LTC1148HVCS-5#TRPBF Linear Technology, LTC1148HVCS-5#TRPBF Datasheet - Page 9

• Manufacturer Part Number: LTC1148HVCS-5#TRPBF
• Description: IC SW REG STEP-DOWN 5V 14-SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1148HVCS-5#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 50mA
• Frequency - Switching: 250kHz
• Voltage - Input: 3.5 ~ 20 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 14-SOIC (3.9mm Width), 14-SOL
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIO S I FOR ATIO
where:
t
V
the measured output voltage. Thus V
regulation.
Note that as V
When the input to output voltage differential drops
below 1.5V, the LTC1148 series reduces t
creasing the discharge current in C
audible operation prior to dropout.
Once the frequency has been set by C
must be chosen to provide no more than 25mV/R
of peak-to-peak inductor ripple current. This results in
a minimum required inductor value of:
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.
OFF
REG
L
f =
MIN
= 1.3(10
is the desired output voltage (i.e., 5V, 3.3V). V
t
OFF
= 5.1(10
1
1000
800
600
400
200
0
)
1 –
0
4
Figure 3. Timing Capacitor Value
)C
IN
V
IN
V
T
5
decreases, the frequency decreases.
= 7V
V
)R
) )
OUT
V
V
IN
U U
SENSE
REG
OUT
100
FREQUENCY (kHz)
)
(C
V
V
T
SENSE –
IN
)V
= 12V
200
REG
= V
V
W
IN
OUT
= 10V
LTC1148 • F03
T
= 5V
REG
T
. This prevents
, the inductor L
300
/V
OUT
OFF
U
= 1 in
by in-
SENSE
OUT
is
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
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 “hard,” 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 Mµ (from Magnetics, Inc.) is a very good, low loss
core material for toroids, with a “soft” 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
tance R
The minimum input voltage determines whether standard
threshold or logic-level threshold MOSFETs must be used.
For V
may be used. If V
Kool Mµ is a registered trademark of Magnetics, Inc.
IN
> 8V, standard threshold MOSFETs (V
DS(ON)
.
LTC1148-3.3/LTC1148-5
IN
is expected to drop below 8V, logic-
®
on molypermalloy (MPP)
GS(TH)
LTC1148
and on resis-
GS(TH)
114835fd
< 4V)
9