Datasheet

9
LTC1550L/LTC1551L
APPLICATIONS INFORMATION
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Figure 4 shows a marked decrease in peak-to-peak output
ripple when a 0.1µF 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 0.1µF 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 0.1µF ceramic
capacitor will usually keep the output ripple well below
1mV
P-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.
5µs/DIV 1550L/51L F05
Figure 5. Output Ripple with 10µF Tantalum Capacitor
V
OUT
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
OUTPUT RIPPLE (mV
P-P
)
1
100
V
CC
= 5V
T
A
= 25°C
C
IN
= 2.2µF
WITHOUT 0.1µF
WITH 0.1µF
1
2
V
CC
8
7
REG
CP
OUT
SHDN
V
CC
C1
+
V
OUT
LTC1551L
GND
C1
–
C
OUT
10µF
1550L/51L F03
V
OUT
–4.1V
C
CP
0.1µF
C1
0.1µF
C
IN
2.2µF
3
4
6
5
C
L
0.1µF
+
+
R1
10k
10µs/DIV 1550L/51L F06
V
OUT
AC COUPLE
2mV/DIV
Figure 6. Output Ripple with 10µF Tantalum
Capacitor Paralleled with 0.1µF Ceramic Capacitor