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ELM327DSH Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
Modifications for Low Power Standby Operation
The ELM327 may be placed in a low power
standby mode in which it consumes very little current.
This will find its greatest use with semi-permanent
vehicle installations where you want the current
consumption to be as low as possible (ideally zero)
when the ELM327 is not needed.
The following uses the Example Application circuit
of Figure 9, and modifies it slightly in order to show a
few ways of further reducing current consumption. The
resulting circuit (Figure 12) is shown on the next page.
Note that portions of the circuit that are the same as
Figure 9 are shown in grey, while the changes that we
are suggesting are shown in black.
To know how effective the changes are, we need
a base current reading for what the entire circuit
consumes. With 12.0V applied as ‘Battery Positive’,
the measured current for Fig. 9 is:
base current = 31.9 mA
Without making any wiring changes, you can
reduce this current by sending the Low Power
command (AT LP). The resulting current is:
current after AT LP = 19.7 mA
This change represents almost all of the ELM327’s
current (it needs a little in order to stay in standby). 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, U2. By disconnecting the
common connections from R31 and R1, and then
returning both to pin 16 of the ELM327, we can switch
the current that these two use. With this change
(shown as modification #1), the LP current becomes:
current after mod #1 = 10.3 mA
If we continue to reduce the load currents, we can
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, we recommend that you add either a zener
diode or a transient voltage suppressor (TVS) as
shown, directly across the 5V supply. Suggestions for
devices to consider are the 1N5232B zener diode or
the SA5.0AG TVS.
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 78L05) 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 this change:
current after mod #2 = 6.2 mA
That worked well. We’ll now change the 317L
regulator (U4) for an LP2951ACM, but leave it’s pin 3
solidly connected to circuit common. Again, the 4.7uF
tantalum capacitor is required for stability (and it helps
with transient capability too). Note that the circuit
shown 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 measure the new current
when in LP mode:
current after mod #3 = 1.2 mA
OK. Now we tie pin 3 of the LP2951 to pin 16 of
the ELM327, so that the regulator can be disabled
entirely. The LP current is:
current after mod #4 = 1.0 mA
Why is there still current flowing? This is due to a
number of things, some that you can change, others
which you can not. The MCP2551 and the ELM327
are not completely shut off, just in low power mode.
The LP2950 regulator is operating normally, as the
MCP2551 and the ELM327 need it. There are currents
flowing into pin 3 of the LP2951, and through both the
R34/R35 and R20/R21 pairs. All of these little currents
eventually add up (to the 1 mA).
These few changes have reduced the total power
from 383 mW to 12 mW - a considerable savings. We
leave any more improvements to you…
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