ltc3729euh-trpbf Linear Technology Corporation, ltc3729euh-trpbf Datasheet - Page 13

• Manufacturer Part Number: ltc3729euh-trpbf
• Description: 550khz, Polyphase, High Efficiency, Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

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
inductor ripple currents are constant determined 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
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
Kool M is a registered trademark of Magnetics, Inc.
OUT
I
I
L
L
= 0.4(I
occurs at the maximum input voltage. The individual
is the total load current. Remember, the maximum
Figure 3. Normalized Peak Output Current vs
Duty Factor [I
OUT
)/N, where N is the number of channels and
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.1
U
0.2
RMS
0.3
DUTY FACTOR (V
U
0.4
0.3 ( I
0.5
L
allows the use of low
O(P–P)
OUT
0.6
W
®
/V
cores. Actual core
IN
0.7
)]
1-PHASE
2-PHASE
3-PHASE
4-PHASE
6-PHASE
)
0.8
3729
F03
0.9
U
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 manu-
facturer is Kool M . Toroids are very space efficient,
especially when you can use several layers of wire. Be-
cause 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 LTC3729: 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
age. This voltage is typically 5V during start-up (see
EXTV
old MOSFETs must be used in most applications. The only
exception is if low input voltage is expected (V
then, sublogic-level threshold MOSFETs (V
should be used. Pay close attention to the BV
cation for the MOSFETs as well; most of the 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
LTC3729 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:
Main Switch Duty Cycle
Synchronous Switch Duty Cycle
CC
Pin Connection). Consequently, logic-level thresh-
DS(ON)
, reverse transfer capacitance C
V
V
OUT
IN
LTC3729
V
IN
GS(TH)
–
V
DSS
IN
V
sn3729 3729fas
OUT
IN
13
CC
specifi-
< 5V);
< 3V)
volt-
RSS
,