Switching Power Supply Question (LT1940)

ry.david

Does anyone have much experience with switching DC-DC step down regulators? I am using Linear LT1940, specifically a similar circuit to the example given on page 17 for 5v and 3.3v. It seems like the feedback pins is completely ignoring the voltage divider for both the 5-volt output and 3.3-volt ouput. A specific example:

Vin : 12.08v
Vout1 : 6.37v (expecting 3.3v)
Vout2 : 7.35v (expecting 5.5v)
FB1 : 2.06v (supposed to be regulated to 1.25v, right?)
FB2 : 1.71v (again, supposed to be regulated?)

Thanks in advance!
Ryan

sam_sam_sam

These three are the most important


Figure 1.<<<I would use this setup>> > 3.3V and 5V Dual Output Step-Down Converter with Output Sequencing (LT1940) PCB Layout

Although two inductors are required, each will be smaller than the inductor required for a single-phase regulator.
·This may be important when there are tight height restrictions on the circuit.
·The Typical Applications section shows circuits with maximum heights of 1.4mm, 1.8mm and 2.1mm.



For proper operation and minimum EMI, care must be taken during printed circuit board (PCB) layout. Figure 7 shows the high-di/dt paths in the buck regulator circuit.
Note that large, switched currents flow in the power switch, the catch diode and the input capacitor. The loop formed by these components should be as small as possible.
These components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer.
Place a local, unbroken ground plane below these components, and tie this ground plane to system ground at one location, ideally at the ground terminal of the output capacitor C2. Additionally, the SW and BOOST nodes should be kept as small as possible.
Figure 8 shows recommended component placement with trace and via locations.

Figure 8. A Good PCB Layout Ensures Proper Low EMI Operation· <<<<<< Look at this

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Sam

Post Edited (sam_sam_sam) : 2/14/2010 8:17:49 PM GMT

sam_sam_sam

Yes i have played with switching regulators before and if you do not follow there directions about ground loops and the like then you will have noise
on your out put or it will not work right or both

I hope this help also read all of the data sheet to make sure you have not miss something I did not read all of it


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··Thanks for any··that you may have and all of your time finding them

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Sam

ry.david

@sam_sam_sam,

Thanks for the quick reply. I believe my board layout is halfway decent. It's a 4 layer board (signal, gnd, power, signal). The entire power supply of the board is within 1"x1" so the traces are not long.

sam_sam_sam

Can you post your board layout Thanks

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··Thanks for any··that you may have and all of your time finding them

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Sam

ry.david

Attached... Thanks again for all your time!