### Power Dissipation in Case of Bus Failure

```AND8362/D
Power Dissipation in Case
of Bus Failure
Prepared by: Peter Cox
ON Semiconductor
http://onsemi.com
Recessive State
AMIS−30660 High Speed CAN transceiver is designed to
withstand bus failures. Without any damage to the IC the
CANH or CANL line may be shorted to ground, VCC or the
battery supply. However in some bus failure conditions an
increase in power dissipation might occur. This will lead to
a rise in junction temperature.
Two bus states can be distinguished: recessive and
dominant. In both states both CANH and CANL can be
shorted to GND, VCC or VBAT. In this application note we
are investigating the worst case conditions therefore short to
VCC is not discussed.
APPLICATION NOTE
In the recessive state TxD = 1 and both CANH and CANL
drivers are disabled. The figure below illustrates the
equivalent schematic. RBUS is the total impedance of the
(split) termination on both end−sides of the CAN bus. The
typical value is 60 W. Ri,cm is the common mode input
impedance with a typical value of 25 kW. VCC is the 5 V
supply. Without power (VCC = 0 V) the common mode
voltage is still kept by a passive clamp but can be higher than
VCC/2. This particular condition is not taken into account in
the calculations.
V CC
3
AMIS−30660
7
COMP
CANH
R i,cm
V cc / 2
+
R BUS = RT / 2
R i,cm
6
CANL
2
PC20040825.1
GND
Figure 1. Equivalent Schematic in Recessive State
The power dissipation for the different bus−error conditions is given in the table below.
Table 1. POWER DISSIPATION FOR CAN−BUS ERRORS IN RECESSIVE STATE
Short to
BUS
GND
CANL
P [+
CANH
P [+
© Semiconductor Components Industries, LLC, 2008
December, 2008 − Rev. 0
VBAT
V CC 2
P [+
2R i,cm
V CC 2
P [+
2R i,cm
1
2(V BAT * V CCń2) 2
2R i,CM
2(V BAT * V CCń2) 2
2R i,CM
Publication Order Number:
AND8362/D
AND8362/D
Calculated for VCC = 5 V, VBAT = 24 V, Ri,cm.= 25 kW and RBUS << Ri,cm yields in:
Table 2. CALCULATED POWER DISSIPATION FOR CAN−BUS ERRORS IN RECESSIVE STATE
Short to
BUS
GND
VBAT
CANL
0.5 mW
37 mW
CANH
0.5 mW
37 mW
Dominant State
In dominant state TxD = 0 and both drivers are active. In case of a short circuit the currents for both CANH and CANL are
limited to Io(sc) which is 120 mA in worst case condition. The figure below illustrates the equivalent schematic.
V
AMIS−30660
I
CC
3
V BAT
o(sc)(CANH)
7
CANH
R
COMP
BUS
V BAT
6
CANL
I
1
o(sc)(CANL)
2
2
3
4
PC20040825.2
GND
Figure 2. Equivalent Schematic in Dominant State
The power dissipation for the different bus−error conditions is given in the table below.
Table 3. POWER DISSIPATION FOR CAN−BUS ERRORS IN DOMINANT STATE
Short to
GND
BUS
VBAT
• See Figure 2 Case (1)
• Bus communication possible but with
CANL
bit timing limitations
♦
♦
P+
No communication possible
Time−out by master
V O(dom)CANH(V CC * VSubO(dom)CANH)
P + V BAT @ IO(sc)(CANHL)
R BUS
• See Figure 2 Case (3)
• Both CANL / CANH are on GND
CANH
• See Figure 2 Case (2)
• Both CANL / CANH are on VBAT level through RBUS
level through RBUS →
♦ No communication possible
♦ Time−out by master
• See Figure 2 Case (4)
• Bus communication possible but with bit timing
limitations
P + V BAT @ I O(sc)(CANL) * R BUS @ I O(sc)(CANL) 2
P + V CC @ I O(sc)(CANH)
http://onsemi.com
2
AND8362/D
Calculated for VCC = 5 V, VBAT = 24 V, RBUS.= 60 W, Io(SC)(CANL) = 120 mA, |Io(SC)(CANH)| = 95 mA and
Vo(dom)CANH = 3.6 V yields in:
Table 4. CALCULATED POWER DISSIPATION FOR CAN−BUS ERRORS IN DOMINANT STATE
Short to
BUS
GND
VBAT
CANL
84 mW
2.88 W (Note 1)
CANH
475 mW
2.02 W
1. Because no communication is possible, the master (depending on the application software) will cease the communication (= permanent
recessive state) and the dissipated power drops to 37 mW.
Average Power Dissipation and Related Increase in
Junction Temperature
power dissipation is 1.01 W (neglecting the 37 mW
dissipation in recessive state)
The thermal resistance of the package is 150 K/W in free
air. Soldered on a 2 layer PCB Rth(vj−a) < 100 K/W is
expected. Calculating with 100 K/W yields in a worst case
expected temperature increase of 101°C.
The worst case condition from application point of view
is a short to VBAT on the CANH pin in dominant state.
Communication is still possible but the dissipation is 2.02 W
giving the boundary conditions as stipulated in Table 4.
Calculating with a duty cycle of 50% (meaning 50% of the
transmission time the bus is in dominant state) the average
ON Semiconductor and
are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice
to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability
arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.
“Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All
operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights
nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications
intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. Should
Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates,
and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death
associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal
Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.
PUBLICATION ORDERING INFORMATION
LITERATURE FULFILLMENT:
Literature Distribution Center for ON Semiconductor
P.O. Box 5163, Denver, Colorado 80217 USA
Phone: 303−675−2175 or 800−344−3860 Toll Free USA/Canada
Fax: 303−675−2176 or 800−344−3867 Toll Free USA/Canada
Email: [email protected]
N. American Technical Support: 800−282−9855 Toll Free