7
Rev. 0
THEORY OF OPERATION
oscilloscope channel terminated to 50Ω at the scope. This
will help to reduce the noise coupling onto and displaying
on the scope. The demo board is set up to solder an
U.FL,RECEPT,ST SMD,0Hz to 6GHz 50Ω connector (TP1)
near the output cap C3 & C4. These pads can also be
used to solder a coax cable or other oscilloscope probe
connector if desired.
The DC3123A uses 3-terminal capacitors, C9 and C10,
on the output to reduce the high frequency spikes that
can be observed with high bandwidth oscilloscopes.
The high frequency spikes are partially attributed to the
interwinding capacitance of the inductor and the voltage
step is partially attributed to the inductance in the output
capacitors. The step can be reduced by choosing low ESL
capacitors as close to the inductor as possible. The high
frequency spikes are reduced with using a small low ESL
3-terminal capacitor with the self-resonant frequency
close to the high frequency spikes. Below is the output
ripple using a 500MHz scope, 50Ω probe measured at
TP1 without the 3-terminal capacitor populated in C9.
TP1
50Ω PROBE
V
IN
= 3.3V
V
OUT1
= 1.2V
I
OUT1
= 2A
C9 = 1nF 3-TERMINAL CAP
1µs/DIV
V
OUT1
10mV/DIV
dc3132a G05
TP2
50Ω PROBE
V
IN
= 3.3V
V
OUT2
= 1.8V
I
OUT2
= 2A
C10 = 1nF 3-TERMINAL CAP
1µs/DIV
V
OUT2
10mV/DIV
dc3132a G06
TP1
50Ω PROBE
V
IN
= 3.3V
V
OUT1
= 1.2V
I
OUT1
= 2A
C9 = OPEN
1µs/DIV
V
OUT1
10mV/DIV
dc3132a G07
