Low Inductance Capacitors Introduction As switching speeds increase and pulse rise times decrease INTERDIGITATED CAPACITORS the need to reduce inductance becomes a serious limitation Multiple terminations of a capacitor will also help in reducing for improved system performance. Even the decoupling the parasitic inductance of the device. The IDC is such a capacitors, that act as a local energy source, can generate device. By terminating one capacitor with 8 connections the unacceptable voltage spikes: V = L (di/dt). Thus, in high ESL can be reduced even further. The measured inductance speed circuits, where di/dt can be quite large, the size of the of the 0612 IDC is 60 pH, while the 0508 comes in around voltage spike can only be reduced by reducing L. 50 pH. These FR4 mountable devices allow for even higher Figure 1 displays the evolution of ceramic capacitor toward clock speeds in a digital decoupling scheme. Design and lower inductance designs over the last few years. AVX has product offerings are shown on pages 48 and 49. been at the forefront in the design and manufacture of these newer more effective capacitors. SpinGuard 2000 -+ - + 2000 pH 1500 1206 MLC 1200 pH 1000 0612 LICC 170 pH 500 0508 LICC 130 pH 0306 LICC 0612 IDC 105pH 0508 IDC 60 pH LICA 50 pH 25 pH LOW INDUCTANCE CHIP ARRAYS (LICA ) Further reduction in inductance can be achieved by designing 0 alternative current paths to minimize the mutual inductance 1990s 1980s factor of the electrodes (Figure 3). This is achieved by AVXs Figure 1. The evolution of Low Inductance Capacitors at AVX LICA product which was the result of a joint development (values given for a 100 nF capacitor of each style) between AVX and IBM. As shown in Figure 4, the charging current flowing out of the positive plate returns in the opposite direction along adjacent negative plates. This minimizes the LOW INDUCTANCE CHIP CAPACITORS mutual inductance. The total inductance of a chip capacitor is determined both The very low inductance of the LICA capacitor stems from by its length to width ratio and by the mutual inductance the short aspect ratio of the electrodes, the arrangement of coupling between its electrodes. Thus a 1210 chip size has the tabs so as to cancel inductance, and the vertical aspect lower inductance than a 1206 chip. This design improve- of the electrodes to the mounting surface. ment is the basis of AVXs low inductance chip capacitors, LI Caps, where the electrodes are terminated on the long side of the chip instead of the short side. The 1206 becomes an 0612 as demonstrated in Figure 2. In the same manner, an 0805 becomes an 0508 and 0603 becomes an 0306. This results in a reduction in inductance from around 1200 pH for conventional MLC chips to below 200 pH for Low Inductance Chip Capacitors. Standard designs and perfor- mance of these LI Caps are given on pages 46 and 47. Net Net Inductance Inductance 1206 Figure 3. Net Inductance from design. In the standard Multilayer capacitor, the charge currents entering and leaving the capacitor create complementary 0612 flux fields, so the net inductance is greater. On the right, however, if the design permits the currents to be opposed, there is a net cancellation, and the Figure 2. Change in aspect ratio: 1206 vs. 0612 inductance is much lower. 44 pH Charges entering - plate Charges leaving + plate Charges entering - plate Charges leaving + plate - + +-Low Inductance Capacitors Introduction Also the effective current path length is minimized because the current does not have to travel the entire length of both electrodes to complete the circuit. This reduces the self inductance of the electrodes. The self inductance is also min- imized by the fact that the charging current is supplied by both sets of terminals reducing the path length even further The inductance of this arrangement is less than 30 pH, causing the self-resonance to be above 100 MHz for the same popular 100 nF capacitance. Parts available in the LICA design are shown on pages 50 and 51. Figure 5 compares the self resonant frequencies of various capacitor designs versus capacitance values. The approxi- mate inductance of each style is also shown. Figure 4. LICAs Electrode/Termination Construction. Active development continues on low inductance The current path is minimized this reduces self-inductance. capacitors. C4 termination with low temperature solder Current flowing out of the positive plate, returns in the is now available for plastic packages. Consult AVX opposite direction along the adjacent negative plate for details. this reduces the mutual inductance. 1000.00 LICA (25 pH) 0508 IDC (50 pH) 0612 IDC (60 pH) 100.00 0306 LICC (110 pH) 0508 LICC (130 pH) 0612 LICC (170 pH) 0603 (700 pH) 0805 (800 pH) 10.00 1206 (1200 pH) 1.00 10.00 100.00 1000.00 Capacitance, (nF) Figure 5. Self Resonant Frequency vs. Capacitance and Capacitor Design 45 Self Resonant Frequency (MHz)