LTC3727LX-1_15 Datasheet, PDF (13/28 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3727LX-1
2.5
2.0
1.5
1.0
0.5
0
200 250 300 350 400 450 500 550
OPERATING FREQUENCY (kHz)
3727LX1 F05
Figure 5. PLLFLTR Pin Voltage vs Frequency
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 ∆I_L decreases with higher induc-
tance or 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 transi-
tion 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 inductance value is determined, the type of
inductor must be selected. Actual core loss is independent
of core size for a fixed inductor value, but it is very
dependent on inductance selected. As inductance in-
creases, 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 concen-
trate on reducing I²R loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design cur-
rent 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/
current and price/current relationship of an inductor.
Toroid or shielded pot cores in ferrite or permalloy mate-
rials are small and don’t radiate much energy, but gener-
ally cost more than powdered iron core inductors with
similar characteristics. The choice of which style inductor
to use mainly depends on the price vs size requirements
and any radiated field/EMI requirements. New designs
for high current surface mount inductors are available
from numerous manufacturers, including Coiltronics,
Vishay, TDK, Pulse, Panasonic, Wuerth, Coilcraft, Toko
and Sumida.
Power MOSFET and D1 Selection
Two external power MOSFETs must be selected for each
controller in the LTC3727LX-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 INTV_CC
voltage. This voltage is typically 7.5V during start-up (see
EXTV_CC Pin Connection). Consequently, logic-level
threshold MOSFETs must be used in most applications.
The only exception is if low input voltage is expected
(V_IN < 5V); then, sub-logic level threshold MOSFETs
(V_GS(TH) < 3V) should be used. Pay close attention to the
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