13
LTC1436A
LTC1436A-PLL/LTC1437A
14367afb
APPLICATIONS INFORMATION
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frequency operation down to lower currents before cycle
skipping occurs.
The R
DS(ON)
recommended for the small MOSFET is
around 0.5Ω. Be careful not to use a MOSFET with an
R
DS(ON)
that is too low; remember, we want to conserve
gate charge. (A higher R
DS(ON)
MOSFET has a smaller gate
capacitance and thus requires less current to charge its
gate). For cost sensitive applications the small MOSFET
can be removed. The circuit will then begin Burst Mode
operation as the load current is dropped.
The peak-to-peak gate drive levels are set by the INTV
CC
voltage. This voltage is typically 5V during start-up (see
EXTV
CC
Pin Connection). Consequently, logic level
threshold MOSFETs must be used in most LTC1436A/
LTC1437A applications. The only exception is applications
in which EXTV
CC
is powered from an external supply
greater than 8V (must be less than 10V), in which standard
threshold MOSFETs [V
GS(TH)
< 4V] may be used. Pay close
attention to the BV
DSS
specification for the MOSFETs as
well; many of the logic level MOSFETs are limited to 30V
or less.
Selection criteria for the power MOSFETs include the “ON”
resistance R
SD(ON)
, reverse transfer capacitance C
RSS
,
input voltage and maximum output current. When the
LTC1436A/LTC1437A are operating in continuous mode
the duty cycles for the top and bottom MOSFETs are
given by:
Main Switch Duty Cycle =
V
V
OUT
IN
Synchronous Switch Duty Cycle =
V
IN
−
()
V
V
OUT
IN
Kool Mµ is a registered trademark of Magnetics, Inc.
P
V
V
IR
kV I C f
P
VV
V
IR
MAIN
OUT
IN
MAX DS ON
IN MAX RSS
SYNC
IN OUT
IN
MAX DS ON
=
()
+
()
+
() ( )( )()
=
−
()
+
()
()
()
2
185
2
1
1
δ
δ
.
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron
cores, forcing the use of more expensive ferrite,
molypermalloy, or Kool Mµ
®
cores. Actual core loss is
independent of core 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 prefered at
high switching frequencies, so design goals can concen-
trate 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 Mµ. Toroids are very space efficient,
especially when you can use several layers of wire.
Because 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 use
with the LTC1436A/LTC1437A: a pair of N-channel MOS-
FETs for the top (main) switch and an N-channel MOSFET
for the bottom (synchronous) switch.
To take advantage of the Adaptive Power output stage, two
topside MOSFETs must be selected. A large (low R
SD(ON)
)
MOSFET and a small (higher R
DS(ON)
) MOSFET are
required. 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. This increases midcurrent efficiencies
while continuing to operate at constant frequency. Also, by
using the small MOSFET the circuit can maintain constant
The MOSFET power dissipations at maximum output
current are given by:
