
voltage will rise and damage might result. To prevent
the voltage from rising too high, we recommend that
you add either a zener diode or a transient voltage
suppressor (TVS), directly across the 5V supply, as
shown in the top right corner of Figure 15. Suggestions
for devices to consider are the 1N5232B zener diode
or the SA5.0AG TVS.
Another integrated circuit that is not changing
current during low power mode is the 317L (U4). In
fact, a quick calculation shows that it is likely using
about 2.6 mA when idle, which is very significant. If we
replace this IC with another that uses less current, we
will be close to getting the total circuit current to less
than 1 mA.
Figure 15, modification #3 shows an LP2951ACM
regulator in the circuit, as a replacement for the 317L.
It uses much less current than the 317 during normal
operation, and offers a shutdown control input as well
to further reduce current when it is not required. Note
that the LP2951 circuit needs the ELM327 to provide a
high level at pin 3 for a 5V output, and a low for 8V, so
an inversion is needed. To do this, set PP 12 to 00
with:
>AT PP 12 SV 00
>AT PP 12 ON
then reset the chip, and the voltages will always be
correct for J1850 from that point on.
Note that the LP2951 regulator also requires a
84 of 94
ELM327
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