AMIS-30660 - Power Dissipation in Case of Bus Failure

```AND8377/D
AMIS-30660 - Power
Dissipation in Case of Bus
Failure
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APPLICATION NOTE
Introduction
The power dissipation for the different bus−error conditions
is given in Table 1.
The 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.
Table 1. POWER DISSIPATION FOR CAN−BUS
ERRORS IN RECESSIVE STATE
Short To
Bus
GND
CANL
P[
CANH
P[
Recessive State
In the recessive state TxD = 1 and both CANH and CANL
drivers are disabled. Figure 1 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.
2R i,cm
V CC 2
2R i,cm
P[
P[
2ǒV BAT * V CCń2Ǔ 2
2R i,cm
2ǒV BAT * V CCń2Ǔ 2
2R i,cm
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
GND
VBAT
CANL
0.5 mW
37 mW
CANH
0.5 mW
37 mW
Bus
VCC
AMIS−30660
V CC 2
VBAT
3
Dominant State
7
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. Figure 2 illustrates the equivalent schematic.
CANH
Ri,cm
Vcc /2
+
RBUS = RT / 2
COMP
Ri,cm
6
CANL
2
PC20040825.1
GND
Figure 1. Equivalent Schematic in Recessive State
© Semiconductor Components Industries, LLC, 2009
January, 2009 − Rev. 2
1
Publication Order Number:
AND8377/D
AND8377/D
V
CC
3
AMIS−30660
I o(sc)
V BAT
7
CANH
R
COMP
BUS
V BAT
6
CANL
1
2
3
4
I o(sc)
2
PC20040825.2
GND
Figure 2. Equivalent Schematic in Dominant State
The power dissipation for the different bus−error conditions is given in Table 3.
Table 3. POWER DISSIPATION FOR CAN−BUS ERRORS IN DOMINANT STATE
Short To
GND
Bus
CANL
See Figure 2 Case (2)
Both CANL/CANH are on VBAT Level through RBUS →
No Communication Possible
Time−Out by Master
See Figure 2 Case (1)
Bus Communication Possible but with Bit Timing Limitations
P+
CANH
VBAT
(V CC * V O(dom)CANH) 2
P + V BAT @ I O(sc)
R BUS
See Figure 2 Case (4)
Bus Communication Possible but with Bit Timing Limitations
See Figure 2 Case (3)
Both CANL/CANH are on GND Level through RBUS →
No Communication Possible
Time−Out by Master
P + V BAT @ I O(sc) * R BUS @ I O(sc) 2
P + V CC @ I O(sc)
Average Power Dissipation and Related Increase in
Junction Temperature
Calculated for VCC = 5 V, VBAT = 24 V, RBUS = 60 W,
Io(SC) = 120 mA and Vo(dom)CANH = 3.6 V yields in:
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 .
Calculating with a duty cycle of 50% (meaning 50% of the
transmission time the bus is in dominant state) the average
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 two 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.
Table 4. CALCULATED POWER DISSIPATION FOR
CAN−BUS ERRORS IN DOMINANT STATE
Short To
GND
VBAT
CANL
108 mW
2.88 W (Note 1)
CANH
350 mW
2.02 W
Bus
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.
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