LTC1266 Ver la hoja de datos (PDF) - Linear Technology
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LTC1266 Datasheet PDF : 20 Pages
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LTC1266
LTC1266-3.3/LTC1266-5
APPLICATIO S I FOR ATIO
As the operating frequency is increased the gate charge
losses will be higher, reducing efficiency (see Efficiency
Considerations). The complete expression for operating
frequency of the circuit in Figure 1 is given by:
) f = 1 1 – VOUT
tOFF
VIN
where:
) tOFF = 1.3 • 104 • CT •
VREG
VOUT
VREG is the desired output voltage (i.e., 5V, 3.3V). VOUT is the
measured output voltage. Thus VREG/VOUT = 1 in regulation.
Once the frequency has been set by CT, the inductor L
must be chosen to provide no more than 25mV/RSENSE
of peak-to-peak inductor ripple current. This results in
a minimum required inductor value of:
LMIN = 5.1 • 105 • RSENSE • CT • VREG
As the inductor value is increased from the minimum
value, the ESR requirements for the output capacitor
are eased at the expense of efficiency. If too small an
inductor is used, the inductor current will decrease past
zero and change polarity. A consequence of this is that
the LTC1266 series may not enter Burst Mode operation
and efficiency will be slightly degraded at low currents.
Inductor Core Selection
Once the minimum value for L is known, the type of
inductor must be selected. The highest efficiency will be
obtained using ferrite, Kool Mµ® on molypermalloy (MPP)
cores. Lower cost powdered iron cores provide suitable
performance but cut efficiency by 3% to 7%. 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 increase.
Ferrite designs have very low core loss, so design goals
can concentrate on copper loss and preventing satura-
tion. Ferrite core material saturates “hard,” which means
that inductance collapses abruptly when the peak design
Kool Mµ is a registered trademark of Magnetics, Inc.
current is exceeded. This results in an abrupt increase in
inductor ripple current and consequent output voltage
ripple which can cause Burst Mode operation to be falsely
triggered. Do not allow the core to saturate!
Kool Mµ is a very good, low loss core material for toroids,
with a “soft” saturation characteristic. Molypermalloy is
slightly more efficient at high (> 200kHz) switching fre-
quency. 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, new
designs for surface mount are available from Coiltronics
and Beckman Industrial Corp. which do not increase the
height significantly.
Power MOSFET and D1 Selection
Two external power MOSFETs must be selected for use
with the LTC1266 series: either a P-channel MOSFET or an
N-channel MOSFET for the main switch and an N-channel
MOSFET for the synchronous switch. The main selection
criteria for the power MOSFETs are the type of MOSFET,
threshold voltage VGS(TH) and on-resistance RDS(ON).
The cost and maximum output current determine the type
of MOSFET for the topside switch. N-channel MOSFETs
have the advantage of lower cost and lower RDS(ON) at the
expense of slightly increased circuit complexity. For lower
current applications where the losses due to RDS(ON) are
small, a P-channel MOSFET is recommended due to the
lower circuit complexity. However, at load currents in
excess of 3A where the RDS(ON) becomes a significant
portion of the total power loss, an N-channel is strongly
recommended to maximize efficiency.
The maximum output current IMAX determines the RDS(ON)
requirement for the two MOSFETs. When the LTC1266
series is operating in continuous mode, the simplifying
assumption can be made that one of the two MOSFETs is
always conducting the average load current. The duty
cycles for the two MOSFETs are given by:
Topside Duty Cycle =
VOUT
VIN
Bottom-Side Duty Cycle =
VIN – VOUT
VIN
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