LTC3612_15 Datasheet, PDF (15/30 Pages) Linear Technology – 3A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter

LTC3612
Applications Information
Frequency Synchronization
The LTC3612’s internal oscillator can be synchronized to
an external frequency by applying a square wave clock
signal to the RT/SYNC pin. During synchronization, the top
switch turn-on is locked to the falling edge of the external
frequency source. The synchronization frequency range
is 300kHz to 4MHz. During synchronization all operation
modes can be selected.
It is recommended that the regulator is powered down
(RUN pin to ground) before removing the clock signal on
the RT/SYNC pin in order to reduce inductor current ripple.
AC coupling should be used if the external clock generator
cannot provide a continuous clock signal throughout start-
up, operation and shutdown of the LTC3612. The size of
capacitor C_SYNC depends on parasitic capacitance on the
RT/SYNC pin and is typically in the range of 10pF to 22pF
V_{IN LTC3612}
SV_IN
RT/SYNC
f_OSC
2.25MHz
V_{IN LTC3612}
0.4V ^SVIN
RT/SYNC
R_T
SGND
f_OSC ∝1/R_T
V_{IN LTC3612}
SV_IN
f_OSC
RT/SYNC ^1/TP
SGND
1.2V
0.3V
T_P
V_{IN LTC3612}
C_SYNC
SV_IN
f_OSC
RT/SYNC ^1/TP
SGND
R_T
3612 F02
Figure 2. Setting the Switching Frequency
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current ∆I_L increases with higher V_IN and decreases
with higher inductance:
∆I_L
=


V_OUT
f_SW •L


•


1–
V_OUT
V_IN


Having a lower ripple current reduces the core losses
in the inductor, the ESR losses in the output capacitors
and the output voltage ripple. A reasonable starting point
for selecting the ripple current is ∆I_L = 0.3 • I_OUT(MAX).
The largest ripple current occurs at the highest V_IN. To
guarantee that the ripple current stays below a specified
maximum, the inductor value should be chosen according
to the following equation:
L
=


f_SW
V_OUT
• ∆I_L(MAX)


•


1–
V_OUT
V_IN


The inductor value will also have an effect on Burst Mode
operation. The transition to low current operation begins
when the peak inductor current falls below a level set by the
burst clamp. Lower inductor values result in higher ripple
current which causes this to occur at lower load currents.
This causes a dip in efficiency in the upper range of low
current operation. In Burst Mode operation, lower induc-
tance values will cause the burst frequency to increase.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. Actual core loss is independent of core size for
fixed inductor value, but it is very dependent on the induc-
tance selected. As the inductance increases, core losses de-
crease. Unfortunately, increased inductance requires more
turns of wire and therefore, copper losses will increase.
Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing satura-
tion. Ferrite core material saturates “hard,” meaning
that inductance collapses abruptly when the peak design
current is exceeded. This results in an abrupt increase in
For more information www.linear.com/LTC3612
3612fc
15