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My device is powered at 12V and I have some components which need 5.5V power supply. In order to drop the voltage I have used a step down switcher (figure 13 of datasheet). That circuit works very good but as soon as I plug a load at Vout, the tensions drops of severals hundreds of mV.

Do you thing it is a good idea to decouple this step down circuit from the load using an ampli op with unitary gain? If yes do you know some example of alpi op which would do the job? Pretty much all those I have found so far (such as datasheet, note 3) have low output currents. I might need them to support currents as high as 3A (extremely worse scenario).

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  • \$\begingroup\$ An opamp is not the answer. Sounds like you might be overloading your switcher, or have not designed the circuit for it optimally. How much load (current) does it need to start dropping the voltage? \$\endgroup\$ – Oli Glaser Sep 17 '15 at 12:00
  • \$\begingroup\$ The only thing I have tried was 10kOhm and gave me a drop of 90mV which is still acceptable, but going to 5k I have a drop of 1V09. The problem is that my load can be very small (I'm controlling some valves with low resistance). Could you tell me in more detail why an ampli op is not the answer (just to understand it better). \$\endgroup\$ – Worldsheep Sep 17 '15 at 12:04
  • \$\begingroup\$ If at 5k you have a drop of over 1V something is wrong with the switcher circuit (or whatever's powering it). Can you post your circuit? \$\endgroup\$ – Oli Glaser Sep 17 '15 at 12:20
  • \$\begingroup\$ It is exactly the one in figure 13 of the datasheet i've posted! The only difference is that instead of the two 10uF ceramic capacitors i am using a 22uF electrolytic capacitor... Could that be the problem? \$\endgroup\$ – Worldsheep Sep 17 '15 at 12:33
  • \$\begingroup\$ Changing the capacitor reduced the drop to 80mV... Still too much! \$\endgroup\$ – Worldsheep Sep 17 '15 at 12:40
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To sum this all up, the breadboard is almost certainly the issue. The part in question need a tight layout, and requires a thermal sink for it's heat dissipation (see figure below with layout considerations from datasheet) Also, the extra capacitance and resistance of breadboard contacts is likely to cause issues, especially with the feedback loop.

LAYOUT CONSIDERATIONS
1. Keep the input switching current loop as small as possible.
2. Keep the SW node as physically small and short as possible to minimize parasitic capacitance and inductance and to minimize radiated emissions. Kelvin connections should be brought from the output to the feedback pin of the device.
3. Keep analog and non-switching components away from switching components. 4. Make a single point connection from the signal ground to power ground.
5. Do not allow switching current to flow under the device.
6. Keep the pattern lines for VIN and PGND broad.
7. Exposed pad of device must be connected to PGND with solder.
8. VREG5 capacitor should be placed near the device, and connected PGND.
9. Output capacitor should be connected to a broad pattern of the PGND.
10. Voltage feedback loop should be as short as possible, and preferably with ground shield.
11. Lower resistor of the voltage divider which is connected to the VFB pin should be tied to SGND.
12. Providing sufficient via is preferable for VIN, SW and PGND connection.
13. PCB pattern for VIN, SW, and PGND should be as broad as possible.
14. VIN Capacitor should be placed as near as possible to the device.

enter image description here

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  • \$\begingroup\$ Thank you very much for your help. How do you suggest to test this without designing and producing a PCB? There is no way, right? \$\endgroup\$ – Worldsheep Sep 17 '15 at 14:34
  • \$\begingroup\$ You might check out an evaluation board like this: digikey.com/catalog/en/partgroup/tps54327-evaluation-module/… That is the complete circuit for 1.05V, but you could modify it for 5Volts. \$\endgroup\$ – JRE Sep 17 '15 at 14:44
  • \$\begingroup\$ Unless you really know what you are doing it will be difficult without a proper PCB yes. You could etch your own board or something similar, or if it's just for a personal project maybe just buy a ready made module. \$\endgroup\$ – Oli Glaser Sep 17 '15 at 14:57
  • \$\begingroup\$ That's actually great! I didn't know that that was possible! I'll first have a look if it already exist a PCB with my requirements! \$\endgroup\$ – Worldsheep Sep 17 '15 at 15:07

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