LTC3731H_15 Datasheet, PDF (22/34 Pages) Linear Technology – 3-Phase, 600kHz, Synchronous Buck Switching Regulator Controller

LTC3731H
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, in-
cluding 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 10 is the most straightforward
approach to protect a DC/DC converter from the ravages
of an automotive battery 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 IC has a maximum input voltage of 32V on
the SW pins, most applications will be limited to 30V by
the MOSFET BV_DSS.
V_BAT
12V
V_CC
5V
+
LTC3731H
3731H F10
Design Example
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.
L
=
V_OUT
f(∆I)


1−
V_OUT
V_IN


=
(400kHz
1.3V
)(30%)(15A)
 _1−
1.3V
20V


≥ 0.68µH
Using L = 0.6µH, a commonly available value results in
34% ripple current. The worst-case output ripple for the
three stages operating in parallel will be less than 11% of
the peak output current.
R_SENSE1, R_SENSE2 and R_SENSE3 can be calculated by using
a conservative maximum sense current threshold of 65mV
and taking into account half of the ripple current:
R_SENSE
=
65mV
15A^{}1+ ^{342%}
=
0.0037Ω
Use a commonly available 0.003Ω sense resistor.
Next verify the minimum on-time is not violated. The
minimum on-time occurs at maximum V_CC:
t_ON(MIN)
=
V_OUT
( ) ^V_IN(MAX) ^f
=
1.3V
20V₍400kHz₎
=
162ns
The output voltage will be set by the resistive divider from
the DIFFOUT pin to SGND, R1 and R2 in the Functional
Diagram. Set R1 = 13.3k and R2 = 11.3k.
Figure 10. Automotive Application Protection
3731Hfb
22