ltc3729euh-trpbf Linear Technology Corporation, ltc3729euh-trpbf Datasheet - Page 22

• Manufacturer Part Number: ltc3729euh-trpbf
• Description: 550khz, Polyphase, High Efficiency, Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3729
Design Example (Using Two Phases)
As a design example, assume V
(max), V
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
for 300kHz operation. The minimum inductance for 30%
ripple current is:
A 2 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
The R
maximum current sense voltage specification with some
accomodation for tolerances:
Choosing 1% resistors: R1 = 16.5k and R2 = 13.2k yields
an output voltage of 1.80V.
22
R
L
t
ON MIN
SENSE
SENSE
1 35
V
f I
300
.
OUT
OUT
kHz
resistors value can be calculated by using the
= 1.8V, I
H
50
11 5
1
V
V f
1 8
OUT
mV
.
IN
.
30
V
A
V
V
OUT
U
IN
MAX
%
IN
0 005
10
5 5
.
:
= 20A, T
.
U
A
V
1 8
IN
.
1
300
= 5V (nominal), V
V
A
= 70 C and f = 300kHz.
1 8
5 5
kHz
W
CC
.
.
V
V
pin to generate 1V
1 1
.
s
U
IN
= 5.5V
The power dissipation on the topside MOSFET can be
easily estimated. Using a Siliconix Si4420DY for example;
R
voltage with T
temperature:
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:
A short-circuit to ground will result in a folded back current
of:
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:
DS(ON)
P
P
I
P
SC
MAIN
SYNC
SYNC
= 0.013 , C
0 005
25
.
0 013
1 8
1 29
5 5
310
360
5 5
5 5
.
mV
.
.
.
.
.
j
(estimated) = 110 C at an elevated ambient
V
V
V
V
5 5
5 5
kHz
mW
W
.
.
10 1
V
V
1 8
1 8
2
1
.
.
2
1 7 5 5
RSS
V
V
0 61
.
200
.
10
5 28
= 300pF. At maximum input
.
ns
.
W
2
0 005 110
A
.
V
A
H
2
5 5
2
.
1 48 0 013
2
V
10
.
1 48 0 013
.
A
300
.
5 28
C
.
.
pF
25
A
sn3729 3729fas
C