LTC3707IGN datasheet(25/32 Pages) LINER

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LTC3707

3707fb

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

Figure 9. Automotive Application Protection

VIN

3707 F09

LTC3707

TRANSIENT VOLTAGE

SUPPRESSOR

GENERAL INSTRUMENT

1.5KA24A

50A IPK RATING

12V

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.

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