dient across the case-top, the use of a thermal couple is
generally not recommended. If a thermal couple has to be
used, try to locate the hottest spot on the case-top first and
then secure the thermal couple at exactly the same location.
The thermal couple needs to be a light-gauge type (such as
40-gauge). Apply a small blob of thermal compound to the
contact point and then secure the thermal couple on the case-
top using thermally non-conductive glue.
3. The SW pins are high current carrying pins so traces con-
nected to them should be short and fat.
4. Keep feedback resistors close to the FB pins.
5. Keep the AVIN RC filter close to the AVIN pin.
6. Keep the voltage feedback traces away from the switch
nodes.
7. Use six or more vias next to the ground pad of the catch
diode.
If the maximum allowed junction temperature is exceeded,
load current has to be lowered to bring the temperature back
in specification. Or better thermal management such as more
air flow needs to be provided.
8. Use at least four vias next to the ground pad of output ca-
pacitors.
9. Use at least four vias next to each pad of the input capac-
itors.
As a summary, here is a list of important items to consider:
1. Use multi-layer PC boards with internal ground planes.
For low EMI emission, try not to assign large areas of copper
to the noisy switch nodes as a heat sinking method. Instead,
assign a lot of copper to the output nodes.
2. Use nine or more thermal vias to connect the top-layer
thermal pad to internal ground plane(s) and ground copper on
the bottom layer.
3. Generate as large a ground plane as allowable on outer
layers, especially near the package.
4. Use 2 oz. copper whenever possible.
5. Try to spread out heat generating components.
6. The inductors and diodes are heat generating components
and should be connected to power or ground planes using
many vias.
LAYOUT GUIDELINES
There are mainly two considerations for PCB layout - thermal
and electrical. For thermal details, refer to the section THER-
MAL CONSIDERATIONS. Electrical wise, follow the rules
below as much as possible. In general, the LM26400Y is a
quite robust part in terms of insensitivity to different layout
patterns or even abuses.
20200243
FIGURE 9. PCB Layout Example
1. Keep the input ceramic capacitor(s) as close to the PVIN
pins as possible.
2. Use internal ground planes when available.
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