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LTC1550CGN Datasheet(PDF) 8 Page - Linear Technology |
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LTC1550CGN Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LTC1550/LTC1551 APPLICATIONS INFORMATION output capacitors provide lower output ripple. To keep ripple below 1mVP-P, 10µF or greater, with a 0.1µF ceramic capacitor in parallel, is required. At least 4.7 µF is required at the output under all conditions to guarantee loop stability. Figure 3 shows a marked decrease in peak-to-peak output ripple when a 0.1 µF ceramic capacitor added in parallel with the tantalum output capacitor. The additional ripple with the tantalum output capacitor alone is mostly very high order harmonics of the 900kHz clock, which appear as sharp "spikes" at the output. The energy in these spikes is very small and they do not contribute to the RMS output voltage, but their peak-to-peak amplitude can be several Figure 5. Output Ripple with 10 µF Tantalum Capacitor Paralleled with 0.1 µF Ceramic Capacitor 10 µs/DIV LTC1550/51 • F04 VOUT AC COUPLE 2mV/DIV millivolts under some conditions. A garden variety 0.1 µF ceramic capacitor has significantly lower impedance at the spike frequency than even a large tantalum capacitor, and helps eliminate most of these left-over switching spikes that the tantalum capacitor leaves behind. Figure 4 and 5 show scope photos of the output of Figure 3 with and without the additional ceramic capacitor at the output. A series RC or LC filter can reduce high frequency output noise even further. Due to the high 900kHz switching frequency, not much R or L is required; a ferrite bead or a relatively long PC board trace in series with 0.1 µF ceramic capacitor will usually keep the output ripple well below 1mVP-P. The cover page shows an example of an ultralow 5 µs/DIV LTC1550/51 • F03 Figure 4. Output Ripple with 10 µF Tantalum Capacitor VOUT AC COUPLE 5mV/DIV Figure 2. Output Ripple Test Circuit 1 2 VCC 8 7 SENSE CPOUT SHDN VCC C1+ VOUT LTC1551 GND C1– COUT 10 µF 1550/51 F03 VOUT – 4.1V CCP 0.1 µF C1 0.1 µF CIN 4.7 µF 3 4 6 5 CL 0.1 µF* *CL IS OPTIONAL + Figure 3. Output Ripple vs Output Capacitance OUTPUT CAPACITANCE ( µF) 8 7 6 5 4 3 2 1 0 10 LTC1550/51 • F04 1 100 VCC = 6V TA = 25°C CIN = 4.7µF WITHOUT 0.1 µF WITH 0.1 µF |
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