LTC1876
28
1876fa
Automotive Considerations: Plugging into the
Cigarette Lighter
As battery-powered devices go mobile, there is a natural
interest in plugging into the cigarette lighter in order to
conserve or even recharge battery packs during operation.
But before you connect, be advised: you are plugging into
the supply from hell. The main battery line in an automobile
is the source of a number of nasty potential transients,
including load-dump, reverse-battery, and double-bat-
tery.
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 12 is the most straight
forward 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 LTC1876 step-down controllers
have a maximum input voltage of 36V, most applications
will be limited to 30V by the MOSFET BVDSS.
APPLICATIO S I FOR ATIO
WUU
U
Figure 12. Automotive Application Protection
Design Example
As a design example for one channel, assume V
IN
= 12V
(nominal), V
IN
= 22V(max), V
OUT
= 1.8V, I
MAX
= 5A, and
f = 300kHz, R
SENSE
can immediately be calculated:
R
SENSE
= 50mV/5A = 0.01Ω
Tie the FREQSET pin to the INTV
CC
pin for 300kHz opera-
tion.
Assume a 4.7µH inductor and check the actual value of the
ripple current. The following equation is used:
∆I
V
fL
V
V
L
OUT OUT
IN
=
()()
–1
The highest value of the ripple current occurs at the
maximum input voltage:
∆I
V
kHz H
V
V
A
L
=
µ
=
18
300 4 7
1
18
22
117
.
(. )
–
.
.
The ripple current is 23% of maximum output current,
which is below the 30% guideline. This means that a 3.3µH
inductor can be used.
Increasing the ripple current will also help ensure that the
minimum on-time of 200ns is not violated. The minimum
on-time occurs at maximum V
IN
:
t
V
Vf
V
V kHz
ns
ON MIN
OUT
IN MAX
()
()
.
()
== =
18
22 300
273
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 pins current.
Rk
V
VV
k
V
VV
k
MAX
OUT
124
08
24
24
08
24 18
32
()
.
.–
.
.–.
=
=
=
V
IN
1876 F09
LTC1876
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
50A I
PK
RATING
12V
