LTC1550 데이터 시트보기 (PDF) - Linear Technology
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LTC1550 Datasheet PDF : 12 Pages
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LTC1550/LTC1551
APPLICATIONS INFORMATION
output capacitors provide lower output ripple. To keep
ripple below 1mVP-P, 10µF or greater, with a 0.1µF ceramic
capacitor in parallel, is required. At least 4.7µF 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 0.1µF 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
millivolts under some conditions. A garden variety 0.1µF
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 0.1µF ceramic
capacitor will usually keep the output ripple well below
1mVP-P. The cover page shows an example of an ultralow
VCC
+ CIN
4.7µF
1
SHDN
2
VCC
8
SENSE
7
CPOUT
LTC1551
3 C1+
GND 6
4
VOUT
C1– 5
CCP
0.1µF
*CL IS OPTIONAL
C1
0.1µF
COUT
10µF
VOUT
– 4.1V
CL
0.1µF*
1550/51 F03
Figure 2. Output Ripple Test Circuit
8
VCC = 6V
7
TA = 25°C
WITHOUT 0.1µF
CIN = 4.7µF
6
5
4
3
2
WITH 0.1µF
1
0
1
10
100
OUTPUT CAPACITANCE (µF)
LTC1550/51 • F04
Figure 3. Output Ripple vs Output Capacitance
VOUT
AC COUPLE
5mV/DIV
VOUT
AC COUPLE
2mV/DIV
5µs/DIV
LTC1550/51 • F03
Figure 4. Output Ripple with 10µF Tantalum Capacitor
8
10µs/DIV
LTC1550/51 • F04
Figure 5. Output Ripple with 10µF Tantalum
Capacitor Paralleled with 0.1µF Ceramic Capacitor
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