LMZ21700 Input Cap Question

Hello Gary, 

This is an interesting idea. 

If the circuits are right next to each other, I would think that this would work. A few caveats though:

The capacitor has to be as close as possible to the switcher. As you may know, the input capacitor in a buck converter along with the switching elements inside the module form a high di/dt current path. You need to keep the area of this current loop as small as possible to minimize the stay inductance. Placing the capacitor as close as possible to the LMZ device and on the same layer as the LMZ device is very important.

Another point to make is the capacitance value. It looks like the LDO typical schematic suggests 22uF as Cout and the switcher suggests a minimum of 10uF as input capacitance. Be sure to check the ceramic capacitor capacitance de-rating under DC bias. Your 22uF 25V cap in a small case size at 15V bias may only have about 10uF left. You may end up having to use two capacitors anyway. 

Hope this helps. 

Cheers, 
Denislav

Denislav,
Thanks for the information.
I checked with the PCB layout Eng and he stated the Cin Cap is 0.043 Inches away from the LMZ21700. Cin is on the same side as LMZ21700. We need to keep the area of this current loop small. Is this close enough to get proper performance of the LMZ21700.

I am reviewing the Cin Part selection and also evaluating using Electrolytic Case Size C1210 TPSB226K020R0400. It has a 0.4 Ohm ESR, 22UF 10%, but DC rating is 20V. Should I add additional Pads for Cin on the Opposite side of the board as a mitigation plan?

I do have an LMZ21700 evaluation board if there is any circuit mods I need to try. Thanks, Gary

Hello Gary, 

43mil (about 1.1mm) sounds very close - it should work. 

I would recommend staying with ceramic for the input capacitor. The electrolytic cap will come with higher ESR and ESL and that would make it ineffective at high frequency.

If you have room for an additional footprint on the other side of the PCB, I think it is a good idea to have it. 

Cheers, 

Denislav

Denislav:
Just to make sure I understand the importance of the Cin for my circuit app (Vin = 15 V Vout = 5.5 V approx. max current 0.2 A):
1) Is 12063D226KAT2A in series with a Yageo RL0603FR-070R5L resistor preferred over an C1210 TPSB226K020R0400 Electrolytic with 400 mOhms ESR?

2) Do you have a phone contact so I can discuss the specific PCB layout issues?

Thank you for your assistance. Regards, Gary