7
DC3054A-B
Rev. 0
THEORY OF OPERATION
Introduction to the DC3054A-B
The DC3054A-B demonstration circuit features the
LTC3308B, a low voltage synchronous step-down silent
switcher. The LTC3308B is a monolithic, constant fre-
quency, current mode step-down DC/DC converter. An
oscillator, with frequency set using a resistor on the RT
pin, turns on the internal top power switch at the beginning
of each clock cycle. Current in the inductor then increases
until the top switch comparator trips and turns off the
top power switch. If the EN pin is low, the LTC3308B is
in shutdown and in a low quiescent current state. When
the EN pin is above its threshold, the switching regulator
will be enabled.
The MODE/SYNC pin sets the switching mode to pulse
skip, forced continuous, or Burst Mode operation. If an
external 3MHz to 10MHz clock is connected to the MODE/
SYNC turret while the JP1 is set to the FC/SYNC position,
the LTC3308B switching frequency will sync to the exter-
nal clock while operating in forced continuous mode. See
the LTC3308B data sheet for more detailed information.
The maximum allowable operating frequency is influenced
by the minimum on time of the top switch, the ratio of
V
OUT
to V
IN
. The maximum allowable operating frequency
may be calculated using a minimum t
ON
of 22ns in the
formula below.
f
SW(MAX)
=
V
OUT
V
IN(MAX)
• t
ON(MIN)
Select an operating switching frequency below f
SW(MAX)
.
The recommended ripple current in the output inductor is
1.2A peak-to-peak for the LTC3308B. This determines the
recommended inductor value for the application.
Accurately Measuring Output Ripple of the LTC3308B
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 bandwidth
and above a 50Ω coax cable connected as close to the
output caps as possible should be used with the oscillo-
scope 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 C4. These pads can also be used to sol-
der a coax cable or other oscilloscope probe connector
if desired.
The DC3054A-B uses 3-terminal capacitors, C18 and
C19, 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 out-
put 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 on with the 3-terminal capacitor populated.
200ns/DIV
V
OUT
5mV/DIV
dc3054b F04
V
IN
= 3.3V
V
OUT
= 1.8V
I
OUT
= 4A
C4, C5 = 4,7µF 0603
C18, C19 3-TERMINAL CAPACITOR NOT
POPULATED
200ns/DIV
V
OUT
5mV/DIV
dc3054b F05
V
IN
= 3.3V
V
OUT
= 1.8V
I
OUT
= 4A
C4, C5 = 4,7µF 0603
C18, C19 = 1nF 3-TERMINAL CAP
