ELM327_13 Datasheet, PDF (77/82 Pages) ELM Electronics – OBD to RS232 Interpreter

ELM327
Modifications for Low Power Standby Operation
If you wish to semi-permanently install your
ELM327 circuit in a vehicle, current consumption
becomes an issue. The typical 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 great, but is generally not enough - you
will still need to consider other ways to reduce power
consumption further.
Consider the circuit of Figure 9, in the Example
Applications section. With 12V to 15V applied to the
‘Battery Positive’ input, our tests showed that the total
current used by the circuit is typically:
base current = 25.7 mA
Without making any wiring changes, you can
reduce this current by sending the Low Power
command (AT LP). The resulting current is about:
current after AT LP = 3.6 mA
This large change is not only due to the drop in
ELM327 current, but is also from that of the MCP2551
and the ‘power on’ led (L5). All of these change
because of the connection to pin 16. While the 3.6 mA
represents a considerable reduction, it would be nice
to bring the total circuit current to less than 1 mA.
If you look at Figure 9, the one remaining IC that
does not change current during low power mode is the
317L (U4). In fact, a quick calculation shows that it is
likely using about 2.7 mA when idle. If we replace this
IC with another that uses less current, we will be close
to getting the total current to less than 1 mA.
Figure 14 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 minimize
current when it is not required. Using this IC as shown,
you should obtain a total circuit current of less than
1 mA.
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.
Reducing current further will require careful design
and parts selection. As a point to start, perhaps you
could find better choices for the voltage regulators,
and possibly look at different values for the R25-R26
and R20-R21 resistor pairs, maybe turn off the flashing
led while in low power mode, or else use a more
efficient version. There are many little currents that all
make up the total.
+12V
28 27 26 25
+5V
327
12345
U4
8765
LP2951
ELM327
pin 16
1234
47KΩ
+
4.7µF
16V
Q2
D2
22KΩ
R8
10KΩ
R7
4.7KΩ _Q1
4.7KΩ
R6
4.7KΩ
Figure 14. Replacing U4 with an LP2951
ELM327DSI
Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
77 of 82