The data sheet-specified junction-to-ambient
thermal resistance, RθJA, is primarily useful for comparing various
drivers or approximating thermal performance. However, the actual system performance
is better or worse than this value depending on PCB stack-up, routing, number of
vias, and copper area around the thermal pad. The length of time the driver drives a
particular current also impacts power dissipation and thermal performance. This
section considers how to design for steady-state and transient thermal
conditions.
The data in this section is simulated using the
following criteria:
HTSSOP (DDW package)
- 2-layer PCB (size 114.3 x 76.2 x 1.6mm), standard FR4, 1oz (35mm copper
thickness) or 2oz copper thickness. Thermal vias are only present under the
thermal pad (13 x 5 thermal via array, 1.1mm pitch, 0.2mm diameter, 0.025mm Cu
plating).
- Top layer: HTSSOP package footprint and copper plane
heat sink. Top layer copper area is varied in simulation.
- Bottom layer: ground plane thermally connected through
vias under the thermal pad for the driver. Bottom layer copper area
varies with top copper area.
- 4-layer PCB (size 114.3 x 76.2 x 1.6mm), standard
FR4. Outer planes are 1oz (35mm copper thickness) or 2oz copper thickness. Inner
planes are kept at 1oz. Thermal vias are only present under the thermal pad (13
x 5 thermal via array, 1.1mm pitch, 0.2mm diameter, 0.025mm Cu plating).
- Top layer: HTSSOP package
footprint and copper plane heat sink. Top layer copper area is varied in
simulation.
- Mid layer 1: GND plane thermally connected to thermal
pad through vias. The area of the ground plane varies with top copper
area.
- Mid layer 2: power plane, no thermal connection. The
area of the power plane varies with top copper area.
- Bottom layer: signal layer thermally connected through
via stitching from the TOP and internal GND plane. Bottom layer thermal
pad is the same size as the top layer copper area.
Figure 8-1 shows an example of the simulated board for the DDW package.
Table 8-1 shows the dimensions of the board that are varied for each
simulation.
Table 8-1 Dimension A for DDW package| Cu area (cm2) | Dimension A (mm) |
|---|
| 2 | 19.79 |
| 4 | 26.07 |
| 8 | 34.63 |
| 16 | 46.54 |
32 | 63.25 |