LTC1435A Linear Technology, LTC1435A Datasheet - Page 9

• Manufacturer Part Number: LTC1435A
• Description: High Efficiency Low Noise Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS
a fixed inductor value, but it is very dependent on induc-
tance 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
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that in-
ductance 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 manufac-
turer is Kool M . 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, 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 LTC1435A: an N-channel MOSFET for the top (main)
switch and an N-channel MOSFET for the bottom (synchro-
nous) 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 LTC1435A applications.
The only exception is applications in which EXTV
powered from an external supply greater than 8V (must be
less than 10V), in which standard threshold MOSFETs
(V
specification for the MOSFETs as well; many of the logic level
MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance R
put voltage and maximum output current. When the
LTC1435A is operating in continuous mode the duty cycles
for the top and bottom MOSFETs are given by:
GS(TH)
CC
Pin Connection). Consequently, logic level thresh-
< 4V) may be used. Pay close attention to the BV
DS(ON)
, reverse transfer capacitance C
U
INFORMATION
U
W
U
RSS
CC
, in-
DSS
CC
is
The MOSFET power dissipations at maximum output cur-
rent are given by:
where is the temperature dependency of R
is a constant inversely related to the gate drive current.
Both MOSFETs have I
N-channel equation includes an additional term for tran-
sition losses, which are highest at high input voltages.
For V
proves with larger MOSFETs, while for V
transition losses rapidly increase to the point that the use
of a higher R
vides higher efficiency. The synchronous MOSFET losses
are greatest at high input voltage or during a short circuit
when the duty cycle in this switch is nearly 100%. Refer
to the Foldback Current Limiting section for further appli-
cations information.
The term (1 + ) is generally given for a MOSFET in the form
of a normalized R
voltage MOSFETs. C
characteristics. The constant k = 2.5 can be used to esti-
mate the contributions of the two terms in the main switch
dissipation equation.
The Schottky diode D1 shown in Figure 1 conducts during
the dead-time between the conduction of the two large
power MOSFETs. This prevents the body diode of the bot-
tom MOSFET from turning on and storing charge during the
dead-time, which could cost as much as 1% in efficiency.
A 1A Schottky is generally a good size for 3A regulators.
= 0.005/ C can be used as an approximation for low
Main Switch Duty Cycle =
Synchronous Switch Duty Cycle =
P
P
MAIN
SYNC
IN
< 20V the high current efficiency generally im-
k V
V
V
V
OUT
IN
DS(ON)
IN
IN
V
1 85
IN
V
.
I
MAX
OUT
DS(ON)
RSS
device with lower C
I
MAX
is usually specified in the MOSFET
2
I
2
MAX
R losses while the topside
1
vs Temperature curve, but
C
V
RSS
2
V
OUT
R
IN
1
DS ON
f
LTC1435A
R
DS ON
V
RSS
IN
IN
DS(ON)
V
actual pro-
> 20V the
IN
V
OUT
and k
9