SA5778 Datasheet, PDF (8/16 Pages) NXP Semiconductors – Serial triple gauge driver STGD

Philips Semiconductors
Serial triple gauge driver (STGD)
Product specification
SA5778
3.0
POWER
(W)
2.5
2.0
1.5
1.0
POWER DISSIPATION FOR COIL
RESISTANCE IN OHMS AND OP-
ERATING BATTERY VOLTAGE
125
150
175
235
325
LOAD RESISTANCE
(Ω)
0.5
0.0
V_SWBATT
(V)
Figure 7. Power Dissipation of the STGD as a Function of Coil Resistance and Operating Voltage
SR01430
The STGD is specified to operate up to V_BATTmax. The over voltage
shutdown circuit will turn off the output buffers and the switched
battery supply when the battery voltage reaches V_OVSD. Over
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
Θ_JA (thermal resistance, junction to ambient) was determined using
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
sided board also assumes 8 thermal bias with 0.36 mm² cross
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.
1998 Apr 03
8