AN557 datasheet(50/52 Pages) STMICROELECTRONICS

50 / 52 page

AN557 APPLICATION NOTE

50/52

Figure 70.

LAYOUT CONSIDERATIONS

Both for linear and switching power supplies when the current exceeds 1A a careful layout becomes im-

portant to achieve a good regulation. The problem becomes more evident when designing switching reg-

ulators in which pulsed currents are over imposed on dc currents. In drawing the layout, therefore, special

care has to be taken to separate ground paths for signal currents and ground paths for load currents,

which generally show a much higher value.

When operating at high frequencies the path lenght becomes extremely important. The paths introduce

distributed inductances, producing ringing phenomena and radiating noise into the surrounding space.

The recirculation diode must be connected close to output pin, to avoid giving rise to dangerous extra

negative voltages, due to the distributed inductance.

HEATSINK DIMENSIONING

The heatsink dissipates the heat produced by the device to prevent the internal temperature from reaching

values which could be dangerous for device operation and reliability.

Integrated circuits in plastic package must never exceed 150°C even in the worst conditions. This limit has

been set because the encapsulating resin has problems of vitrification if subjected to temperatures of

more than 150°C for long periods or of more than 170°C for short periods. In any case the temperature

accelerates the ageing process and therefore influences the device life. A well designed heatsink should

keep the junction temperature between 90°C and 110°C. Fig 71 shows the structure of a power device.

As demonstrated in thermo-dynamics, a thermal circuit can be considered to be an electrical circuit where

R1, R2 represent the thermal resistance of the elements (expressed in °C/W) (see fig. 72).

C1, C2

are the thermal capacitance (expressed in

°C/W).

is the dissipated power.

is the temperature difference with respect to

the reference (ground).

This circuit can be simplified as shown in fig. 74, where:

is the thermal capacitance of the die plus that of the tab.

is the thermal capacitance of the heatsink

Rjc

is the junction case thermal resistance

Rth

is the heatsink thermal resistance

HIGH POWER

PREREGULATOR

L7805

L7812

L4805

5V/1A

12V/1A

5V/0.4A

D93IN010A

L2 and C2 are necessary to reduce the switching frequency spikes

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