LTC3734_15 Datasheet, PDF (22/28 Pages) Linear Technology – Single-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs

LTC3734
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 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-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 7 is the most straightforward
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 LTC3734 has a maximum input
voltage of 32V, most applications will be limited to 30V
by the MOSFET BV_DSS.
V_BAT
12V
PV_CC
+
PV_CC
LTC3734
3734 F07
Design Example
As a design example, assume V_IN = 12V (nominal),
V_IN = 21V (max), V_OUT = 1.5V, I_MAX = 20A, and f = 350kHz.
The inductance value is chosen first based on a 40% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. The minimum
inductance for 40% ripple current is:
L
≥
V_OUT
f • ∆I
•


1–
V_OUT
V_IN


=
350kHz
1.5V
• ( 40%
•
20A
)
•


1–
1.5V
21V


=
0.5µH
The peak inductor current will be the maximum DC current
plus one half of the ripple current, or 24A.
Tie the FREQSET pin to 1.2V, resistively divided down from
SV_CC to have 350kHz operation.
The minimum on-time also occurs at maximum input
voltage:
t_ON(MIN)
=
V_OUT
V_IN • f
=
1.5V
21V • 350kHz
=
204ns
which is larger than 150ns, the typical minimum on time
of the LTC3734.
R_SENSEcan be calculated by using a conservative maximum
sense voltage threshold of 40mV and taking into account
of the peak current:
R_SENSE
=
40mV
24A
=
0.002Ω
Figure 7. Automotive Application Protection
3734fa
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