LTC3773 Datasheet, PDF (15/32 Pages) Linear Technology – Triple Output Synchronous 3-Phase DC/DC Controller with Up/Down Tracking

LTC3773
APPLICATIONS INFORMATION
The basic application circuit is shown on the first page
of this data sheet. External component selection is
driven by the load requirement, and normally begins
with the selection of an inductance value based upon the
desired operating frequency, inductor current and output
voltage ripple requirements. Once the inductors and oper-
ating frequency have been chosen, the current sensing
resistors can be calculated. Next, the power MOSFETs and
Schottky diodes are selected. Finally, C_IN and C_OUT are
selected according to the required voltage ripple require-
ments. The circuit on the front page can be configured for
operation up to a MOSFET supply voltage of 36V (limited
by the external MOSFETs, V_IN capacitor voltage rating and
possibly the minimum on-time).
Operating Frequency and Synchronization
The choice of operating frequency, f_OSC, is a trade-off
between efficiency and component size. Low frequency op-
eration improves efficiency by reducing MOSFET switching
losses, both gate charge loss and transition loss. However,
lower frequency operation requires more inductance for a
given amount of ripple current. The internal oscillator for
each of the LTC3773’s controllers runs at a nominal 400kHz
frequency when the PLLFLTR pin is left floating and the
PLLIN/FC pin input is not switching. Pulling PLLFLTR to
V_CC selects 560kHz operation; pulling PLLFLTR to SGND
selects 220kHz operation. Alternatively, the LTC3773 will
phase-lock to a clock signal applied to the PLLIN/FC pin
with a frequency between 160kHz and 700kHz (see Phase-
Locked Loop and Frequency Synchronization).
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because of
MOSFET gate-charge losses. In addition to this basic trade-
off, the effect of inductor value on ripple current and low
current operation must also be considered. The inductor
value has a direct effect on ripple current. The inductor
ripple current ΔI_L decreases with higher inductance or
frequency and increases with higher V_IN or V_OUT:

I_L
=
V_OUT
(f)(L)


1–
V_OUT
V_IN


Accepting larger values of ΔI_L allows the use of low in-
ductances, but results in higher output voltage ripple and
greater core losses. A reasonable starting point for setting
ripple current is ΔI_L = 0.3 to 0.6 (I_MAX). Remember, the
maximum ΔI_L occurs at the maximum input voltage. The
inductor value also has an effect on low current operation.
The transition to low current operation begins when the
inductor current reaches zero while the bottom MOSFET
is on. Burst Mode operation begins when the average
inductor current required results in a peak current below
20% of the current limit determined by R_SENSE. Lower
inductor values (higher ΔI_L) will cause this to occur at
higher load currents, which can cause a dip in efficiency
in the upper range of low current operation. In Burst Mode
operation, lower inductance values will cause the burst
frequency to decrease.
Inductor Core Selection
Once the inductance value is determined, the type of in-
ductor must be selected. Actual core loss is independent
of core size for a fixed inductor value, but it is very de-
pendent on inductance selected. As inductance increases,
core losses go down. Unfortunately, increased inductance
requires more turns of wire and therefore copper (I²R)
losses will increase.
Ferrite designs have very low core loss and are preferred at
high switching frequencies, so designers can concentrate
on reducing I²R loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Different core materials and shapes will change the size/cur-
rent and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and do not radiate much energy, but generally cost
more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
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