Questions & Answers - Transistor Outline (TO, DPAK) query

Transistor Outline (TO, DPAK) query

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Category: PCB Footprint Expert
Forum Name: Questions & Answers
Forum Description: issues and technical support
URL: https://www.PCBLibraries.com/forum/forum_posts.asp?TID=474
Printed Date: 06 Oct 2024 at 10:24am

Topic: Transistor Outline (TO, DPAK) query

Posted By: jameshead
Subject: Transistor Outline (TO, DPAK) query
Date Posted: 24 Jul 2012 at 2:49am

Could you confirm that this view and calculation is okay for National Semiconductor part LM2595S?

The datasheet is here:

http://www.ti.com/lit/ds/symlink/lm2595.pdf" rel="nofollow - http://www.ti.com/lit/ds/symlink/lm2595.pdf

But there's a better drawing here:

http://www.national.com/packaging/mkt/ts5b.pdf" rel="nofollow - http://www.national.com/packaging/mkt/ts5b.pdf

Entered dimensions were:

D1 and D were put in as the same value.

E Min was put in as 13.85 mm based on using a +/- 0.25 mm tolerance which I used based on the main tolerance they'd used of +/- 0.25 else where on the drawing - not too disimilar to the +/- 0.2 mm tolerance suggested by Tom earlier for missing dimensions.

I took L1 from the datasheet as 1.27 max and made a similar application of a +/- 0.25 mm tolerance, then worked out E1, E, and E2.

E1 max = 1.27 + (8.64 + 0.25) = 10.16 mm
E1 min = (1.27 - 0.5) + (8.64 - 0.25) = 9.16 mm

E2 max = 1.27 + (5.08 + 0.5) = 6.85 mm
E2 min = (1.27 - 0.5) + 5.08 = 5.85 mm

L was given as 1.98 with no tolerance so again I put in a +/- 0.25 mm tolerance.

L1 max = 1.27 mm
L1 min = 1.27 - 0.5 = 0.77 mm

Dimensions shown after entry were:

Footprint view is:

Normally I'd not be too bothered if I was just doing this footprint for myself but I am conciously aware that these may end up in FPX files used by others so just wanted some feedback on what's expected for these packages.

Replies:

Posted By: Tom H
Date Posted: 23 Oct 2012 at 8:27pm

The mfr. does not provide the Maximum Thermal pad length (dimension "E2").

In that case, you should always Tweak the E2 dimension so that the Pad Length is at least equal to the mfr. recommended footprint (land pattern).

The recommended footprint E2 dimension for one of the parts is -

6.99 mm - http://www.national.com/packaging/mkt/ts5b.pdf" rel="nofollow - http://www.national.com/packaging/mkt/ts5b.pdf

10.41 mm - http://www.ti.com/lit/ds/symlink/lm2595.pdf" rel="nofollow - http://www.ti.com/lit/ds/symlink/lm2595.pdf

So there are 2 different footprint patterns (2 different FPX files).

If you personally want to make a singe pattern that will fit both parts then you need to use the largest E2 value of the 2 components. You can also use the "Footprint" tab and enter the mfr recommended footprint values to create that unique pattern.