LTC1709EG-8 Linear Technology, LTC1709EG-8 Datasheet - Page 23

• Manufacturer Part Number: LTC1709EG-8
• Description: IC REG SW 2PH SYNC STPDWN 36SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1709EG-8

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 1.3 ~ 3.5 V
• Current - Output: 3A
• Voltage - Input: 4 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 36-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Frequency - Switching: -

APPLICATIO S I FOR ATIO
Design Example
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 the INTV
inductance for 30% ripple current is:
A 1.5 H inductor will produce 27% ripple current. The
peak inductor current will be the maximum DC value plus
one half the ripple current, or 11.4A. The minimum on-
time occurs at maximum V
The R
maximum current sense voltage specification with some
accomodation for tolerances:
The power dissipation on the topside MOSFET can be
easily estimated. Using a Siliconix Si4420DY for example;
R
voltage with T
temperature:
DS(ON)
R
L
t
ON MIN
SENSE
SENSE
1 35
V
f I
300
.
OUT
OUT
= 0.013 , C
CC
kHz
resistors value can be calculated by using the
= 1.8V, I
H
50
11 4
J
1
V
(estimated) = 110 C at an elevated ambient
pin for 300kHz operation. The minimum
V f
OUT
mV
.
1 8
IN
.
30
V
A
V
OUT
U
V
IN
MAX
%
RSS
0 004
5 5
.
= 20A, T
20
.
U
2
= 300pF. At maximum input
A
IN
V
1 8
:
IN
.
300
= 5V (nominal), V
1
V
A
= 70 C and f = 300kHz.
kHz
W
1 8
5 5
.
.
V
V
1 1
.
s
U
IN
= 5.5V
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 about:
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:
which is less than half of the normal, full-load dissipation.
Incidentally, since the load no longer dissipates power in
the shorted condition, total system power dissipation is
decreased by over 99%.
The duty factor for this application is:
P
MAIN
I
P
P
DF
SC
SYNC
SYNC
0 013
V
0 004
V
1 8
5 5
300
.
25
IN
.
O
.
.
1 29
564
5 5
5 5
V
mV
V
kHz
.
.
.
1 8
10 1
V
V
5 5
5 5
mW
5
.
W
.
.
LTC1709-8/LTC1709-9
V
V
2
1 7 5 5
V
V
1 8
1 8
2
1
.
.
.
0 65
.
V
V
0 36
200
.
.
0 005 110
W
6 6
20
.
1 5
V
ns
.
2
.
A
2
A
5 5
H
.
2
20
2
2
V
1 48 0 013
1 48 0 013
A
.
.
C
300
6 6
.
.
.
25
pF
A
C
23