LTC3731CUH#TR Linear Technology, LTC3731CUH#TR Datasheet - Page 27

• Manufacturer Part Number: LTC3731CUH#TR
• Description: IC CTRLR SW REG 3PH BUCK 32-QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3731CUH#TR

• 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: Contains lead / RoHS non-compliant
• Current - Output: -
• Power - Output: -

PACKAGE DESCRIPTIO
This totals 1W at V
at V
Total of all three synchronous MOSFETs’ AC gate 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.
2 • 3(0.7V)(15A)(50ns)(4E5)
( )
3
IN
Q
= 20V.
G
V
DSSPEC
V
IN
IN
= 20V. The bottom MOSFET does not
IN
( ) ( )(
f
= 8V, 2.25W at V
=
IN
6 15
= 8V, 0.12W at V
nC
)
V
DSSPEC
V
U
IN
IN
= 12V and 6.25W
(
4 5
IN
E
= 12V and
)
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.
Main switch’s AC loss (V
Main switch’s DC loss
Synchronous switch AC loss
Synchronous switch DC loss
Power path loss
MILLER
IN
= 12V) 2.25W 3.75%
to better balance the AC
0.87W 1.5%
0.19W 0.3%
7.2W
3.7W
LTC3731
IN
= 12V. It
12%
6.1%
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