Analog Devices DC2990A Manual page 7

THEORY OF OPERATION
Accurately Measuring Output Ripple of the LTC3307A
With the fast edge rates of the circuit, high frequency
noise can be observed when measuring the output voltage
with 1MΩ terminated oscilloscope probes. To better view
the output ripple with oscilloscopes of 400MHz band-
width and above a 50Ω coax cable connected as close
to the output capacitor as possible should be used with
the oscilloscope channel terminated to 50Ω at the scope.
This will help to reduce the noise coupling onto and dis-
playing 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 capacitor C4. These pads can also
be used to solder a coax cable or other oscilloscope probe
connector if desired.
TP1
50Ω PROBE
1.2V
10mV/DIV
V = 3.3V
IN
V = 1.2V
OUT
I = 3A
OUT
Figure 5.
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. This can be reduced by choosing low ESL
capacitors or adding small low ESL capacitors in parallel
dc2990a F05
500ns/DIV
DEMO MANUAL DC2990A
to the output capacitors as close to the inductor as pos-
sible. Adding capacitors close to the load creates a π filter
between the output capacitors, trace inductance, and load
decoupling capasitors and will also help to reduce the
ripple. Figure 6 shows the output ripple using a 500MHz
scope, 50Ω probe with C4 and C5 reduced to 22μF 0603
capacitors. The capacitors near the V
bottom of the board were also populated with C17 = 1μF
0402, plus C18 and C19 = 10μF 0603 capacitors. The
output ripple was measured at TP3 on the bottom of the
board near the V turrets.
OUT
TP3
50Ω PROBE
1.2V
10mA/DIV
= 3.3V
V
IN
V = 1.2V
OUT
I = 3A
OUT
C4, C5 = 22µF 0603
C17 = 1µF
C18, SC19 = 10µF 0603
Figure 6.
turret on the
OUT
dc2990a F06
500ns/DIV
7
Rev. 0