Philips Semiconductors

Product specification

SA5778

Serial triple gauge driver (STGD)

1998 Apr 03

8

SR01430

3.0

2.5

2.0

1.5

1.0

0.5

0.0

125

150

175

235

325

POWER DISSIPATION FOR COIL

RESISTANCE IN OHMS AND OP-

ERATING BATTERY VOLTAGE

LOAD RESISTANCE

(

Ω)

(V)

POWER

(W)

Figure 7.

Power Dissipation of the STGD as a Function of Coil Resistance and Operating Voltage

shutdown circuit will turn off the output buffers and the switched

temperature conditions will also cause the output buffers to be

disabled.

The STGD employs a thermally enhanced SO-28 package. The

center four pins on each side are fused to the die pad to create a

path for removal of heat from the package to the copper foil on the

PC board. An area of copper foil is required on the PC board for

heat dissipation at higher power dissipation levels.

In order to determine the size of the copper foil required, both

thermal testing and thermal modeling were used. The effective

both single and double sided PCBs with heat-sinking copper foil

areas. Figures 8 and 9 show the effect of PCB copper foil area on

the effective thermal resistance of the STGD part/PCB system.

Figure 8 shows the thermal resistance of the STGD mounted on a

PC board with heat-sinking copper on the component side only.

Figure 9 is a similar plot for a two sided PC board (same size copper

areas on each side). Both plots assume a 60 x 60 x 1.57 mm FR4

board with varying square-shaped sizes of 2 oz. copper. The two

section.

It is important to note that at such a high ambient temperature (worst

case of 105

°C assumed), radiation is just as significant as

convection in the dissipation of heat. Good radiation is highly

dependent on the emissivity of the heated surface, so the thermal

radiation properties of the copper foil should be considered. Bare,

clean copper is a good thermal conductor, but it has a low emissivity,

and is therefore a bad radiator. It is recommended that the copper

areas intended for heat dissipation be left covered with solder mask

or otherwise blackened to increase the emissivity, thereby improving

the heat radiating ability of the board.