Datasheet

17
LTC1625
APPLICATIONS INFORMATION
WUU
U
automobile is the source of a number of nasty potential
transients, including load dump, reverse and double
battery.
Load dump is the result of a loose battery cable. When the
cable breaks connection, the field collapse in the alternator
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 finding that a 24V jump start cranks
cold engines faster than 12V.
The network shown in Figure 8 is the most straightforward
approach to protect a DC/DC converter from the ravages
of an automotive battery line. The series diode prevents
current from flowing 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 LTC1625 has a maximum input voltage of
36V, most applications will be limited to 30V by the
MOSFET V
(BR)DSS
.
For 40% ripple current at maximum V
IN
the inductor
should be:
L
V
kHz A
V
V
H≥






=µ
33
225 0 4 2
1
33
22
16
.
( )( . )( )
–
.
Choosing a standard value of 15µH results in a maximum
ripple current of:
∆I
V
kHz H
V
V
A
L MAX()
.
()()
–
.
.=
µ






=
33
225 15
1
33
22
083
Next, check that the minimum value of the current limit is
acceptable. Assume a junction temperature close to a
70°C ambient with ρ
80°C
= 1.3.
I
mV
AA
LIMIT
≥
Ω






=
150
0 042 1 3
1
2
083 23
(. )(.)
–. .
This is comfortably above I
O(MAX)
= 2A. Now double-check
the assumed T
J
:
P
V
V
A
A pF kHz
mW mW mW
TOP
=Ω+
=+=
33
22
23 13 0042
1 7 22 2 3 180 225
43 77 120
2
2
.
(. )(.)(. )
(.)( )(. )( )( )
T
J
= 70°C + (120mW)(50°C/W) = 76°C
Since ρ(76°C) ≅ ρ(80°C), the solution is self-consistent.
A short circuit to ground will result in a folded back
current of:
I
mV V s
H
A
SC
=
Ω
+






µ
µ
=
30
003 11
1
2
15 0 5
15
12
(. )(.)
()(.)
.
with a typical value of R
DS(ON)
and ρ(50°C) = 1.1. The
resulting power dissipated in the bottom MOSFET is:
P
VV
V
AmW
BOT
=Ω=
15 3 3
15
12 11 003 37
2
–.
(. )(.)(. )
which is less than under full load conditions.
V
IN
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
12V
LTC1625
50A I
PK

RATING
1625 F08
PGND
Figure 8. Automotive Application Protection
Design Example
As a design example, take a supply with the following
specifications: V
IN
= 12V to 22V (15V nominal), V
OUT
=
3.3V, I
O(MAX)
= 2A, and f = 225kHz. The required R
DS(ON)
can immediately be estimated:
R
mV
A
DS ON()
()(.)
.==Ω
120
213
0 046
A 0.042Ω Siliconix Si4412DY MOSFET (θ
JA
= 50°C/W) is
close to this value.