LTC3734 Datasheet, PDF (22/28 Pages) Linear Technology – Single-Phase, High Efficiency DC/DC Controller for Intel Mobile CPUs

LTC3734
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 opera-
tion. 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 alternator
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 straightfor-
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 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
Figure 7. Automotive Application Protection
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 • ΔI
⎛
• _⎝⎜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_SENSE can be calculated by using a conservative maxi-
mum sense voltage threshold of 40mV and taking into
account of the peak current:
R_SENSE
=
40mV
24A
=
0.002Ω
The power loss dissipated by the top MOSFET can be
calculated with equations 3 and 7. Using a Fairchild
FDS7760 as an example: R_DS(ON) = 8mΩ, Q_G = 55nC at 5V
V_GS, C_RSS = 307pF, V_TH(MIN) = 1V. At maximum input
3734f
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