AN1596 datasheet(3/24 Pages) STMICROELECTRONICS

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AN1596 - APPLICATION NOTE

of 13V and the output pin grounded, is limited to a typical value of 10µA for a one channel HSD. In figure

3 a plot of typical IS values versus Tj is shown for single channel and double channel monolithic HSDs.

Figure 3: Stand-by current for single and double channel single chip HSDs Vs. junction temperature

Thermal considerations

In order to choose the suitable HSD for a given load some important points must be highlighted. In the

worst-case operation (Tj=150ºC), for a single channel HSD and in steady state conditions, the Joule effect

power developed by the device equals the Power dissipated according to the following equation:

Assuming that the second term can be neglected, for a given load current IOUT a given package and heat

sink and a given ambient temperature (fixed at 85°C in automotive environment) the result is:

This is the maximum value of RDS(on) which can be chosen. The steady state on-resistance of HSDs is a

function of the junction temperature and in the datasheet its value is given at 25°C and this is

approximately doubled at 150°C. In some cases it may be convenient to use an HSD with a bigger RDS(on)

in the same package. To still comply with the above equation we must reduce Rthi-amb and have a better

heatsink. The trend from through-hole packages to low-cost SMD applications has led to think of the PCB

as a heatsink itself. In earlier packages (like PENTAWATT) a solid heatsink was either screwed or

clamped to the power package and it was easy to calculate the thermal resistance from the geometry of

the heatsink. In SMDs the heat path must be evaluated: chip (junction) - leadframe - case or pin - footprint

- PCB materials - PCB volume - surroundings. To evaluate static thermal properties of an SMD an

associated static equivalent circuit (see figure 4) can be considered. The power dissipation of the chip is

symbolized by a current source whilst the ambient temperature is represented by a voltage source. By

estimating the PCB heatsink area in a real application, the user can easily determine Rthj-amb in still air,

-50

100

150

Tcase(ºC)

Is(uA)

Double channel

Single Channel

amb

thj

amb

OUT

−

⋅

⋅ 2

)

(

()

amb

thj

OUT

amb

−

⋅

−

)

(

150

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