33486A Datasheet, PDF (12/30 Pages) Freescale Semiconductor, Inc – Dual High-Side Switch for H-Bridge Applications

FUNCTIONAL DESCRIPTION
FUNCTIONAL INTERNAL BLOCK DESCRIPTION
Thermal Model
The junction-to-ambient thermal resistance of the circuit
mounted on a printed circuit board can be spit into two main
parts: junction-to-case and case-to-ambient resistances.
Figure 8 shows a simplified steady state model.
Power (W)
(1.0 A = 1.0 W of
Power Dissipation)
Junction Temperature Node
(Volts represent Die
Surface Temperature)
Switch
R_θJC
Case Temperature Node
R_θCA
(1.0 Ω = 1.0°C/W)
Ambient Temperature Node
(1.0 V = 1.0°C Ambient Temperature)
Figure 8. Simplified Thermal Model
(Electrical Equivalent)
The use of this model is similar to the electrical Ohm law
(voltage = resistance x current), where:
•Voltage represents temperature.
•Current represents power dissipated by the device.
•Resistance represents thermal resistance.
We finally have:
Temperature or delta temperature = power dissipation
times thermal resistance; that is, °C = W x °C/W.
Any node temperature can easily be calculated knowing
the amount of power flowing through the thermal resistances.
Example
1. Numerical Value
•Junction-to-case thermal resistance (R_θJC): 2.0°C/W
•Power into the switch: Assuming the device is driving
8.0 A at 150°C junction temperature (R_DS(ON) at
150°C is 40 mΩ), the total power dissipation is 0.04 *
8 * 8 = 2.56 W
•Case-to-ambient thermal resistance (R_θCA): 20°C/W
2. Results
•Junction-to-case delta temperature: 5.0°C (2.5 W x
2.0°C/W)
•Case delta temperature from ambient: 50°C (20°C/W x
2.5 W)
•Actual junction temperature node will be:
50°C + 5.0°C = 55°C above the ambient
temperature.
Assuming an 85°C ambient temperature, the junction
temperature is 85°C + 55°C = 140°C.
The above example takes into account the junction-to-
ambient thermal resistance, assuming that ambient
temperature is 85°C.
In the case where the device plus its printed circuit board
are located inside a module, the ambient temperature of the
module should be taken into account. Or an additional
thermal resistance from inside module to external ambient
temperature must be added. The calculation method remains
the same.
The low-side block is packaged into D²PAK or DPAK
package. Junction-to-case thermal resistance is
approximately 2/0°C/W. The junction-to-ambient thermal
resistance follows the same rules as for the high-side block
and is in the same range.
33486A
12
Analog Integrated Circuit Device Data
Freescale Semiconductor