Operation Manual

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ELM327
ELM327DSJ Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
Modifications for Low Power Standby Operation (continued)
4.7uF tantalum capacitor for stability (and it helps with
transient capability too).
Leaving U4’s pin 3 solidly connected to circuit
common, we now find that the current is about:
current after mod #3 = 1.1 mA
and, if we tie pin 3 of the LP2951 to pin 16 of the
ELM327, the LP current becomes:
current after mod #4 = 0.9 mA
We can suggest one more change at this point.
The MCP2551 shown draws about 0.3 mA when in
standby mode, and this can be improved upon. By
replacing this chip with the newer MCP2561, the
standby current will be reduced even further. Typically,
you will see a change of at least 0.2 mA, giving:
current after mod #5 = 0.7 mA
The current has been reduced considerably
through the circuit modifications, but 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 MCP2561 and the ELM327 are never completely
shut off, but are instead in a low power mode, while
the LP2950 regulator is operating normally (as the
MCP2551 and the ELM327 need it). You can not do
anything about that.
There are some currents that you may reduce, by
choice in your design. The R25/R26 resistor pair uses
current, but do you really need to monitor battery
voltage? Similarly, the R20/R21 pair passes current,
but does your application need ISO9141 or ISO14230
support? The OBD Tx LED flashes when in Low
Power mode, but do you need it to? (You may turn it
off with PP 0F b0, but it only uses about 25 µA on
average). All of these little currents eventually add up
to what we’ve measured here.
These few changes that we have shown have
reduced the total current from about 30 mA to less
than 1 mA (or power from 358 mW to 8 mW) - a
considerable savings, which is enough for most
applications. We leave any further improvements to
you.