LTC1435CS#TRPBF Linear Technology, LTC1435CS#TRPBF Datasheet - Page 9

• Manufacturer Part Number: LTC1435CS#TRPBF
• Description: IC SW REG SYNC STEP-DOWN 16-SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1435CS#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 1.19 ~ 9 V
• Current - Output: 50mA
• Frequency - Switching: 125kHz
• Voltage - Input: 3.5 ~ 30 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

Kool M is a registered trademark of Magnetics, Inc.
APPLICATIONS
MOSFET is on. Lower inductor values (higher I
cause this to occur at higher load currents, which can
cause a dip in efficiency in the upper range of low current
operation. In Burst Mode operation, lower inductance
values will cause the burst frequency to decrease.
The Figure 3 graph gives a range of recommended induc-
tor values vs operating frequency and V
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool M
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 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
®
Figure 3. Recommended Inductor Values
60
50
40
30
20
10
0
cores. Actual core loss is independent of core
0
50
OPERATING FREQUENCY (kHz)
U
100
INFORMATION
U
150
200
V
V
V
W
OUT
OUT
OUT
250
= 5.0V
= 3.3V
= 2.5V
OUT
1435 F03
300
.
U
L
) will
ferrite. A reasonable compromise from the same manu-
facturer is Kool M . Toroids are very space efficient,
especially when you can use several layers of wire. Be-
cause 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
Two external power MOSFETs must be selected for use
with the LTC1435: an N-channel MOSFET for the top
(main) switch and an N-channel MOSFET for the bottom
(synchronous) switch.
The peak-to-peak gate drive levels are set by the INTV
voltage. This voltage is typically 5V during start-up (see
EXTV
old MOSFETs must be used in most LTC1435 applica-
tions. The only exception is applications in which EXTV
is powered from an external supply greater than 8V (must
be less than 10V), in which standard threshold MOSFETs
(V
BV
logic level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance R
input voltage and maximum output current. When the
LTC1435 is operating in continuous mode the duty cycles
for the top and bottom MOSFETs are given by:
The MOSFET power dissipations at maximum output
current are given by:
GS(TH)
DSS
Main Switch Duty Cycle =
Synchronous Switch Duty Cycle =
P
P
MAIN
SYNC
CC
specification for the MOSFETs as well; many of the
Pin Connection). Consequently, logic level thresh-
< 4V) may be used. Pay close attention to the
k V
V
V
SD(ON)
V
OUT
IN
IN
IN
V
1 85
IN
V
.
I
, reverse transfer capacitance C
MAX
OUT
I
MAX
2
I
MAX
1
C
V
RSS
2
V
OUT
R
IN
1
DS ON
f
R
DS ON
V
LTC1435
IN
V
IN
V
OUT
RSS
9
CC
CC
,