LM 3478 no switching output !

6076.webench_design_938544_17.pdf

3187.Envy rev H schem.pdf

Hi Mike,

I will let the application engineer know to look at this post.

I have a couple questions. 

1) What is the component U1 which is tied to the FB trace?  Anything connected to the FB node I would check to see if it is holding the pin high which would turn off the switching action.

2) Are you sure that CIN and Cout are oriented like this in the schematic, with the positive side connected to ground?  If so, the capacitors might have shorted.

Regards,

Marc

U1 is a AD5228 10k ohm digital pot , I changed R4 to a 4.7 k , and have a range of  0.3 - 2 volts at FB on LM3478  , the caps just drawn wrong  , they are ok on the PCB ! ( fixed that ) 

thanks ! 

OK as I adjust the feedback by changing the digital resistor value , as the FB pin goes past the 1.26 volts the output from DR changes from 0 volts to .57 volts FYI but never goes into switching mode . 

I would try taking out the digital pot and add wirewound lower resistor.  If the part works with this then we will try getting the digital pot working.  If the part still doesn't work at least one thing is off the possibility list.

 Also, what input voltage are you currently using to test the part with?

Marc

Hi MArc , I thought the same thing , I am using a Li-ion battery 3.7 volts Nom. , I will try replacing the Digi pot with a fixed res. I do not have any Wirewound ones just SMT types . 

I assume the DR pin should OSC with or without a load correct ? 

_{OK I tried 2 3.3 k ohm res for the feedback R's Fb1- Fb2 same results no switching ! lost here} 

how does the circuit look >? I could not find any errors 

Thanks , I have a few questions , 

1,. I am testing this under a NO LOAD condition with a Li-ION battery that has about a 4 amp current capacity 

2. UVLO is something I will need ! , is there another device that incorporate the UVLO ? 

3. the device in this design uses a single Li-Ion battery and has VERY limited PCB space 1"x.8" size so space is limited 

I am burning up components right now with no load , is this a normal condition ??? if so I need to change parts for a UVLO circuit.

Hello Mike,

please see my answers:

1.there may be some inrush current causing that problem or something in your set up. Furthermore 4amp are not enoug for a 5V@3A with 3V at the input. I am sure that a big bulk cap should solve the problem.

2. I really encourage you to use the LM3481 inplace of the LM3478. The LM3481 is an improved version of the LM3478 and it implements UVLO that you can set.

3. I would like to take a look at your PCB layout for a quick review.

Anyway please use the LM3481 instead of the LM3478.

Regards,

Giuseppe

Here is the PCB layout , this was a prototype to just test the circuit, the final design will be much different 

I am going to try this with a much larger battery ( 6 amp ) to see if we get any switching to take place 

Hello Mike,

in any case I encourage you to use the LM3481 instead of the LM3478.

Anyway I took a look at your PCB and schematic.

It is suggested to have a bypass cap (like the one used to solve the previous problem) right on the VIN pin.

Furthermore you should move the C3Csense right on ISEN pin (between ISEN pin and GND) for a better current filtering.

Also please consider the following suggestions for an optimized PCB layout for a boost converter:

FILTER CAPACITORS

Place CBYP as close as possible to the VIN pin and GND pins of the LM3478

Place CSN  as close as possible to the ISEN pin and GND pins of the LM3478

SENSE LINES

The current sensing circuit in current mode devices can be easily effected by switching noise. This noise can cause duty cycle jitter which leads to increased spectral noise. RSN should be connected to the ISEN pin with a separate trace made as short as possible. Route this trace away from the inductor and the switch node (where D1, Q1, and L1 connect). For the voltage loop, keep RFBB/T close to the LM3476 and run a trace as close as possible to the positive side of CO. As with the ISEN line, the FB line should be routed away from the inductor and the switch node. These measures minimize the length of high impedance lines and reduce noise pickup.

COMPACT LAYOUT

The most important layout rule is to keep the AC current loops as small as possible. In a boost regulator the primary switching loop consists of the output capacitor, diode and MOSFET. Minimizing the area of this loop reduces the stray inductances and minimizes noise and possible erratic operation. The output capacitor(s) should be placed as close as possible to the diode cathode and the ground side of RSN.

GROUND PLANE AND VIAS

A ground plane in the printed circuit board is recommended as a means to connect the quiet end (input voltage ground side) of the input filter capacitor to the output filter capacitors and the PGND pin of the controller. Connect all the low power ground connections directly to the regulator AGND. Connect the AGND and PGND pins together through a copper area covering the entire underside of the device. Place several vias in this underside copper area to ground plane. If a via is needed to connect the sensing resistor to the ISEN pin, then place that via in the inner side of the sensing resistor such that no current flow occurs. Place several vias from the ground side of the output capacitor(s) to ground place, that will minimize the path for AC current. The PGND and AGND pins have to be connected to the same ground very close to the IC. To avoid ground loop currents attach all the grounds of the system only at one point.

Here an example:

Regards,

Giuseppe