LTC1709-9 Linear Technology, LTC1709-9 Datasheet - Page 23

• Manufacturer Part Number: LTC1709-9
• Description: 2-Phase/ 5-Bit VID/ Current Mode/ High Efficiency/ Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

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 FREQSET 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)
L
R
P
t
ON MIN
MAIN
SENSE
SENSE
1 35
V
f I
300
.
OUT
OUT
= 0.013 , C
0 013
1 8
5 5
CC
kHz
resistors value can be calculated by using the
300
= 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
V
V
1 8
OUT
kHz
mV
.
IN
.
30
V
10
A
V
OUT
V
U
IN
%
MAX
2
1 7 5 5
RSS
0 004
.
1
0 65
5 5
.
10
= 20A, T
.
.
U
= 300pF. At maximum input
A
IN
V
.
0 005 110
W
1 8
:
.
V
IN
.
300
1
= 5V (nominal), V
2
V
A
10
= 70 C and f = 300kHz.
kHz
1 8
5 5
W
.
.
A
V
V
300
C
1 1
.
pF
25
s
U
IN
C
= 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:
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
I
I
P
P
DF
INRMS
SC
SYNC
SYNC
V
0 004
V
25
= (20A)(0.23) = 4.6A
IN
.
O
1 29
564
5 5
5 5
mV
.
.
.
1 8
V
V
5 5
5 5
mW
5
.
W
.
.
LTC1709-8/LTC1709-9
V
V
V
V
1 8
1 8
2
1
.
.
V
V
0 36
200
.
10
6 6
1 5
ns
.
.
A
A
5 5
RMS
2
H
.
2
RMS
1 48 0 013
V
1 48 0 013
.
.
ripple current rating
6 6
.
.
.
A
23