LTC3731 Linear Technology, LTC3731 Datasheet - Page 26

• Manufacturer Part Number: LTC3731
• Description: Synchronous Buck Switching Regulator Controller
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
LTC3731
output ripple current for a 3-phase design is:
The ripple frequency is also increased by three, further
reducing the required output capacitance when V
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
implemented. Designing a system with multiple stages
close to the V
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
tor resistance R
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:
26
Main MOSFET R
Sync MOSFET R
C
C
R
R
V
V
V
I
P-P
INESR
OUTESR
SCHOTTKY
OUT
IN
L
SENSE
OUT
= 2.5mΩ
= 12V
=
= 1.3V
/V
( )( )
= 20mΩ
V
= 3mΩ
IN
f L
OUT
= 3mΩ
= 0.8V at 15A (0.7V at 90°C)
ratios equal to the number of stages
OUT
(
L
1 3
, the sense resistance R
/V
–
DS(ON)
DS(ON)
U
IN
DC
ratio will significantly reduce the
)
U
= 7mΩ (9mΩ at 90°C)
= 7mΩ (9mΩ at 90°C)
V
IN
>
3
V
W
OUT
SENSE
DS(ON)
U
CC
< 3V
and the
, induc-
OUT
The main MOSFET is on for the duty factor V
the synchronous MOSFET is on for the rest of the period
or simply (1 – V
small, the AC loss in the inductor can be made small if a
good quality inductor is chosen. The average current,
I
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.
The temperature of the MOSFET’s die temperature may
require interative calculations if one is not familiar typical
performance. A maximum operating junction tempera-
ture of 90° to 100°C for the MOSFETs is recommended
for high reliability applications.
Common output path DC loss:
This totals 3.7W + C
Total of all three main MOSFET’s DC loss:
This totals 0.87W + C
Total of all three synchronous MOSFET’s DC loss:
This totals 7.2W at 90°C.
Total of all three main MOSFET’s AC loss:
P
P
OUT
COMPATH
MAIN
I
δ = 0.5%°C (MOSFET temperature coefficient)
N = 3
f = 400kHz
P
MAX
SYNC
is used to simplify the calaculations. The equation
=
= 45A
N
=
⎛
⎜
⎝
≈
V
N
N
V
OUT
⎛
⎜
⎝
IN
⎛
⎜
⎝
1
I
–
OUT
MAX
⎞
⎟
⎠
N
V
⎛
⎜
⎝
V
OUT
/V
I
IN
MAX
OUTESR
⎞
⎟
⎠
N
INESR
2
IN
(
⎞
⎟
⎠
R
). Assuming the ripple current is
⎛
⎜
⎝
⎞
⎟
⎠
L
I
2
MAX
(
+
N
loss (at 90°C).
1 δ
loss.
R
+
SENSE
⎞
⎟
⎠
2
)
(
R
1
DS ON
+
)
(
δ
+
)
C
R
)
OUTESR
DS ON
+
(
C
OUT
INESR
)
/V
Loss
IN
Loss
3731fa
and