LTC3731CUH#PBF Linear Technology, LTC3731CUH#PBF Datasheet - Page 26

• Manufacturer Part Number: LTC3731CUH#PBF
• Description: IC SW REG CTRLR SYNC BUCK 32QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3731CUH#PBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 6 V
• Frequency - Switching: 225kHz ~ 680kHz
• Voltage - Input: 4 ~ 36 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -

APPLICATIO S I FOR ATIO
LTC3731
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
δ = 0.5%°C (MOSFET temperature coefficient)
N = 3
f = 400kHz
MAX
INESR
OUTESR
SCHOTTKY
OUT
IN
L
SENSE
OUT
= 2.5mΩ
= 12V
= 45A
= 1.3V
/V
= 20mΩ
= 3mΩ
IN
= 3mΩ
= 0.8V at 15A (0.7V at 90°C)
ratios equal to the number of stages
OUT
L
, the sense resistance R
/V
DS(ON)
DS(ON)
U
IN
ratio will significantly reduce the
U
= 7mΩ (9mΩ at 90°C)
= 7mΩ (9mΩ at 90°C)
W
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.
Determining the MOSFETs’ die temperature may require
iterative calculations if one is not familiar with 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 MOSFETs’ DC loss:
This totals 0.87W + C
Total of all three synchronous MOSFETs’ DC loss:
This totals 7.2W at 90°C.
Total of all three main MOSFETs’ AC loss:
P
P
OUT
COMPATH
MAIN
P
P
SYNC
MAIN
, is used to simplify the calculations. The equation
=
N
=
≈
⎛
⎜
⎝
≈
⎛
⎜
⎝
V
N
3
N
V
OUT
(
5
⎛
⎜
⎝
V
IN
⎛
⎜
⎝
V
1
IN
I
–
OUT
MAX
– .
)
⎞
⎟
⎠
N
1
2
V
1 8
⎛
⎜
⎝
V
OUT
( )( )
/V
I
IN
45
2 3
MAX
⎞
⎟
⎠
OUTESR
N
V
2
INESR
IN
A
(
⎞
⎟
⎠
+
R
). Assuming the ripple current is
⎛
⎜
⎝
⎞
⎟
⎠
(
L
2
I
1 8
2
MAX
+
(
Ω
.
N
loss (at 90°C).
1
1 δ
loss.
R
V
+
)(
SENSE
⎞
⎟
⎠
1000
⎞
⎟
⎠
2
(
)
400
(
R
1
DS ON
+
pF
)
kHz
(
δ
+
)
)
C
R
)
)
DS ON
OUTESR
+
=
(
C
6 3
OUT
INESR
.
)
W
/V
Loss
IN
Loss
3731fb
and