LTC1550 Linear Technology, LTC1550 Datasheet - Page 8

• Manufacturer Part Number: LTC1550
• Description: Low Noise/ Switched Capacitor Regulated Voltage Inverters
• Manufacturer: Linear Technology

output capacitors provide lower output ripple. To keep
ripple below 1mV
capacitor in parallel, is required. At least 4.7 F 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 F 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
LTC1550/LTC1551
APPLICATIONS
8
AC COUPLE
5mV/DIV
Figure 4. Output Ripple with 10 F Tantalum Capacitor
+
*C
V
V
CC
OUT
L
IS OPTIONAL
C
4.7 F
IN
Figure 2. Output Ripple Test Circuit
1
2
3
4
SHDN
V
C1
V
P-P
CC
OUT
+
LTC1551
U
, 10 F or greater, with a 0.1 F ceramic
0.1 F
C1
SENSE
CP
GND
INFORMATION
OUT
C1
U
–
5 s/DIV
6
5
8
7
+
W
C
10 F
OUT
LTC1550/51 • F03
C
0.1 F*
C
0.1 F
L
CP
U
1550/51 F03
V
– 4.1V
OUT
millivolts under some conditions. A garden variety 0.1 F
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 F ceramic
capacitor will usually keep the output ripple well below
1mV
AC COUPLE
2mV/DIV
P-P
V
OUT
. The cover page shows an example of an ultralow
Figure 3. Output Ripple vs Output Capacitance
Figure 5. Output Ripple with 10 F Tantalum
Capacitor Paralleled with 0.1 F Ceramic Capacitor
8
7
6
5
4
3
2
1
0
1
OUTPUT CAPACITANCE ( F)
WITHOUT 0.1 F
WITH 0.1 F
10
10 s/DIV
V
T
C
A
CC
IN
LTC1550/51 • F04
= 25 C
= 4.7 F
= 6V
100
LTC1550/51 • F04