LTC1929CG View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1929CG' PDF : 24 Pages View PDF
LTC1929/LTC1929-PG
APPLICATIO S I FOR ATIO
Accepting larger values of ∆IL allows the use of low
inductances, but can result in higher output voltage ripple.
A reasonable starting point for setting ripple current is ∆IL
= 0.4(IOUT)/2, where IOUT is the total load current. Remem-
ber, the maximum ∆IL occurs at the maximum input
voltage. The individual inductor ripple currents are deter-
mined by the inductor, input and output voltages.
Inductor Core Selection
Once the values for L1 and L2 are 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 induc-
tance 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 Mµ. Toroids are very space efficient,
especially when you can use several layers of wire. Be-
cause they lack a bobbin, mounting is more difficult.
However, designs for surface mount are available which
do not increase the height significantly.
Power MOSFET, D1 and D2 Selection
Two external power MOSFETs must be selected for each
output stage with the LTC1929: 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 INTVCC volt-
age. This voltage is typically 5V during start-up (see
EXTVCC Pin Connection). Consequently, logic-level thresh-
old MOSFETs must be used in most applications. The only
exception is if low input voltage is expected (VIN < 5V);
then, sublogic-level threshold MOSFETs (VGS(TH) < 3V)
should be used. Pay close attention to the BVDSS specifi-
cation for the MOSFETs as well; most of the logic-level
MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance RDS(ON), reverse transfer capacitance CRSS,
input voltage, and maximum output current. When the
LTC1929 is operating in continuous mode the duty factors
for the top and bottom MOSFETs of each output stage are
given by:
Main Switch Duty Cycle = VOUT
VIN
Synchronous
Switch
Duty
Cycle
=
VIN
– VOUT
VIN
The MOSFET power dissipations at maximum output
current are given by:
( ) PMAIN
=
VOUT
VIN
IMAX 2
2
1+ δ
RDS(ON)
+
( ) ( )( ) k
VIN
2 IMAX
2
CRSS
f
( ) PSYNC
=
VIN
– VOUT
VIN
IMAX 2
2
1+ δ
RDS(ON)
where δ is the temperature dependency of RDS(ON) and k
is a constant inversely related to the gate drive current.
Both MOSFETs have I2R losses but the topside N-channel
equation includes an additional term for transition losses,
which peak at the highest input voltage. For VIN < 20V the
high current efficiency generally improves with larger
MOSFETs, while for VIN > 20V the transition losses rapidly
increase to the point that the use of a higher RDS(ON) device
with lower CRSS actual provides higher efficiency. The
Kool Mµ is a registered trademark of Magnetics, Inc.
13
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