LTC3729_15 Datasheet, PDF (13/30 Pages) Linear Technology – 550kHz, PolyPhase, High Efficiency, Synchronous Step-Down Switching Regulator

LTC3729
APPLICATIONS INFORMATION
1.0
0.9
1-PHASE
2-PHASE
0.8
3-PHASE
4-PHASE
0.7
6-PHASE
0.6
0.5
0.4
0.3
0.2
0.1
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
DUTY FACTOR (V_OUT/V_IN)
3729 F03
Figure 3. Normalized Peak Output Current vs
Duty Factor [I_RMS ≈ 0.3 (∆I_O(P–P))]
Accepting larger values of ∆I_L allows the use of low in‑
ductances, but can result in higher output voltage ripple.
A reasonable starting point for setting ripple current is
∆I_L = 0.4(I_OUT)/N, where N is the number of channels and
I_OUT is the total load current. Remember, the maximum
∆I_L occurs at the maximum input voltage. The individual
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 Mµ^® cores. Actual core loss
is independent of core 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 cop‑
per 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
Kool Mµ is a registered trademark of Magnetics, Inc.
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 Mµ. 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 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_CC voltage.
This voltage is typically 5V during start-up (see EXTV_CC Pin
Connection). Consequently, logic-level threshold MOSFETs
must be used in most applications. The only exception
is if low input voltage is expected (V_IN < 5V); then, sub‑
logic-level threshold MOSFETs (V_GS(TH) < 3V) should be
used. Pay close attention to the BV_DSS specification 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_DS(ON), reverse transfer capacitance
C_RSS, 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:
Main
Switch
Duty
Cycle
=
V_OUT
V_IN
Synchronous
Switch
Duty
Cycle
=


V_IN
– V_OUT
V_IN


The MOSFET power dissipations at maximum output
current are given by:
3729fb
13