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LTC1148L Datasheet

  • LTC1148L

  • Linear Technology [High Efficiency Synchronous Step-Down Sw...

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LTC1148
LTC1148-3.3/LTC1148-5
APPLICATIO S I FOR ATIO
1000
V
SENSE 鈥?/div>
= V
OUT
= 5V
800
CAPACITANCE (pF)
600
400
V
IN
= 12V
200
V
IN
= 7V
V
IN
= 10V
0
0
200
100
FREQUENCY (kHz)
300
LTC1148 鈥?F03
Figure 3. Timing Capacitor Value
f=
1
t
OFF
)
1鈥?/div>
V
OUT
V
IN
)
where:
t
OFF
= 1.3(10
4
)C
T
) )
V
REG
V
OUT
V
REG
is the desired output voltage (i.e., 5V, 3.3V). V
OUT
is
the measured output voltage. Thus V
REG
/V
OUT
= 1 in
regulation.
Note that as V
IN
decreases, the frequency decreases.
When the input to output voltage differential drops
below 1.5V, the LTC1148 series reduces t
OFF
by in-
creasing the discharge current in C
T
. This prevents
audible operation prior to dropout.
Once the frequency has been set by C
T
, the inductor L
must be chosen to provide no more than 25mV/R
SENSE
of peak-to-peak inductor ripple current. This results in
a minimum required inductor value of:
L
MIN
= 5.1(10
5
)R
SENSE
(C
T
)V
REG
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 LTC1148 series may not enter Burst Mode operation
and efficiency will be severely degraded at low currents.
U
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 M碌
廬
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 saturation.
Ferrite core material saturates 鈥渉ard,鈥?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 which can cause Burst Mode operation to be falsely
triggered. Do not allow the core to saturate!
Kool M碌 (from Magnetics, Inc.) is a very good, low loss
core material for toroids, with a 鈥渟oft鈥?saturation charac-
teristic. Molypermalloy is slightly more efficient at high
(>200kHz) switching frequencies, but quite a bit more
expensive. Toroids are very space efficient, especially
when you can use several layers of wire. Because they
generally lack a bobbin, mounting is more difficult. How-
ever, 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 LTC1148 series: a P-channel MOSFET for the
main switch, and an N-channel MOSFET for the synchro-
nous switch. The main selection criteria for the power
MOSFETs are the threshold voltage V
GS(TH)
and on resis-
tance R
DS(ON)
.
The minimum input voltage determines whether standard
threshold or logic-level threshold MOSFETs must be used.
For V
IN
> 8V, standard threshold MOSFETs (V
GS(TH)
< 4V)
may be used. If V
IN
is expected to drop below 8V, logic-
Kool M碌 is a registered trademark of Magnetics, Inc.
W
U U
9

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