ELM327LP datasheet(87/94 Pages) ELM

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ELM327L

ELM327L DSA

Elm Electronics – Circuits for the Hobbyist

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Modifications for Low Power Standby Operation (continued)

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

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 24 mA to less than 1 mA

(and power from about 300 mW to 8 mW) - a

considerable

savings.

leave

any

further

improvements to you.

ELM327LP Datasheet(PDF) 87 Page - ELM Electronics

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