LTC3707 Datasheet, PDF (25/32 Pages) Linear Technology – High Effi ciency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3707
APPLICATIONS INFORMATION
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 opera-
tion. But before you connect, be advised: you are plug-
ging into the supply from hell. The main power line in an
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 field 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 finding 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 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 LTC3707 has a maximum input
voltage of 30V, most applications will be limited to 28V
by the MOSFET BVDSS.
50A I_PK RATING
V_IN
12V
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
LTC3707
3707 F09
Figure 9. Automotive Application Protection
3707fb
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