TH8056 Datasheet, PDF (15/26 Pages) Melexis Microelectronic Systems – Enhanced Single Wire CAN Transceiver

TH8056
Enhanced Single Wire CAN Transceiver
3.9 Power Dissipation
The TH8056 has an integrated protection against thermal overload. If the junction temperature reaches the
thermal shutdown threshold the TH8056 disables the transmitter driver to reduce the power dissipation to
protect the IC itself from thermal overload. The function of the transceiver will become again available if the
junction temperate drops below the thermal recovery temperature.
To secure a stable functioning within the application and to avoid a transmitter switch off due to thermal
overload under normal operating conditions, the application must take care of the maximum power
dissipation of the IC. The junction temperature can be calculated with:
T_J = T_a + P_d * θ_ja
T_J
Junction temperature
T_a
Ambient temperature
P_d
Dissipated power
θ_ja
Thermal resistance
The Junction temperature shouldn’t exceed under normal operating conditions the limit specified in chapter
2.3 Static Characteristics.
The power dissipation of an IC is the major factor determining the junction temperature. The TH8056
consumes current in different functions. A part of the supply current goes to the load and the other part
dissipates internally. The internal part has a constant passive part and an active part which depends on the
actual bus transmission. The complete internal part causes and increasing of the junction temperature.
P_tot = P_{INT_a} + P_INT__P
P_{INT_a} Internal power dissipation active
P_{INT_p} Internal power dissipation passive
P_tot Overall power dissipation
D
Duty cycle for data transmission
The internal passive part can be calculated with the operating voltage and the normal mode supply current
recessive. The active part can be calculated with the voltage drop of the driving transistor and the current of
the CAN bus. The active part generates only during data transmission power dissipation. Therefore the duty
cycle has to be taken into account.
P_{INT_p} = V_BAT * I_BAT
P_{INT_a} = (V_BAT – V_CANH) * I_load * D
V_BAT Battery supply voltage
I_BAT Normal mode supply current recessive
I_load Can network current
D
Duty cycle for data transmission
V_CANH Voltage at CANH pin
The power dissipation of the load can be calculated with the CANH voltage and the CAN bus current.
where
P_load = V_CANH * I_load * D
I_load = V_CANH / R_{load_net}
P_load Power dissipation of the load resistor
I_load Current of CAN network
V_CANH Voltage at CANH pin
R_{load_net} Network total resistance
TH8056 – Datasheet
3901008056
Page 15 of 26
March 2007
Rev 012