Datasheet

22
LTC1929/LTC1929-PG
Design Example
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 FREQSET pin
to the INTV
CC
pin for 300kHz operation. The minimum
inductance for 30% ripple current is:
L
V
fI
V
V
V
kHz A
V
V
H
OUT OUT
IN
≥
∆
()
−






≥
()()()
−






≥µ
1
18
300 30 10
1
18
55
135
.
%
.
.
.
A 1.5µH 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
IN
:
t
V
Vf
V
V kHz
s
ON MIN
OUT
IN
()
==
()( )
=µ
18
5 5 300
11
.
.
.
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 4
0 004
.
.
The power dissipation on the topside MOSFET can be
easily estimated. Using a Siliconix Si4420DY for example;
R
DS(ON)
= 0.013Ω, C
RSS
= 300pF. At maximum input
voltage with T
J
(estimated) = 110°C at an elevated ambient
temperature:
P
V
V
CC
VApF
kHz W
MAIN
=
()
+
()
°− °
()
[]
+
()()( )
()
=
18
55
10 1 0 005 110 25
0 013 1 7 5 5 10 300
300 0 65
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
VV
V
A
W
SYNC
=
−
()()
Ω
()
=
55 18
55
10 1 48 0 013
129
2
..
.
..
.
A short-circuit to ground will result in a folded back current
of about:
I
mV
ns V
H
A
SC
=
Ω
+
()
µ








=
25
0 004
1
2
200 5 5
15
66
.
.
.
.
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
VV
V
A
mW
SYNC
=
−
()()
Ω
()
=
55 18
55
66 148 0013
564
2
..
.
...
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:
DF
V
V
V
V
O
IN
== =
18
5
036
.
.
Using Figure 4, the RMS ripple current will be:
I
INRMS
= (20A)(0.23) = 4.6A
RMS
An input capacitor(s) with a 4.6A
RMS
ripple current rating
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
APPLICATIO S I FOR ATIO
WUU
U