33486A datasheet(12/30 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33486A

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.

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 (RθJC): 2.0°C/W

•Power into the switch: Assuming the device is driving

8.0 A at 150

°C junction temperature (RDS(ON) at

150

°C is 40 mΩ), the total power dissipation is 0.04 *

8 * 8 = 2.56 W

•Case-to-ambient thermal resistance (RθCA): 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:

°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.

RθCA

RθJC

Junction Temperature Node

Ambient Temperature Node

Case Temperature Node

Switch

Power (W)

(1.0 A = 1.0 W of

(1.0 V = 1.0

°C Ambient Temperature)

(Volts represent Die

Surface Temperature)

(1.0

Ω = 1.0°C/W)

Power Dissipation)

33486A Datasheet(PDF) 12 Page - Freescale Semiconductor, Inc

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