ltc3730 Linear Technology Corporation, ltc3730 Datasheet - Page 25

• Manufacturer Part Number: ltc3730
• Description: 3-phase, 5-bit Intel Mobile Vid, 600khz, Synchronous Buck Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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:
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
OUT
Main MOSFET R
Sync MOSFET R
C
C
R
R
V
V
V
I
δ = 0.5%/°C
N = 3
f = 400kHz
MAX
INESR
OUTESR
SCHOTTKY
OUT
IN
L
SENSE
, is used to simplify the calaculations. The equation
= 2.5mΩ
= 12V
= 45A
= 1.3V
= 20mΩ
= 3mΩ
= 3mΩ
= 0.8V at 15A (0.7V at 90°C)
OUT
L
, the sense resistance R
DS(ON)
DS(ON)
U
/V
IN
). Assuming the ripple current is
U
= 7mΩ (9mΩ at 90°C)
= 7mΩ (9mΩ at 90°C)
W
SENSE
DS(ON)
OUT
U
/V
and the
, induc-
IN
and
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:
This totals 3.375W + 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:
This totals 1W at V
at V
P
P
COMPATH
MAIN
P
P
IN
SYNC
MAIN
= 20V.
=
N
≈
=
⎛
⎜
⎝
≈
3
N
⎛
⎜
⎝
V
(
N
V
5
OUT
⎛
⎜
⎝
V
IN
⎛
⎜
⎝
1
V
IN
I
–
MAX
– .
)
2
N
⎞
⎟
⎠
1
V
1 8
⎛
⎜
⎝
IN
V
OUT
( )( )
45
2 3
IN
I
MAX
⎞
⎟
⎠
= 8V, 2.25W at V
V
N
INESR
2
A
OUTESR
⎞
⎟
⎠
(
+
R
⎛
⎜
⎝
(
⎞
⎟
⎠
2
1 8
L
I
2
MAX
Ω
.
1
+
( )
N
loss at 90°C.
1 δ
)(
V
R
+
1000
⎞
⎟
⎠
loss.
SENSE
⎞
⎟
⎠
(
2
400
(
R
1
pF
DS ON
+
kHz
)
IN
δ
(
)
+
)
= 12V and 6.25W
R
C
)
LTC3730
)
DS ON
=
OUTESR
+
6 3
(
C
.
INESR
W
)
25
Loss
Loss
3730fa