ELM327P datasheet(70/76 Pages) ELM

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ELM327

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…

ELM327P Datasheet(PDF) 70 Page - ELM Electronics

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