Datasheet

LM26400Y

SNVS457C –FEBRUARY 2007–REVISED APRIL 2013

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When prototyping the design, it is necessary to know the junction temperature of the LM26400Y to assess the

thermal margin. The best way to measure the LM26400Y's junction temperature when the board is working in its

usual mode is to measure the package-top temperature using an infrared thermal imaging camera. Look for the

highest temperature reading across the case-top. Add two degrees to the measurement result and the number

should be a pretty good estimate of the junction temperature. Due to the high temperature gradient 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.

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.

As a summary, here is a list of important items to consider:

1. Use multi-layer PC boards with internal ground planes.

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 THERMAL 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.

1. Keep the input ceramic capacitor(s) as close to the PVIN pins as possible.

2. Use internal ground planes when available.

3. The SW pins are high current carrying pins so traces connected 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.

8. Use at least four vias next to the ground pad of output capacitors.

9. Use at least four vias next to each pad of the input capacitors.

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.

Figure 38. PCB Layout Example

Product Folder Links: LM26400Y