LTC3731H Datasheet, PDF (22/32 Pages) Linear Technology – 3-Phase, 600kHz, Synchronous Buck Switching Regulator Controller

LTC3731H
APPLICATIO S I FOR ATIO
Automotive Considerations: Plugging into the
Cigarette Lighter
Design Example
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
As a design example, assume V_CC = 5V, V_IN = 12V(nominal),
V_IN = 20V(max), V_OUT = 1.3V, I_MAX = 45A and f = 400kHz.
The inductance value is chosen first based upon a 30%
ripple current assumption. The highest value of ripple
current in each output stage occurs at the maximum input
voltage.
transients, including load dump, reverse battery and
double battery.
Load dump is the result of a loose battery cable. When the
L
=
V_OUT
f(∆I)
⎛
⎝⎜ ^1−
V_OUT
V_IN
⎞
⎠⎟
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
=
(
1.3V
400kHz)(30%)(15A)
⎛
⎝⎜
1−
1.3V
20V
⎞
⎠⎟
≥ 0.68µH
tow-truck operators finding that a 24V jump start cranks
cold engines faster than 12V.
Using
L
= 0.6µH,
a
commonly
available value results
in
34% ripple current. The worst-case output ripple for the
The network shown in Figure 10 is the most straightfor- _{three stages operating in parallel will be less than 11% of}
ward approach to protect a DC/DC converter from the _{the peak output current.}
ravages of an automotive battery line. The series diode
prevents current from flowing during reverse battery, ^{RSENSE1, RSENSE2 and RSENSE3 can be calculated by using}
while the transient suppressor clamps the input voltage ^{a conservative maximum sense current threshold of 65mV}
during load dump. Note that the transient suppressor ^{and taking into account half of the ripple current:}
should not conduct during double-battery operation, but
must still clamp the input voltage below breakdown of the
converter. Although the IC has a maximum input voltage
of 32V on the SW pins, most applications will be limited to
R_SENSE
=
65mV
15A^{⎛⎝⎜}1+ ^{342%⎞⎠⎟}
=
0.0037Ω
30V by the MOSFET BV_DSS.
Use a commonly available 0.003Ω sense resistor.
V_CC
5V
+
Next verify the minimum on-time is not violated. The
V_BAT
12V
minimum on-time occurs at maximum V_CC:
t_ON(MIN)
=
V_OUT
( ) ^V_IN(MAX) ^f
=
1.3V
20V₍400kHz₎
=
162ns
LTC3731H
The output voltage will be set by the resistive divider from
the DIFFOUT pin to SGND, R1 and R2 in the Functional
3731H F10
Diagram. Set R1 = 13.3k and R2 = 11.3k.
Figure 10. Automotive Application Protection
3731hf
22