Datasheet
LTC3731
23
3731fc
APPLICATIONS INFORMATION
the SW pins, most applications will be limited to 30V by
the MOSFET BV
DSS
.
Design Example
As a design example, assume V
CC
= 5V, V
IN
= 12V(nominal),
V
IN
= 20V(max), V
OUT
= 1.3V, I
MAX
= 45A and f = 400kHz. 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.
L =
V
OUT
f ∆I
( )
1−
V
OUT
V
IN
=
1.3V
400kHz
( )
30%
( )
15A
( )
1−
1.3V
20V
≥ 0.68µH
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
SENSE1,
R
SENSE2
and R
SENSE3
can be calculated by using
a conservative maximum sense current threshold of 65mV
and taking into account half of the ripple current:
R
SENSE
=
65mV
15A 1+
34%
2
= 0.0037Ω
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
CC
:
t
ON MIN
( )
=
V
OUT
V
IN(MAX)
f
( )
=
1.3V
20V 400kHz
( )
= 162ns
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
DS(ON)
= 7mΩ, C
MILLER
= 15nC/15V = 1000pF. At maximum input
voltage with T(estimated) = 50°C:
P
MAIN
≈
1.8V
20V
15
( )
2
1+ 0.005
( )
50°C − 25°C
( )
0.007Ω + 20
( )
2
45A
2
( )
3
( )
2Ω
( )
1000pF
( )
1
5V – 1.8V
+
1
1.8V
400kHz
( )
= 2.2W
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:
P
SYNC
=
20V − 1.3V
20V
15A
( )
2
1.25
( )
0.007Ω
( )
= 1.84W
A short-circuit to ground will result in a folded back cur-
rent of:
I
SC
≈
25mV
2+ 3
( )
mΩ
+
1
2
150ns 20V
( )
0.6µH
= 7.5A
with a typical value of R
DS(ON)
and d = (0.005/°C)(50°C)
= 0.25. The resulting power dissipated in the bottom
MOSFET is:
P
SYNC
= (7.5A)
2
(1.25)(0.007Ω) ≈ 0.5W
+
LTC3731
V
CC
5V
V
BAT
12V
3731 F10
Figure 10. Automotive Application Protection