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LTC1628/LTC1628-PG

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 power 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

Figure 9. Automotive Application Protection

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 forward

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 LTC1628 has a maximum input voltage of

36V, most applications will be limited to 30V by the

MOSFET BVDSS.

VIN

1628 F09

LTC1628

TRANSIENT VOLTAGE

SUPPRESSOR

GENERAL INSTRUMENT

1.5KA24A

50A IPK RATING

12V

APPLICATIO S I FOR ATIO

LTC1628 Datasheet(PDF) 25 Page - Linear Technology