LTC3731_15 Datasheet, PDF (16/34 Pages) Linear Technology – 3-Phase, 600kHz, Synchronous Buck Switching Regulator Controller

LTC3731
APPLICATIONS INFORMATION
where N is the number of output stages, δ is the tempera-
ture dependency of R_DS(ON), R_DR is the effective top driver
resistance (approximately 2Ω at V_GS = V_MILLER), V_IN is
the drain potential and the change in drain potential in the
particular application. V_TH(IL) is the data sheet specified typi-
cal gate threshold voltage specified in the power MOSFET
data sheet at the specified drain current. C_MILLER is the
calculated capacitance using the gate charge curve from
the MOSFET data sheet and the technique described above.
Both MOSFETs have I²R losses while the topside N‑channel
equation includes an additional term for transition losses,
which peak at the highest input voltage. For V_IN < 12V,
the high current efficiency generally improves with larger
MOSFETs, while for V_IN > 12V, the transition losses rapidly
increase to the point that the use of a higher R_DS(ON) device
with lower C_MILLER actually provides higher efficiency. The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
a short-circuit when the synchronous switch is on close
to 100% of the period.
The term (1 + δ ) is generally given for a MOSFET in the
form of a normalized R_DS(ON) vs temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
The Schottky diodes (D1 to D3 in Figure 1) conduct 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, storing charge during the
dead time and requiring a reverse recovery period which
could cost as much as several percent in efficiency. A 2A
to 8A Schottky is generally a good compromise for both
regions of operation due to the relatively small average
current. Larger diodes result in additional transition loss
due to their larger junction capacitance.
C_IN and C_OUT Selection
In continuous mode, the source current of each top
N‑channel MOSFET is a square wave of duty cycle V_OUT/
V_IN. A low ESR input capacitor sized for the maximum
RMS current must be used. The details of a close form
equation can be found in Application Note 77. Figure 6
shows the input capacitor ripple current for different phase
configurations with the output voltage fixed and input volt-
age varied. The input ripple current is normalized against
the DC output current. The graph can be used in place of
tedious calculations. The minimum input ripple current
can be achieved when the product of phase number and
output voltage, N(V_OUT), is approximately equal to the
input voltage V_IN or:
V_{OUT =} k _{where k = 1, 2, ..., N – 1}
V_IN N
So the phase number can be chosen to minimize the input
capacitor size for the given input and output voltages.
In the graph of Figure 4, the local maximum input RMS
capacitor currents are reached when:
V_OUT
V_IN
=
2k – 1
N
where
k = 1,
2, ..., N
These worst-case conditions are commonly used for design
because even significant deviations do not offer much relief.
Note that capacitor manufacturer’s ripple current ratings
are often based on only 2000 hours of life. This makes
it advisable to further derate the capacitor or to choose
a capacitor rated at a higher temperature than required.
Several capacitors may also be paralleled to meet size or
0.6
0.5
1-PHASE
0.4
2-PHASE
3-PHASE
4-PHASE
0.3
6-PHASE
12-PHASE
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)
3731 F06
Figure 6. Normalized Input RMS Ripple Current
vs Duty Factor for One to Six Output Stages
3731fc
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