AND8377/D AMIS-30660 - Power Dissipation in Case of Bus Failure http://onsemi.com 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. http://onsemi.com 2 AND8377/D 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. 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