Theory Of Operation - Analog Devices DC2745A Demo Manual

THEORY OF OPERATION

Introduction to the DC2745A
The DC2745A demonstration circuit features the
LTC3309A, a low voltage synchronous step-down silent
switcher. The LTC3309A 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 begin-
ning 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 LT3309A
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. If an external
1MHz to 3MHz clock is connected to the MODE/SYNC
turret while the JP1 is set to the FC/SYNC position, the
LTC3309A switching frequency will sync to the external
clock while operating in forced continuous mode. See
the LTC3309A 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 to V . The maximum allowable operating frequency
OUT IN
may be calculated using a minimum t
formula below.
V
OUT
f =
SW(MAX)
V • t
IN(MAX)
Select an operating switching frequency below f
The recommended ripple current in the output inductor is
1.8A peak-to-peak for the LTC3309A. This determines the
recommended inductor value for the application.
Accurately Measuring Output Ripple of the LTC3309A
With the fast edge rates of the circuit, high frequency
noise can be observed when measuring the output volt-
age with 1MΩ terminated oscilloscope probes. To bet-
ter 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 oscilloscope channel terminated to 50Ω at the scope.
This will help to reduce the noise coupling onto and dis-
of 42ns in the
ON
ON(MIN)
SW(MAX)
DEMO MANUAL DC2745A
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 cap C4. These pads can also be
used to solder a coax cable or other oscilloscope probe
connector if desired.
10mV/DIV
V = 3.3V
IN
V = 1.2V
OUT
I = 6A
OUT
The high frequency spikes are partially attributed to the
inter-winding 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
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 caps, trace inductance, and load decou-
pling caps and will also help to reduce the ripple. Below is
the output ripple using a 500MHz scope, 50Ω probe with
C4 and C5 reduced to 22µF 0603 caps. The capacitors near
the V turret on the bottom of the board were also popu-
OUT
lated 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
10mV/DIV
V = 3.3V
IN
V = 1.2V
OUT
I = 6A
OUT
C4, C5 = 22µF 0603
C17 = 1µF 0402
C18, C19 = 10µF 0603
29mV
dc2745a G06
500ns/DIV
turrets.
OUT
9mV
dc2745a G07
500ns/DIV
Rev. 0
7