22
LTC3729
sn3729 3729fas
Design Example (Using Two Phases)
As a design example, assume V
IN
= 5V (nominal), V
IN
␣ =␣ 5.5V
(max), V
OUT
= 1.8V, I
MAX
= 20A, T
A
= 70
°
C and f␣ =␣ 300kHz.
The inductance value is chosen first based on a 30% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. Tie the PLLFLTR pin
to a resistive divider using the INTV
CC
pin to generate 1V
for 300kHz operation. The minimum inductance for 30%
ripple current is:
L
V
f
I
V
V
V
kHz
A
V
V
H
OUT
OUT
IN
≥
∆
( )
−
≥
(
)( )( )
−
≥
µ
1
1 8
300
30
10
1
1 8
5 5
1 35
.
%
.
.
.
A 2
µ
H inductor will produce 20% ripple current. The peak
inductor current will be the maximum DC value plus one
half the ripple current, or 11.5A. The minimum on-time
occurs at maximum V
IN
:
t
V
V f
V
V
kHz
s
ON MIN
OUT
IN
( )
=
=
( )(
)
= µ
1 8
5 5
300
1 1
.
.
.
The R
SENSE
resistors value can be calculated by using the
maximum current sense voltage specification with some
accomodation for tolerances:
R
mV
A
SENSE
=
≈
Ω
50
11 5
0 005
.
.
Choosing 1% resistors: R1 = 16.5k and R2 = 13.2k yields
an output voltage of 1.80V.
The power dissipation on the topside MOSFET can be
easily estimated. Using a Siliconix Si4420DY for example;
R
DS(ON)
= 0.013
Ω
, C
RSS
= 300pF. At maximum input
voltage with T
j
(estimated) = 110
°
C at an elevated ambient
temperature:
P
V
V
C
C
V
A
pF
kHz
W
MAIN
=
( )
+
( )
° − °
(
)
[
]
+
( ) ( )( )
(
)
=
1 8
5 5
10
1
0 005 110
25
0 013
1 7 5 5
10
300
310
0 61
2
2
.
.
.
.
.
.
.
Ω
The worst-case power disipated by the synchronous
MOSFET under normal operating conditions at elevated
ambient temperature and estimated 50
°
C junction tem-
perature rise is:
P
V
V
V
A
W
SYNC
=
−
( ) ( )
Ω
(
)
=
5 5
1 8
5 5
10
1 48 0 013
1 29
2
.
.
.
.
.
.
A short-circuit to ground will result in a folded back current
of:
I
mV
ns
V
H
A
SC
=
Ω
+
( )
µ
=
25
0 005
1
2
200
5 5
2
5 28
.
.
.
The worst-case power disipated by the synchronous
MOSFET under short-circuit conditions at elevated ambi-
ent temperature and estimated 50
°
C junction temperature
rise is:
P
V
V
V
A
mW
SYNC
=
−
( ) ( )
Ω
(
)
=
5 5
1 8
5 5
5 28
1 48 0 013
360
2
.
.
.
.
.
.
APPLICATIO S I FOR ATIO
W
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