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LTC1551L Datasheet(PDF) 9 Page - Linear Technology |
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LTC1551L Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LTC1550L/LTC1551L APPLICATIONS INFORMATION Figure 4 shows a marked decrease in peak-to-peak output ripple when a 0.1 µF ceramic capacitor is 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 millivolts under some conditions. A 0.1 µF ceramic capaci- tor has significantly lower impedance at the spike fre- quency than a large tantalum capacitor, and eliminates most of these left-over switching spikes that the tantalum capacitor leaves behind. Figure 5 and 6 show scope photos of the output of Figure 4 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. Figure 1 shows an example of an ultralow noise – 2V generator. The corresponding spectrum and spot noise plots for this circuit are shown in the Typical Perfor- mance Characteristics section. 5 µs/DIV 1550L/51L F05 Figure 5. Output Ripple with 10 µF Tantalum Capacitor VOUT AC COUPLE 5mV/DIV Figure 3. Output Ripple Test Circuit Figure 4. Output Ripple vs Output Capacitance OUTPUT CAPACITANCE ( µF) 8 7 6 5 4 3 2 1 0 10 1550L/51L F04 1 100 VCC = 5V TA = 25°C CIN = 2.2µF WITHOUT 0.1 µF WITH 0.1 µF 1 2 VCC 8 7 REG CPOUT SHDN VCC C1+ VOUT LTC1551L GND C1– COUT 10 µF 1550L/51L F03 VOUT – 4.1V CCP 0.1 µF C1 0.1 µF CIN 2.2 µF 3 4 6 5 CL 0.1 µF + R1 10k 10 µs/DIV 1550L/51L F06 VOUT AC COUPLE 2mV/DIV Figure 6. Output Ripple with 10 µF Tantalum Capacitor Paralleled with 0.1 µF Ceramic Capacitor |
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