LTC1709-9 Linear Technology, LTC1709-9 Datasheet - Page 13

• Manufacturer Part Number: LTC1709-9
• Description: 2-Phase/ 5-Bit VID/ Current Mode/ High Efficiency/ Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Accepting larger values of I
inductances, but can result in higher output voltage ripple.
A reasonable starting point for setting ripple current is I
= 0.4(I
ber, the maximum I
voltage. The individual inductor ripple currents are deter-
mined by the inductor, input and output voltages.
Inductor Core Selection
Once the values for L1 and L2 are known, the type of
inductor must be selected. High efficiency converters
generally cannot afford the core loss found in low cost
powdered iron cores, forcing the use of more expensive
ferrite, molypermalloy, or Kool M
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 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 ferrite. A reasonable compromise from the same
manufacturer is Kool M . Toroids are very space effi-
cient, especially when you can use several layers of wire.
Because they lack a bobbin, mounting is more difficult.
However, designs for surface mount are available which
do not increase the height significantly.
Power MOSFET, D1 and D2 Selection
Two external power MOSFETs must be selected for each
controller with the LTC1709: one N-channel MOSFET for
the top (main) switch, and one N-channel MOSFET for the
bottom (synchronous) switch.
The peak-to-peak drive levels are set by the INTV
voltage. This voltage is typically 5V during start-up
OUT
)/2, where I
U
OUT
L
is the total load current. Remem-
occurs at the maximum input
U
L
allows the use of low
W
®
cores. Actual core
U
CC
L
(see EXTV
threshold MOSFETs must be used in most applications.
The only exception is if low input voltage is expected
(V
(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
LTC1709 is operating in continuous mode the duty factors
for the top and bottom MOSFETs of each output stage are
given by:
The MOSFET power dissipations at maximum output
current are given by:
where is the temperature dependency of R
is a constant inversely related to the gate drive current.
Both MOSFETs have I
equation includes an additional term for transition losses,
which peak at the highest input voltage. For V
high current efficiency generally improves with larger
MOSFETs, while for V
increase to the point that the use of a higher R
with lower C
Kool M is a registered trademark of Magnetics, Inc.
GS(TH)
IN
DSS
Main Switch Duty Cycle
P
P
Synchronous Switch Duty Cycle
MAIN
SYNC
< 5V); then, sublogic-level threshold MOSFETs
specification for the MOSFETs as well; most of the
< 1V) should be used. Pay close attention to the
k V
CC
V
V
DS(ON)
V
OUT
IN
RSS
Pin Connection). Consequently, logic-level
IN
IN
–
V
2
IN
LTC1709-8/LTC1709-9
actual provides higher efficiency. The
V
I
, reverse transfer capacitance C
I
OUT
MAX
MAX
2
2
2
IN
R losses but the topside N-channel
> 20V the transition losses rapidly
I
2
MAX
C
2
1
RSS
V
V
2
OUT
IN
R
1
f
DS ON
(
R
V
)
DS ON
IN
(
DS(ON)
–
V
DS(ON)
IN
IN
V
)
< 20V the
OUT
13
device
and k
RSS
,