LTC3892_15 Datasheet, PDF (18/36 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

LTC3892/LTC3892-1
Applications Information
Ensure that R1 has a power rating higher than this value.
If high efficiency is necessary at light loads, consider this
power loss when deciding whether to use DCR sensing or
sense resistors. Light load power loss can be modestly
higher with a DCR network than with a sense resistor,
due to the extra switching losses incurred through R1.
However, DCR sensing eliminates a sense resistor, reduces
conduction losses and provides higher efficiency at heavy
loads. Peak efficiency is about the same with either method.
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 switching and 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, ∆I_L, decreases with higher induc-
tance or higher frequency and increases with higher V_IN:
∆I_L
=
(
1
f)(L)
V_OUT


1−
V_OUT
V_IN


Accepting larger values of ∆I_L allows the use of low
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ∆I_L = 0.3(I_MAX). The maximum
∆I_L occurs at the maximum input voltage.
The inductor value also has secondary effects. The tran-
sition to Burst Mode operation begins when the average
inductor current required results in a peak current below
25% of the current limit determined by R_SENSE. Lower
inductor values (higher ∆I_L) will cause this to occur at
lower 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 value for L is known, the type of inductor must
be selected. Core loss is independent of core size for a
fixed inductor value, but it is very dependent on inductance
value selected. As inductance increases, core losses go
down. Unfortunately, increased inductance requires more
turns of wire and therefore copper losses will increase.
Ferrite designs have very low core loss and are preferred
for high switching frequencies, so design goals can con-
centrate on copper 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!
Power MOSFET and Schottky Diode
(Optional) Selection
Two external power MOSFETs must be selected for each
controller in the LTC3892/LTC3892-1: one N-channel
MOSFET for the top (main) switch and one N-channel
MOSFET for the bottom (synchronous) switch.
The peak-to-peak drive levels are set by the DRV_CC volt-
age. This voltage can range from 5V to 10V depending
on configuration of the DRVSET pin. Therefore, both
logic-level and standard-level threshold MOSFETs can be
used in most applications depending on the programmed
DRV_CC voltage. Different UVLO thresholds appropriate
for logic-level or standard-level threshold MOSFETs can
be selected by the DRVUV pin. Pay close attention to the
BV_DSS specification for the MOSFETs as well.
The LTC3892/LTC3892-1’s unique ability to adjust the gate
drive level between 5V to 10V (OPTI-DRIVE) allows an
application circuit to be precisely optimized for efficiency.
When adjusting the gate drive level, the final arbiter is the
total input current for the regulator. If a change is made
and the input current decreases, then the efficiency has
improved. If there is no change in input current, then there
is no change in efficiency.
Selection criteria for the power MOSFETs include the
on-resistance R_DS(ON), Miller capacitance C_MILLER, input
18
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38921f