LTC3731 Linear Technology, LTC3731 Datasheet - Page 22

• Manufacturer Part Number: LTC3731
• Description: Synchronous Buck Switching Regulator Controller
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
LTC3731
converter. Although the IC has a maximum input voltage
of 32V on the SW pins, most applications will be limited to
30V by the MOSFET BV
Design Example
As a design example, assume V
V
The inductance value is chosen first based upon a 30%
ripple current assumption. The highest value of ripple
current in each output stage occurs at the maximum input
voltage.
Using L = 0.6µH, a commonly available value results in
34% ripple current. The worst-case output ripple for the
three stages operating in parallel will be less than 11% of
the peak output current.
R
a conservative maximum sense current threshold of 65mV
and taking into account half of the ripple current:
22
IN
SENSE1,
R
= 20V(max), V
L
SENSE
=
=
≥
0 68
V
(
f I
Figure 10. Automotive Application Protection
400
LTC3731
( )
.
R
OUT
∆
V
SENSE2
5V
=
CC
µ
kHz
15 1
⎛
⎜
⎝
H
1
A
−
1 3
)(
⎛
⎜
⎝
.
and R
65
OUT
30
V
U
V
OUT
V
+
mV
IN
%
= 1.3V, I
34
)(
+
SENSE3
DSS
⎞
⎟
⎠
2
15
%
U
⎞
⎟
⎠
.
A
)
=
CC
⎛
⎜
⎝
MAX
can be calculated by using
1
0 0037
= 5V, V
−
.
= 45A and f = 400kHz.
W
1 3
20
.
3731 F10
V
V
IN
Ω
⎞
⎟
⎠
= 12V(nominal),
U
V
12V
BAT
Use a commonly available 0.003Ω sense resistor.
Next verify the minimum on-time is not violated. The
minimum on-time occurs at maximum V
The output voltage will be set by the resistive divider from
the DIFFOUT pin to SGND, R1 and R2 in the Functional
Diagram. Set R1 = 13.3k and R2 = 11.3k.
The power dissipation on the topside MOSFET can be
estimated. Using a Fairchild FDS6688 for example, R
= 7mΩ, C
voltage with T(estimated) = 50°C:
The worst-case power dissipation by the synchronous
MOSFET under normal operating conditions at elevated
ambient temperature and estimated 50°C junction tem-
perature rise is:
A short circuit to ground will result in a folded back current
of:
with a typical value of R
0.25. The resulting power dissipated in the bottom MOSFET
is:
P
t
P
ON MIN
P
I
SYNC
SC
MAIN
SYNC
(
≈
=
(
MILLER
= (7.5A)
≈
)
2 3
0 007
=
⎛
⎜
⎝
20
25
1 8
+
20
.
5
.
V
V
V
mV
V
IN MAX
V
20
)
−
m
= 15nC/15V = 1000pF. At maximum input
– .
V
(
( )
Ω
1
V
15 1
2
OUT
1 3
1 8
Ω
(1.25)(0.007Ω) ≈ 0.5W
.
+
+
2
V
)
V
( )
( )
[
20
2
DS(ON)
f
(
1
+
15
+
⎛
⎜
⎝
1 8
=
2
150
(
A
.
0 005 50
⎛
⎜
⎝
1
20 400
) (
.
V
( )( )
2
0 6
and d = (0.005/°C)(50°C) =
2 3
45
ns
⎞
⎟
⎠
.
V
1 25 0 007
(
(
. 1 3
A
.
(
µ
400
)
20
H
(
⎞
⎟ Ω
⎠
V
)
V
( )(
(
kHz
kHz
2
)
° −
⎞
⎟ =
⎠
.
C
)
CC
)
1000
=
=
7 5
:
25
Ω
.
2 2
162
)
°
.
A
C
=
pF
W
ns
)
1 84
]
DS(ON)
)
.
3731fa
W