Can multiple decoupling capacitors be replaced with a single low-inductance type?

Question

It seems to be usual practice to put in decoupling capacitors of several different values, for example 0.1uF and 1uF (or sometimes 10uF, or 0.01uF depending on the device etc). I'm wondering if there is anything that can replace a 0.1uF/1uF combo with a single device? In particular, would a low inductance 1uF (reversed geometry, LICC, or 2T-LGA) be a suitable replacement?

I have looked at the graphs that show what happens in terms of impedance when combining multiple values, but I haven't seen a similar graph for LICC that I can compare directly.

EDIT An example of the parts that I'm looking at is:

• Taiyo Yuden JWK105BJ105MP-F 1uF, 6.3V, X5R, 0204 size
• Murata LLL153C80G105ME21 1uF, 4V, X6S, 0204 size
• Samsung CLL5X105MR3NLNC
• TDK C0510X5R0J105M030AC

Compare these with totally ordinary 0.1uF 0402 capacitor: if I'm reading the datasheets right, the LICC devices have (far) lower impedance at all frequencies 0.1-100MHz. Hmm...

Answer 1

I only looked at one of your proposed parts, the Murata LLL153C80G105ME21. I compared it with a same-value part in a larger package (GRM21BR71E105KA99#, 0805 size), the key improvement is in the available voltage rating. The 0204 part is rated for 4 V, while the 0805 part is rated for 25 V.

Even if your application only applies 4 V to the cap, take note of the capacitance change with applied voltage charts. The value of the 0204 part will be reduced to a bit above 30% of nominal (e.g. 0.3 uF instead of 1 uF) with 4 V applied. The 0805 part will still be at 95% of its nominal value with 4 V applied, and only loses about 45% of its value at 25 V applied.

So the smaller part can be used if you can accept its reduced temperature range, but its value will be reduced to just a bit more than the 0.1 uF value that has been typically recommended for use as the near-chip bypass capacitor over the past decade or so. If you really want 1.0 uF of bypassing, you'll still have add some larger parts in parallel with the suggested 0204 part.

On the other hand, if you can live with the low WV rating and you use this part in place of the "traditional" 0.1 uF 0402 part (in parallel with additional larger-value caps), you will gain a 3 - 4x increase in effective capacitance, so that is a substantial improvement.

Also, in a high-reliability application, you may want to use a package at least one size up from the minimum needed for the capacitor value and WV you are using. The smallest available size is pushing the limits of what the manufacturers can do, and can have reliability issues.

Answer 2

No, because the two capacitors serve different purposes. The larger capacitor provides power during large bursts of demand, and the smaller capacitor filters high frequency noise from/to the supply. A single capacitor cannot perform both tasks effectively.

Answer 3

If you can accomplish the same filtering and supply requirements with a single capacitor, use one. Generally, it is not the case, as stated above. However, a multiple capacitor solution has several disadvantages: 1. higher cost (2 components, 2 placements). 2. larger board space. 3. If using two capacitors, the frequency response plot has a zero between the two poles, which might cause some problems.