LTC3708_15 Datasheet, PDF (15/32 Pages) Linear Technology – Fast 2-Phase, No RSENSE Buck Controller with Output Tracking

LTC3708
APPLICATIONS INFORMATION
PLL and Frequency Synchronization
In the LTC3708, there are two onboard phase-locked loops
(PLL). One PLL is used to achieve frequency locking and
180° phase shift between the two channels while the sec-
ond PLL locks onto the rising edge of an external clock.
Since the LTC3708 uses a constant on-time architecture,
the error signal generated by the phase detector of the PLL
is used to vary the on time to achieve frequency locking
and phase separation. The variable on-time range is from
0.5 • t_ON to 2 • t_ON, where t_ON is the initial on time set by
the R_ON resistor.
To fully utilize the frequency synchronization range of the
PLL, it is advisable to set the initial on time properly so
that the two channels have close free-running frequencies.
Frequencies far apart may exceed the synchronization
capability of the PLL. If the two output voltages are V_OUT1
and V_OUT2, for example, R_ON resistors should then be
selected proportionally:
R_{ON1 =} V_OUT1
R_ON2 V_OUT2
Similarly, if the external PLL is engaged to synchronize
to an external frequency of f_EXT, R_ON1 should be selected
close to:
R_ON1
=
0.7
•
V_OUT1
f_EXT • 10pF
hence,
⎛⎝⎜_RON2
=
0.7
•
V_OUT2
f_EXT • 10pF
⎞
⎠⎟
In this case, channel 1 will first be synchronized to the
external frequency and channel 2 will then be synchronized
to channel 1 with 180° phase separation.
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and ripples in the
output voltage. Highest efficiency operation is obtained at
low frequency with small ripple current. However, achieving
this requires a large inductor. There is a trade-off between
component size and efficiency.
A reasonable starting point is to choose a ripple current
that is about 40% of I_OUT(MAX). Note that the largest ripple
current occurs at the highest V_IN. To guarantee that ripple
current does not exceed a specified maximum, the induc-
tance should be chosen according to:
L
=
⎛
⎝⎜
f
•
V_OUT
ΔI_L(MAX)
⎞⎛
⎠⎟ ⎝⎜^1–
V_OUT ^⎞
V_{IN(MAX) ⎠⎟}
Once the value for L is known, the type of inductor must be
selected. A variety of inductors designed for high current,
low voltage applications are available from manufacturers
such as Sumida and Panasonic.
Schottky Diode Selection
The Schottky diodes in parallel with both bottom MOSFETs
conduct during the dead time between the conduction of
the power MOSFET switches. They are intended to prevent
the body diode of the bottom MOSFET from turning on
and storing charge during the dead time, which causes a
modest (about 1%) efficiency loss. The diodes can be rated
for about one-half to one-fifth of the full load current since
they are on for only a fraction of the duty cycle. In order
for the diodes to be effective, the inductance between them
and the bottom MOSFETs must be as small as possible,
mandating that these components be placed as close as
possible in the circuit board layout. The diodes can be
omitted if the efficiency loss is tolerable.
Inductor Selection
Given the desired input and output voltages, the induc-
tor value and operating frequency determine the ripple
current:
ΔI_L
=
⎛
⎝⎜
V_OUT
f •L
⎞
⎠⎟
⎛
⎝⎜ ^1–
V_OUT
V_IN
⎞
⎠⎟
C_IN and C_OUT Selection
The selection of C_IN is simplified by the 2-phase architec-
ture and its impact on the worst-case RMS current drawn
through the input network (battery/fuse/capacitor). It can
be shown that the worst-case RMS current occurs when
only one controller is operating. The controller with the
3708fb
15