LTC3731HG Linear Technology, LTC3731HG Datasheet - Page 27

• Manufacturer Part Number: LTC3731HG
• Description: IC REG SW SYNC 3PH STPDWN 36SSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3731HG

• 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: -40°C ~ 140°C
• Mounting Type: Surface Mount
• Package / Case: 36-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Output: -
• Power - Output: -
• Lead Free Status / Rohs Status: Not Compliant

APPLICATIO S I FOR ATIO
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
Total of all three synchronous MOSFETs’ AC gate loss:
P
MAIN
P
P
( )
3
IN
MAIN
SYNC
Q
= 20V.
=
G
N
≈
=
V
⎛
⎝ ⎜
DSSPEC
3
⎛
⎜
⎝
N
V
V
(
5
V
OUT
⎛
⎝ ⎜
IN
V
IN
V
1
IN
–
– .
)
⎞
⎠ ⎟
1
2
1 8
V
⎛
⎝ ⎜
IN
( ) ( )(
V
( )( )
OUT
U
f
45
2 3
I
IN
MAX
= 8V, 2.25W at V
V
=
N
INESR
A
⎞
⎠ ⎟
+
6 15
⎛
⎝ ⎜
⎞
⎠ ⎟
(
1 8
U
2
I
2
MAX
.
Ω
1
(
N
loss (at 90°C).
1 δ
V
)(
nC
+
⎞
⎟
⎠
⎞
⎠ ⎟
1000
(
2
)
400
)
V
(
R
1
W
DSSPEC
DS ON
+
pF
V
kHz
IN
IN
(
δ
)
)
= 12V and 6.25W
R
)
) )
DS
=
+ C
(
4 5
6 3
(
E
O O N)
.
INESR
U
W
)
Loss
This totals 0.08W at V
0.19W at V
experience the Miller capacitance dissipation issue that
the main switch does because the bottom switch turns on
when its drain is close to ground.
The Schottky rectifier loss assuming 50ns nonoverlap
time:
This totals 1.26W.
The total output power is (1.3V)(45A) = 58.5W and the
total input power is approximately 60W so the % loss of
each component is as follows:
The numbers above represent the values at V
can be seen from this simple example that two things can
be done to improve efficiency: 1) Use two MOSFETs on the
synchronous side and 2) use a smaller MOSFET for the
main switch with smaller C
loss with the DC loss. A smaller, less expensive MOSFET
can actually perform better in the task of the main switch.
2 • 3(0.7V)(15A)(50ns)(4E5)
Main switch’s AC loss (V
Main switch’s DC loss
Synchronous switch AC loss
Synchronous switch DC loss
Power path loss
IN
= 20V. The bottom MOSFET does not
IN
= 8V, 0.12W at V
MILLER
IN
= 12V) 2.25W 3.75%
to better balance the AC
LTC3731H
0.87W 1.5%
0.19W 0.3%
7.2W
3.7W
IN
IN
= 12V and
= 12V. It
12%
6.1%
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