LTC3707 Linear Technology, LTC3707 Datasheet - Page 25

• Manufacturer Part Number: LTC3707
• Description: High Efficiency / 2-Phase Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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
U
U
W
Figure 9. Automotive Application Protection
12V
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
U
50A I
PK
RATING
V
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 LTC3707 has a maximum input voltage of
30V, most applications will be limited to 28V by the
MOSFET BVDSS.
IN
LTC3707
3707 F09
LTC3707
25
3707f