8

LTC1550/LTC1551

output capacitors provide lower output ripple. To keep

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

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

5

µs/DIV

LTC1550/51 • F03

Figure 4. Output Ripple with 10

µF Tantalum Capacitor

AC COUPLE

5mV/DIV

Figure 2. Output Ripple Test Circuit

1

2

8

7

SENSE

SHDN

LTC1551

GND

10

µF

1550/51 F03

– 4.1V

0.1

µF

C1

0.1

µF

4.7

µF

3

4

6

5

0.1

µF*

+

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

WITHOUT 0.1

µF

WITH 0.1

µF