9

LTC1550L/LTC1551L

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

– 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

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

WITHOUT 0.1

µF

WITH 0.1

µF

1

2

8

7

REG

SHDN

LTC1551L

GND

10

µF

1550L/51L F03

– 4.1V

0.1

µF

C1

0.1

µF

2.2

µF

3

4

6

5

0.1

µF

+

R1

10k

10

µs/DIV

1550L/51L F06

AC COUPLE

2mV/DIV

Figure 6. Output Ripple with 10

µF Tantalum

Capacitor Paralleled with 0.1

µF Ceramic Capacitor