LTC1539 LINER [Linear Technology], LTC1539 Datasheet - Page 13

• Manufacturer Part Number: LTC1539
• Description: Dual High Efficiency, Low Noise, Synchronous Step-Down Switching Regulators
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
when the inductor current reaches zero while the bottom
MOSFET is on. Lower inductor values (higher I
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 (TGS1, 2 pins open), lower inductance
values will cause the burst frequency to decrease.
The Figure 3 graph gives a range of recommended induc-
tor values vs operating frequency and V
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool M
size for a fixed inductor value, but it is very dependent on
inductance 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
at 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!
Molypermalloy (from Magnetics, Inc.) is a very good, low
loss core material for toroids, but it is more expensive than
ferrite. A reasonable compromise from the same manu-
facturer is Kool M . Toroids are very space efficient,
especially when you can use several layers of wire. Be-
cause they generally lack a bobbin, mounting is more
difficult. However, designs for surface mount are available
which do not increase the height significantly.
Power MOSFET and D1 Selection
Three external power MOSFETs must be selected for each
controller with the LTC1539: a pair of N-channel MOSFETs
for the top (main) switch and an N-channel MOSFET for
the bottom (synchronous) switch. Only one top MOSFET
is required for each LTC1538-AUX controller.
Kool M is a registered trademark of Magnetics, Inc.
®
cores. Actual core loss is independent of core
U
INFORMATION
U
W
OUT
.
L
) will cause
U
To take advantage of the Adaptive Power output stage, two
topside MOSFETs must be selected. A large [low R
MOSFET and a small [higher R
quired. The large MOSFET is used as the main switch and
works in conjunction with the synchronous switch. The
smaller MOSFET is only enabled under low load current
conditions. The benefit of this is to boost low to midcurrent
efficiencies while continuing to operate at constant fre-
quency. Also, by using the small MOSFET the circuit will
keep switching at a constant frequency down to lower
currents and delay skipping cycles.
The R
around 0.5 . Be careful not to use a MOSFET with an
R
gate charge. (A higher R
capacitance and thus requires less current to charge its
gate). For all LTC1538-AUX and cost sensitive LTC1539
applications, the small MOSFET is not required. The circuit
then begins Burst Mode operation as the load current
drops.
The peak-to-peak drive levels are set by the INTV
age. This voltage is typically 5V during start-up (see
EXTV
old MOSFETs must be used in most LTC1538-AUX/
LTC1539 applications. The only exception is applications
in which EXTV
greater than 8V (must be less than 10V), in which standard
threshold MOSFETs (V
attention to the BV
many of the logic level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the "ON"
resistance R
input voltage and maximum output current. When the
LTC1538-AUX/LTC1539 are operating in continuous mode
the duty cycles for the top and bottom MOSFETs are given
by:
DS(ON)
Main Switch Duty Cycle
Synchronous Switch Duty Cycle
CC
DS(ON)
Pin Connection). Consequently, logic level thresh-
that is too low; remember, we want to conserve
SD(ON)
recommended for the small MOSFET is
LTC1538-AUX/LTC1539
CC
DSS
is powered from an external supply
, reverse transfer capacitance C
specification for the MOSFETs as well;
GS(TH)
DS(ON)
< 4V) may be used. Pay close
MOSFET has a smaller gate
V
DS(ON)
V
OUT
IN
] MOSFET are re-
V
IN
V
–
IN
V
OUT
13
CC
SD(ON)
volt-
RSS
]
,