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June 1999
Application Brief 42018 Capacitor Selection for the ML6428-1 S-Video Filter
METHOD
The ML6428 comes in two versions, the -1 and the -2. All outputs must be AC coupled into their loads for the -1 version. The -2 version is DC coupled. Both versions require 0.1F capacitor input coupling. Finding video quality -- i.e., low ESR, equivalent series resistance, low inductance, low leakage, and constant capacitance over a frequency range from 20Hz to 6MHz -- 0.1F capacitors presents no problem. Surface mount capacitors meeting these requirements are both inexpensive and plentiful. The ML6428-1 also requires four 220F output coupling capacitors if all outputs are to be used. See Figure 1. However, some users may use values up to 1000 F to pass "TILT" specifications in the two-field test. In the past, when wideband signals such as video needed coupling capacitors in the hundreds of F's, tantalums were the only choice. This presents a problem. The ML6428 was designed for low-cost applications. In the quantities needed, the cost of the tantalums is greater than the cost of the filter and the other associated components. There is a solution: modern, surface mount, aluminum electrolytic capacitors designed for use in high-frequency switching power supplies have all the necessary qualities. For a given capacitance and voltage rating these electrolytics are about the same size as the tantalums. They also have low equivalent series resistance, low inductance, and low leakage. Since they are intended for use as bypass capacitors in switching power supplies operating up to 1MHz, they have an adequate frequency response as video coupling capacitors. Several types from several manufacturers have been investigated. Some 220F surface mount aluminum electrolytic capacitors from Cornell Dubilier, Elna, and United Chemi-Con were indistinguishable from highquality, high-cost tantalums, as can be seen by comparing Figures 2 and 4 with Figures 3 and 5. It should be pointed out that these are not the only suppliers of usable capacitors. Any physically small, surface mount, 220F, 6V to 10V, aluminum electrolytic capacitor with ESR less than 1 at 6MHz and with less than 2% capacitance change from 20Hz to 6MHz is a candidate for use in ML6428-1 circuits. Many capacitors meet these requirements. With the right capacitors, their cost is no barrier to using the ML6428-1.
ML6428-1 0.1F YIN 1 4th-ORDER FILTER + 220F 8 220F 75 VIDEO CABLES YOUT 75 6 75 CVOUT 75 0.1F CIN 4 + 4th-ORDER FILTER 5 220F 75 CVOUT 75 C* 2 5V 220F 75 COUT 0.1F 1F * C AND R DEPEND ON THE INPUT IMPEDANCE OF LOAD 75 7 3 R* ON-CHANNEL MODULATOR, VCR, AND TV
Figure 1. AC Coupled S-Video and Composite Video Line Driver for NTSC
REV. 1.0 10/25/2000
Application Brief 15
0 1 2 3 AMPLITUDE (dB) 4 5 6 7 8 9 10 1 2 3 4 6 8 10 AMPLITUDE (dB) 0 1 2 3 4 5 6 7 8 9 10 1 2 3 4 6 8 10
FREQUENCY (MHz)
FREQUENCY (MHz)
Figure 2. -3dB Response with Tantalum Capacitor
Figure 3. -3dB Response with Aluminum Electrolytic Capacitor
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Figure 4. Wideband Response with Tantalum Capacitor
Figure 5. Wideband Response with Aluminum Electrolytic Capacitor
DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS. LIFE SUPPORT POLICY FAIRCHILD'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury of the user. www.fairchildsemi.com 2. A critical component in any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness.
(c) 2000 Fairchild Semiconductor Corporation
REV. 1.0 10/25/2000
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