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). 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