Datasheet

LTC3727A-1
24
3727a1fa
automobile is the source of a number of nasty potential
transients, including load-dump, reverse-battery, and
double-battery.
Load-dump is the result of a loose battery cable. When the
cable breaks connection, the fi eld collapse in the alterna-
tor can cause a positive spike as high as 60V which takes
several hundred milliseconds to decay. Reverse-battery is
just what it says, while double-battery is a consequence of
tow-truck operators fi nding that a 24V jump start cranks
cold engines faster than 12V.
The network shown in Figure 9 is the most straight for-
ward approach to protect a DC/DC converter from the
ravages of an automotive power line. The series diode
prevents current from fl owing during reverse-battery,
while the transient suppressor clamps the input voltage
during load-dump. Note that the transient suppressor
should not conduct during double-battery operation, but
must still clamp the input voltage below breakdown of
the converter. Although the LTC3727A-1 has a maximum
input voltage of 36V, most applications will be limited to
30V by the MOSFET BVDSS.
Design Example
As a design example for one channel, assume V
IN
=
24V(nominal), V
IN
= 30V(max), V
OUT
= 12V, I
MAX
= 5A
and f = 250kHz.
The inductance value is chosen fi rst based on a 40% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. Tie the PLLFLTR
pin to the SGND pin for 250kHz operation. The minimum
inductance for 40% ripple current is:
ΔI
V
fL
V
V
L
OUT OUT
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
()( )
–1
A 14μH inductor will result in 40% ripple current. The peak
inductor current will be the maximum DC value plus one
half the ripple current, or 6A, for the 14μH value.
APPLICATIONS INFORMATION
The R
SENSE
resistor value can be calculated by using the
maximum current sense voltage specifi cation with some
accommodation for tolerances:
R
mV
A
SENSE
≤≈Ω
90
6
0 015.
Choosing 1% resistors; R1 = 20k and R2 = 280k yields
an output voltage of 12V.
The power dissipation on the top side MOSFET can be
easily estimated. Choosing a Siliconix Si4412DY results
in: R
DS(ON)
= 0.042Ω, C
RSS
= 100pF. At maximum input
voltage with T(estimated) = 50°C:
P
V
V
CC
MAIN
=
()
+°°
[]
12
30
5 1 0 005 50 25
0 042
2
(. )( – )
. ΩΩ
()
+
()()( )( )
=
1 7 30 5 100 250
664
2
.VA pF kHz
mW
A short-circuit to ground will result in a folded back
current of:
I
mV ns V
μH
A
SC
=
Ω
+
⎛
⎝
⎜
⎞
⎠
⎟
=
45
0 015
1
2
200 30
14
32
.
()
.
with a typical value of R
DS(ON)
and δ = (0.005/°C)(20) = 0.1.
The resulting power dissipated in the bottom MOSFET is:
P
VV
V
A
mW
SYNC
=
()()
Ω
()
=
30 12
30
32 11 0042
284
2
–
...
which is less than under full-load conditions.
Figure 9. Automotive Application Protection
V
IN
3727 F09
LTC3727A-1
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
50A I
PK
RATING
12V