ELM329LP datasheet(80/87 Pages) ELM

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ELM329L

ELM329L DSA

Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

Modifications for Low Power Standby Operation

Using an ELM329L integrated circuit rather than

an ELM329 offers several advantages. One of these is

the ability to operate at lower power supply currents.

This may not always be of primary concern, but if you

are considering building a device that may stay

connected to your vehicle, you should consider this, as

excessive power drain during vehicle off times will

cause battery problems.

The following discusses some of the design

decisions that we made in creating the suggested

circuit of Figure 9. That schematic has been repeated

in Figure 15, with the main changes highlighted with

numbered ‘bubbles’.

The first decision was to use an ELM329L rather

than an ELM329. This results in both lower operating

currents and lower idle current (when in low power

mode). The circuit does require an extra component

(C9) in order to do this.

Bubble 2 highlights our use of higher value voltage

divider resistors. A 47K/10K divider would continuously

use about 0.2 mA, which is significant when in low

power mode. We chose to increase these resistor

values by a factor of 10, in order to reduce the current

by that same factor. The cost is a slight degradation in

the accuracy of the voltage readings dues to the input

leakage currents. If you do not wish to measure the

battery voltage, you may wish to eliminate R10, R11

and C10 altogether. Don’t forget to tie pin 2 to either

VDD or circuit common if you do.

The next bubble (‘3’) shows our use of a low

quiescent current regulator rather than a typical 7805

type unit. This decision saves several mA of current

that is required to maintain bias for the internal circuit.

There is a cost with this however - the regulator is not

stable without a minimum load capacitance (so you

have to add C4).

Bubbles ‘4’ and ‘5’ show how we have returned

both R6 and R9 to pin 16 rather than circuit common.

This allows the ELM329L to turn off the PWR led, and

reduce

the

CAN

transceiver

current

while

the

ELM329L is dormant. Be careful doing this however, if

you wish to have the circuit wake up on CAN activity -

some manufacturer’s transceivers do not pass on the

received CAN signal when they are in low power

mode.

The final change (bubble ‘6’) shows our choice of

MCP2561

CAN

transceiver

rather

than

MCP2551. The ‘2561 device is an improved version

that is recommended for new designs.

To see just how effective these changes are, we

made a few in-circuit measurements, and present

them here. With 12.0V applied at ‘Battery Positive’, the

measured current was typically:

base current (on the bench) = 16.7 mA

when simply powered on the bench, with no PC or

ECU connected.

If you then connect it to a vehicle and a computer,

the current typically rises to:

base current (in the vehicle) = 23.5 mA

which is largely due to the Active LED being on. When

actually monitoring data, this current rises further, as

the OBD Rx and RS232 Tx LEDs are on, and there

are switching losses. We measured :

active current (in the vehicle) = 36.5 mA

in that case.

The “Low Power Mode” of operation section (page

62) discussed the ways in which you might initiate low

power operation, but the easiest is to use the low

power command (AT LP). After sending this, the total

circuit current for is typically:

current after AT LP = 0.19 mA

Out of interest, if the MCP2561 is replaced with

the older MCP2551, low power current is typically:

current after AT LP (with MCP2551) = 0.44 mA

which is low, but not quite as good as with a newer

MCP2561.

The 0.19 mA measured is a very low current, and

is likely all you need for most applications. Note that

whether the Active LED is set to flash or not (by PP 0F

or pin 11) has very little influence on this current as it

uses an average of about 25 µA. Similarly, the CAN

Monitor typically only uses about 20 µA during low

power operation, so does not appreciably affect the

total current.

It is difficult to reduce the standby current further

than this, but for almost all applications reducing the

10’s of mA constant draw to a fraction of a milliAmp

should be more than sufficient.

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