LTC3731_15 Datasheet, PDF (27/34 Pages) Linear Technology – 3-Phase, 600kHz, Synchronous Buck Switching Regulator Controller

LTC3731
APPLICATIONS INFORMATION
The ripple frequency is also increased by three, fur-
ther reducing the required output capacitance when
V_CC < 3V_OUT as illustrated in Figure 6.
The addition of more phases, by phase locking additional
controllers, always results in no net input or output
ripple at V_OUT/V_IN ratios equal to the number of stages
implemented. Designing a system with multiple stages
close to the V_OUT/V_IN ratio will significantly reduce the
ripple voltage at the input and outputs and thereby improve
efficiency, physical size and heat generation of the overall
switching power supply. Refer to Application Note 77 for
more information on PolyPhase circuits.
Efficiency Calculation
To estimate efficiency, the DC loss terms include the
input and output capacitor ESR, each MOSFET R_DS(ON),
inductor resistance R_L, the sense resistance R_SENSE and
the forward drop of the Schottky rectifier at the operating
output current and temperature. Typical values for the
design example given previously in this data sheet are:
Main MOSFET R_DS(ON) = 7mΩ (9mΩ at 90°C)
Sync MOSFET R_DS(ON) = 7mΩ (9mΩ at 90°C)
C_INESR = 20mΩ
C_OUTESR = 3mΩ
R_L = 2.5mΩ
R_SENSE = 3mΩ
V_SCHOTTKY = 0.8V at 15A (0.7V at 90°C)
V_OUT = 1.3V
V_IN = 12V
I_MAX = 45A
δ = 0.5%°C (MOSFET temperature coefficient)
N=3
f = 400kHz
The main MOSFET is on for the duty factor V_OUT/ V_IN and
the synchronous MOSFET is on for the rest of the period
or simply (1 – V_OUT/V_IN). Assuming the ripple current is
small, the AC loss in the inductor can be made small if
a good quality inductor is chosen. The average current,
I_OUT, is used to simplify the calculations. The equation
below is not exact but should provide a good technique
for the comparison of selected components and give a
result that is within 10% to 20% of the final application.
Determining the MOSFETs’ die temperature may require
iterative calculations if one is not familiar with typical
performance. A maximum operating junction temperature
of 90° to 100°C for the MOSFETs is recommended for
high reliability applications.
Common output path DC loss:
( ) ^P_COMPATH
≈
N


I_MAX
N


2
R_L + R_SENSE
+ C_OUTESR Loss
This totals 3.7W + C_OUTESR loss.
Total of all three main MOSFETs’ DC loss:
( ) P_MAIN
=
N


V_OUT
V_IN




I_MAX
N


2
1+ δ
R_DS(ON) + C_INESR Loss
This totals 0.87W + C_INESR loss (at 90°C).
Total of all three synchronous MOSFETs’ DC loss:
( ) P_SYNC

= N _ 1–
V_OUT
V_IN




I_MAX
N


2
1+ δ
R_DS(ON)
This totals 7.2W at 90°C.
Total of all three main MOSFETs’ AC loss:
P_MAIN
≈
3(V_IN
)²
45A
(2)(3)
(2Ω)(1000pF)


5V
1
– 1.8V
+
1
1.8V


(400kHz)
=
6.3W
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