Best way to incorporate Package Types - Page 1
Forum section: electronics / beginners

author: LoveLaika
author_rank: Frequent Contributor
timestamp: July 24, 2021, 05:32:34 pm
content: I'm looking at the LM7372 dual op-amp to use in a circuit, and I'm a bit confused about its package types. The IC holds two op-amps, and it comes in a 16-SOIC package or a 8-pin SO DDA package which seems to have an exposed pad underneath the IC. Looking at the different types, I'm rather confused on the proper way to route it (in the case where I'm using positive and negative power supplies, not a single supply).Regarding the 16-pin SOIC, it has heatsink pins at its top corners and no exposed pad. According to page 17, the datasheet recommends that the remaining pins be used for heatsinking, and they demonstrate this by showcasing a copper pad structure. This seems easily accomplished by isolating the usable pins with a no-fill zone, but if you're putting the IC on top of a copper ground pour, then it's no different than soldering the heatsink pins to ground?Regarding the 8-pin package, it's smaller, but the exposed pad worries me. Page 3 says that the exposed pad should be tied to V- or left floating, as the die attach material is internally tied to V-. That seems straightforward enough, but it seems to me that it creates a large pad that isn't tied to the ground plane but to V-. Is that right, or am I missing something? Seems like this is beneficial for single-supply configurations where the Negative Supply Voltage is tied to ground. Since I'm not using a single-supply configuration, it kind of feels like overkill.Also, with the exposed pad, is it highly recommended that thermal vias be present as shown in Page 28? I thought that board manufacturers charge extra for drilling vias onto exposed pads, so I try to avoid fancy stuff like that.https://www.ti.com/lit/ds/symlink/lm7372.pdf?ts=1627146922319&ref_url=https%253A%252F%252Fwww.google.com%252F

author: cortex_m0
author_rank: Regular Contributor
timestamp: July 24, 2021, 06:03:00 pm
content: Central thermal pads under SO8 packages are pretty common these days, and nearly universal on QFN and QFP packages, so PCB manufacturers are undoubtedly familiar with them. I'm not aware of there being an upcharge for drilling through exposed pads.The data sheet lists the SO16 as having a lower thermal resistance than the SO8 with thermal pad in Section 6.4. This is contradicted by the note below that table, and also Tables 1 & 2. I would expect that the SO8 with power pad would actually have lower thermal resistance, i.e. I think the data in Section 6.4 is mislabeled.The real question for you as the designer is to understand the thermal dissipation of this OP-Amp and whether it will overheat. This OP-Amp is capable of delivering large currents to the output, and more output current implies more power dissipation.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: July 24, 2021, 08:47:02 pm
content: Thanks for your comments. I'm not using it for massive amounts of current; at best, I'm using it as an inverting op-amp summer, with only maybe a few hundred millivolts at the output (and in series with 2k ohms, so low current).Looking at the thermal metric, it's in degrees Celcius per Watt. Is that based on the output voltage and current of the IC? Looking at the footnote on page 5, they mention how if you connect the copper (I'm assuming this is a ground copper pour) to the four heatsink pins and V- (so, a single-supply operation), the thermal resistance decreases? For every watt, you want a small increase in temp, so smaller values are better. I can see why there's a typo now. I'll have to go through and read the note a bit more carefully.As a designer, the 16-pin seems easier to use; if the 8-pin package is necessary, how does one solder it to the exposed pad? Soldering iron can't reach it. Hot air gun? Bake it in the oven? OSHpark seems to offer that via in pad as a service (no blind or buried vias), but they mention that if I do this, solder may get sucked out of the hole, so I should make the hole as tiny as possible.

author: cortex_m0
author_rank: Regular Contributor
timestamp: July 24, 2021, 10:08:12 pm
content: Quote from: LoveLaika on July 24, 2021, 08:47:02 pmThe "watts" value in question is based on total power dissipation, so the voltage between the supply and Vout, times the output current, plus the OP Amp's intrinsic supply current (which is not trivial, almost 20 mA from +-15V)So for +-15V split supply operation, the peak DC power would be when you hold an input at 0V. It should be less power to hold the output at VCC or VEE, because there is less voltage drop inside the OP Amp. Power consumption in AC conditions may vary, e.g. if your load is capacitive/inductive.QuoteYes, I would use my hot air reflow station for the SO8 with thermal pad. You are correct that vias can steal solder, which can cause defects.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: July 25, 2021, 04:57:50 am
content: Let's look at this from a numbers view. I hope I'm running the numbers right.The LM7171 has a Junction-to-ambient thermal resistance depending on the package (assuming 106 deg. C/watt for 16-pin, 47 for 8-pin). Recommending operating temp is max 85 degrees C.In my case, I'm running off of +10 and -10. Assuming the same supply current, round it to 20 mA, that's 0.4 watts (20-volt difference times 20 mA). Using that thermal resistance value, with the 16-pin, the temp rises to 42.4 degrees, whereas its 18.8 degrees C with the 8-pin (and assuming the footnote is accurate with the grounding, its down to 28 degrees with the 16-pin if it is reduced to 70 deg. C/watt).The rest of this depends on the output voltage. Let's assume a max voltage of 8 volts at the output. I envision that this goes to an oscilloscope. Assuming the oscilloscope can be treated as a 50-ohm load, place that with a 50-ohm resistor in series to match impedance, that's 100 ohms seen by the output. So, that's a max current draw of 80 mA, for a output power of 0.64 watts.Using that value, that's an increase of 67.8 degrees with the 16-pin (improperly connected), 30 deg. with the 8-pin, and 44.8 deg. with the 16-pin when it is properly grounded. Combining the sums of all that temperature calculations, the 16-pin will crash and burn unless the heatsinks are properly connected to ground, whereas the 8-pin will be fine.Thinking like this with my other op-amp that I'm planning to use, this thing's gonna get real hot unless the output voltage is limited to a certain extent.