ltc1709-7 Linear Technology Corporation, ltc1709-7 Datasheet - Page 24

• Manufacturer Part Number: ltc1709-7
• Description: 2-phase, 5-bit Vid, Current Mode, High Efficiency, Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

LTC1709-7
APPLICATIO S I FOR ATIO
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 11.5A. 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:
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:
24
DS(ON)
R
P
P
I
t
SC
ON MIN
MAIN
SYNC
SENSE
SENSE
= 0.013 , C
0 004
25
.
0 013
1 8
5 5
1 29
resistors value can be calculated by using the
300
5 5
.
mV
.
.
.
50
11 5
.
J
V
(estimated) = 110 C at an elevated ambient
V f
V
V
V
OUT
5 5
kHz
mV
.
IN
W
.
10
A
V
1 8
U
2
1
.
2
1 7 5 5
RSS
0 004
V
.
200
1
0 65
5 5
.
.
.
10
U
1 5
= 300pF. At maximum input
ns
IN
V
.
.
0 005 110
W
1 8
A
:
.
V
.
5 5
300
2
H
2
.
V
1 48 0 013
10
V
kHz
.
W
A
300
7
C
.
A
1 1
.
pF
25
s
U
C
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 normal, full-load conditions. Inciden-
tally, since the load no longer dissipates power in the
shorted condition, total system power dissipation is de-
creased by over 99%.
The duty factor for this application is:
Using Figure 4, the RMS ripple current will be:
An input capacitor(s) with a 4.6A
is required.
The output capacitor ripple current is calculated by using
the inductor ripple already calculated for each inductor
and multiplying by the factor obtained from Figure 3
along with the calculated duty factor. The output ripple in
continuous mode will be highest at the maximum input
voltage since the duty factor is < 50%. The maximum
output current ripple is:
I
P
DF
V
INRMS
SYNC
OUTRIPPLE
I
I
COUT
COUTMAX
V
V
= (20A)(0.23) = 4.6A
IN
O
630
5 5
.
V
OUT
1 8
fL
V
5 5
mW
5
.
.
1 2
20
V
300
V
.
V
1 8
m
0 3
A
.
.
RMS
kHz
V
0 36
1 8
.
at
1 2
.
7
.
A
33
V
1 5
A
.
2
RMS
%
RMS
1 48 0 013
RMS
D F
H
.
0 3
ripple current rating
.
24
.
mV
RMS