LTC1550LCMS8-2 Linear Technology, LTC1550LCMS8-2 Datasheet - Page 9

• Manufacturer Part Number: LTC1550LCMS8-2
• Description: IC VOLT INV REGLTD SW-CAP 8-MSOP
• Manufacturer: Linear Technology
• Type: Switched Capacitor (Charge Pump), Invertingr

Specifications of LTC1550LCMS8-2

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: -2V
• Current - Output: 20mA
• Frequency - Switching: 900kHz
• Voltage - Input: 2.7 ~ 5.5 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

Figure 4 shows a marked decrease in peak-to-peak output
ripple when a 0.1 F 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 F 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.
APPLICATIONS
AC COUPLE
5mV/DIV
+
V
Figure 5. Output Ripple with 10 F Tantalum Capacitor
CC
V
OUT
C
2.2 F
IN
Figure 3. Output Ripple Test Circuit
1
2
3
4
SHDN
V
C1
V
CC
OUT
+
LTC1551L
0.1 F
U
C1
CP
GND
REG
C1
OUT
INFORMATION
U
–
5 s/DIV
6
5
8
7
R1
10k
+
C
10 F
W
OUT
C
0.1 F
C
0.1 F
L
CP
1550L/51L F05
1550L/51L F03
U
V
– 4.1V
OUT
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
– 2V generator. The corresponding spectrum and spot
noise plots for this circuit are shown in the Typical Perfor-
mance Characteristics section.
AC COUPLE
2mV/DIV
P-P
V
. Figure 1 shows an example of an ultralow noise
OUT
Figure 4. Output Ripple vs Output Capacitance
Figure 6. 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)
LTC1550L/LTC1551L
WITHOUT 0.1 F
WITH 0.1 F
10
10 s/DIV
V
T
C
CC
A
IN
= 25 C
= 2.2 F
1550L/51L F04
= 5V
100
1550L/51L F06
9