Bedienungsanleitung Elm 327 (Seite 84 von 94) (Englisch)

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Modifications for Low Power Standby Operation

If you wish to semi-permanently install your

ELM327 circuit in a vehicle, current consumption

quickly becomes an issue. A typical ELM327 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 generally not enough, however, as you

must also consider the current drawn by the other

components as well.

The following discussion takes the circuit of Figure

9, and modifies it slightly in order to highlight our

suggestions for reducing current consumption. The

resulting circuit (Figure 15) is shown following. Note

that portions of the circuit that are the same as Figure

9 are shown in grey, while the changes that we are

making are shown in black.

Consider the circuit of Figure 9 in the Example

Applications section. With 12V to 15V applied to the

‘Battery Positive’ input, the total current used by this

circuit is typically:

base current = 29.8 mA

Without making any wiring changes, you can

reduce this current by placing the ELM327 into the

low power standby mode. This only requires sending

the Low Power command (AT LP), after which the

current would typically drop to:

current after AT LP = 17.6 mA

This reduction is due entirely to the change in the

ELM327’s operating current (it only needs a very small

current to stay in standby mode). 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 IC, U2. By disconnecting the common

connections from R24 and R31, and then returning

both to pin 16 of the ELM327, we can switch the

current that these two use. With this change (which is

shown as modification #1 in Figure 15), the current

after AT LP drops further:

current after mod #1 = 8.2 mA

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 7805) 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 changing U3 for an LP2950, the current is

typically:

current after mod #2 = 3.5 mA

If we continue to reduce the load currents beyond

this point, we will 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 too high, we recommend that

you add either a zener diode or a transient voltage

suppressor (TVS), directly across the 5V supply, as

shown in the top right corner of Figure 15. Suggestions

for devices to consider are the 1N5232B zener diode

or the SA5.0AG TVS.

Another integrated circuit that is not changing

current during low power mode is the 317L (U4). In

fact, a quick calculation shows that it is likely using

about 2.6 mA when idle, which is very significant. If we

replace this IC with another that uses less current, we

will be close to getting the total circuit current to less

than 1 mA.

Figure 15, modification #3 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 further reduce current when it is not required. 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.

Note that the LP2951 regulator also requires a

84 of 94

ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

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