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We're routing a PCB for a LM5085 buck controller of 8 pin WSON package with a maximum input of 75V. My questions are for the relatively high input voltage

  • 0603 is the smallest input decoupling cap I can use since the distance between uncoated pads is 0.6 mm as per IPC2221 standard says the distance between uncoated external conductors must be above 0.6 mm, right? I'm asking because 10nF or 1nF 100V 0402 caps are available and are of lower ESL.
  • In that case what about the Voltage input pin and the exposed thermal pad of the IC being at 0.3 mm from each other, wouldn't this be at a risk?
  • For tracks covered with solder mask the IPC2221 standard says they can be at a distance more than 0.13mm, right?
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  • \$\begingroup\$ Distance Above 0.6V??? Hmm. Ceramic creapage ought to be < 1kV/mm. You could coat them. \$\endgroup\$ – Tony Stewart EE75 Jan 8 at 19:11
  • \$\begingroup\$ Sorry typo, I meant 0.6 mm. \$\endgroup\$ – EarthLord Jan 8 at 19:58
  • \$\begingroup\$ What was the voltage spec for 0.6 mm.? \$\endgroup\$ – Tony Stewart EE75 Jan 8 at 20:29
  • \$\begingroup\$ smpspowersupply.com/ipc2221pcbclearance.html as you can see here, for uncoated conductors the spacing specified is 0.6 mm for both 50V and 100V. \$\endgroup\$ – EarthLord Jan 9 at 8:08
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    \$\begingroup\$ I don’t know the history or reasons, yet moisture and dust creepage breakdown can cause partial discharge or arcing especially in 5nm semi’s , so I surmise the kV/mm rating of 100V/0.6mm is pretty extreme contam. Level. I would expect designers to conformally coat such small gaps with a silicone spray for long life in extreme dust in moving air boxes on the floor like PC MOBO’s. \$\endgroup\$ – Tony Stewart EE75 Jan 9 at 10:35
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Its really not IPC2221 you need to worry about... that is more of a guideline. What it really comes down to is: Does this need to pass regulatory? And secondly what is the risk of this circuit burning up?

If the circuit needs to pass regulatory then it will need to adhere to IEC61010 creepage and clearance requirements (shown below). The problem with this is the spacing on the WSON package.

The spacing on the WSON between VIN and the thermal pad is ~0.3. The category that circuit would be in would be B2 or B4 (with coating if it were coated). The voltage between conductors would be 51-100V so a spacing of 0.6mm for B2 and a spacing of 0.13mm for B4. If the product is uncoated, then it would fail the IPC creepage spec.

If it were coated... I don't know how the ETL (Electronic Testing Lab) would interpret the conductors on the underside of the WSON (which is really up to them). Because the argument could be that there is no polymer on the underside of the WSON. Barring the specs if it was coated, it would need to be very clean to avoid arcing.

enter image description here Source: https://electronics.stackexchange.com/tags/creepage/info

If you don't need to follow IPC/UL specs, then don't worry about the spacing except it will be dangerous with the WSON. Any pollution (IE dust, solder, flux, moisture) could set off an arcing event on the WSON. If it were me I would reduce my worry and steer clear from that package as I would then have to worry about keeping it clean and arcing. So unless you really have your heart set on the WSON, the HVVSOP has an 0.56mm clearance (which could probably also be made 0.6mm with some "finagling".

As far as the resistors go an 0402 could have just over 0.6mm of clearance (0.635 but it really depends on the footprint), so that should work for 75V (which was actually surprising to me)

enter image description here
Source: https://www.pinterest.com/pin/582019951816051056/

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If you look at the LM5085 data sheet, Vin is on pin 8, the corner pin. So it has only one adjacent pin, which is Vcc pin (pin 7). This is normally at 7.5V below Vin which is 68V for 75V in. The remaining two pins on that side are Pgate and Isens.

Psens does between Vin and Vcc, and Isens is normally connected to a sens resistor on the external FET.

So it seems that TI thought of this - the voltages between adjacent pins on that side of the chip are nowhere near 75V, even with 75V in.

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  • \$\begingroup\$ Yes, pin 5 to pin 8 are close to Vin. What about the Vin pin and the exposed thermal pad in the WSON package? This would be good to use as it has the best thermal performance. \$\endgroup\$ – EarthLord Jan 12 at 17:50

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