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TPS7A4701 and TPS7A33 PCBs?
TPS7A4701 and TPS7A33 PCBs?
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Old 19th June 2018, 06:25 PM   #11
hellokitty123 is offline hellokitty123  United States
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Quote:
Originally Posted by jhofland View Post
And if you are expecting to run a relatively high difference between input and output voltage at currents of more than, say, half an amp, power dissipation is going to be a significant problem.
I'm going to run about 600ma through it @ a couple of volts but I'm going to heatsink the back of it.

Quote:
Originally Posted by JPS64 View Post
Why not big capacitors for in and out externally?
I was told by someone not to cheat with external capacitors if I care about performance.

Quote:
Originally Posted by jhofland View Post
Figure 28 is the way I would generally do it bringing traces directly down to the three pins that go off board. Take the other layout and rotate the whole thing clockwise. Relocate the input cap so it almost butts up the the output cap ground connection.
As I said earlier because of the size of the capacitors the layout you suggest is awkward and creates a longer current path.

For all the trouble I'm wondering if it would be better performance to just create a miniature 3 pin super regulator. There's a lot of weird requirements with this reg.

Last edited by hellokitty123; 19th June 2018 at 06:29 PM.
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Old 19th June 2018, 06:28 PM   #12
OlegSh is online now OlegSh
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TPS7A4701 and TPS7A33 PCBs?
The common GND issue. It matters because the regulator IC takes its reference from the GND next to the IC and regulate the output voltage against this reference. Now you have a single pin connecting the input and output GNDs to your circuit which reg suppose to supply. Depending on the return currents traveling this pin you incur a voltage drop on its impedance. Since return currents related to the input and the output are different they do not cancel each other and the difference between these two return currents is the noise which will not be taken care of by the reg.

To avoid this issue use an analogy of the layout as shown on the second picture in post #8 where input and output GNDs are separate and meet exactly under the IC.

Hope it helps.

Regards,
Oleg

Last edited by OlegSh; 19th June 2018 at 06:31 PM.
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Old 19th June 2018, 06:33 PM   #13
hellokitty123 is offline hellokitty123  United States
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Ah, thanks OlegSh.
Does this configuration satisfy that requirement sufficiently?
Click the image to open in full size.
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Old 19th June 2018, 06:39 PM   #14
OlegSh is online now OlegSh
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TPS7A4701 and TPS7A33 PCBs?
Not really. I would suggest making input on one side and output on the other side of the IC. You can turn the reg by 90 degrees to make it easier.
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Old 19th June 2018, 07:35 PM   #15
hellokitty123 is offline hellokitty123  United States
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Everyone keeps saying that but the big cap prevents the leads from being short between GND and the output.
Click the image to open in full size.
I can stack the 10uf caps on top of each other vertically to solve the issue but that seems like an unelegent solution as there is no easy way to do that.
The big cap represents a relatively large trace distance between the output or ground.
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Old 19th June 2018, 07:56 PM   #16
OlegSh is online now OlegSh
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TPS7A4701 and TPS7A33 PCBs?
To get an idea how to do it right study the TPS7A47 reg evaluation board design and look at the recommended parts.
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Old 20th June 2018, 03:25 PM   #17
hellokitty123 is offline hellokitty123  United States
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The TPS7A33 recommends a feedforward capacitor be connected between the output and the Fb pin
http://www.ti.com/lit/ds/symlink/tps7a33.pdf
but the TPS7A47 does not mention one.
Should I connect this feedforward capacitor to the negative regulator only?
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Old 20th June 2018, 04:13 PM   #18
chris719 is online now chris719  United States
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TPS7A4701 and TPS7A33 PCBs?
Quote:
Originally Posted by hellokitty123 View Post
I'm going to run about 600ma through it @ a couple of volts but I'm going to heatsink the back of it.


I was told by someone not to cheat with external capacitors if I care about performance.


As I said earlier because of the size of the capacitors the layout you suggest is awkward and creates a longer current path.

For all the trouble I'm wondering if it would be better performance to just create a miniature 3 pin super regulator. There's a lot of weird requirements with this reg.
This is an easy to use regulator. The "weird requirements" are not really any different than you would find with any other regulator if you want it to perform well.
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Old 20th June 2018, 04:16 PM   #19
chris719 is online now chris719  United States
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TPS7A4701 and TPS7A33 PCBs?
Quote:
Originally Posted by hellokitty123 View Post
The TPS7A33 recommends a feedforward capacitor be connected between the output and the Fb pin
http://www.ti.com/lit/ds/symlink/tps7a33.pdf
but the TPS7A47 does not mention one.
Should I connect this feedforward capacitor to the negative regulator only?
They are not the same regulator, the design of 7a33 is different. The 7a47 datasheet would have mentioned it if it were applicable.
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Old 20th June 2018, 07:24 PM   #20
OlegSh is online now OlegSh
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TPS7A4701 and TPS7A33 PCBs?
Test module design for TPS7A47 reg shows such capacitor too (C4 in fig. 6 on page 7).
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