LTC3731HG#PBF Linear Technology, LTC3731HG#PBF Datasheet - Page 27

• Manufacturer Part Number: LTC3731HG#PBF
• Description: IC SW REG CTRLR SYNC BUCK 36SSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3731HG#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: -40°C ~ 140°C
• Mounting Type: Surface Mount
• Package / Case: 36-SSOP
• Primary Input Voltage: 7V
• No. Of Outputs: 1
• Output Voltage: 32V
• Output Current: 5A
• No. Of Pins: 36
• Operating Temperature Range: 0°C To +70°C
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -

applicaTions inForMaTion
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
(
V
5
OUT
IN

 
V
IN
V
1
IN
–
– .
)

 
2
1
1 8
V
( ) ( )(
IN

 
V
( )( )
f
OUT
I
2 3
45
IN
MAX
= 8V, 2.25W at V
=
N
INESR
V
A

 
+
6 15

 

 
(
1 8
2
2
I
MAX
.
loss (at 90°C).
(
Ω
1
N
1 d
nC
V
)(
+




 
1000
)
(
2
)
400
V
R
(
DSSPEC
1
DS ON
V
+
pF
IN
kHz
IN
(
d
= 12V and 6.25W
)
)
R
)
) )
(
+ C
=
DS
4 5
E
6 3
(
INESR
O O N)
.
)
W
Loss
This totals 0.08W at V
at V
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
the AC 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 experience the
IN
= 8V, 0.12W at V
IN
= 12V)
MILLER
LTC3731H
IN
to better balance
= 12V and 0.19W
2.25W
0.87W
0.19W
7.2W
3.7W
IN
= 12V. It

3.75%
1.5%
0.3%
6.1%
3731Hfb
12%