Datasheet

22
LTC1629-6
16296f
Although the LTC1629-6 has a maximum input voltage of
36V, most applications will be limited to 30V by the
MOSFET BV
DSS
.
Since the output voltage is below 2.4V the output resistive
divider will need to be sized to not only set the output
voltage but also to absorb the sense pin input current for
both channels.
R
kV
VV
k
V
VV
k
MIN
OUT
OUT
1
20
224
10
18
24 18
30
()
=
−






=
−






=
.
.
..
Choosing 1% resistors: R1 = 10k and R2 = 20k yields an
output voltage of 1.80V and satisfies the above condition.
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
310 0 61
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:
I
mV
ns V
H
A
SC
=
Ω
+
()
µ








=
25
0 005
1
2
200 5 5
2
528
.
.
.
APPLICATIO S I FOR ATIO
WUU
U
Figure 8. Automotive Application Protection
V
IN
1629 F08
12V
50A I
PK
RATING
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
LTC1629-6
Design Example (Using Two Phases)
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 2µH 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
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
60
11 5
0 005
.
.