LTC1735
U
W U U
APPLICATIO S I FOR ATIO
The network shown in Figure 11 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 LTC1735 has a maximum input
voltage of 36V, most applications will be limited to 30V by
VIN = 12V
OUT = 1.5V
FIGURE 8 CIRCUIT
OUTPUT
V
VOLTAGE
1.5V
100mV/DIV
15A
10A/DIV
0A
LOAD
CURRENT
50µs/DIV
1735 F09
the MOSFET BVDSS
.
Figure 9. Normal Transient Response (Without R1, R4)
50A I RATING
PK
FIGURE 8 CIRCUIT
VIN = 12V
OUT = 1.5V
V
IN
V
12V
1.582V
1.5V
OUTPUT
VOLTAGE
LTC1735
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
100mV/DIV
1.418V
15A
1735 F11
LOAD
CURRENT
10A/DIV
Figure 11. Plugging into the Cigarette Lighter
0A
50µs/DIV
1735 F10
Design Example
As a design example, assume VIN = 12V(nominal),
VIN = 22V(max), VOUT = 1.8V, IMAX = 5A and f = 300kHz.
RSENSE and COSC can immediately be calculated:
Figure 10. Transient Response with Active Voltage Positioning
Automotive Considerations: Plugging into the
Cigarette Lighter
RSENSE = 50mV/5A = 0.01Ω
COSC = 1.61(107)/(300kHz) – 11pF = 43pF
As battery-powered devices go mobile, there is a natural
interest in plugging into the cigarette lighter in order to
conserveorevenrechargebatterypacksduringoperation.
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-battery.
Assume a 3.3µH inductor and check the actual value of the
ripple current. The following equation is used:
V
(f)(L)
V
OUT
V
IN
OUT
∆I =
1–
L
The highest value of the ripple current occurs at the
maximum input voltage:
Load-dump is the result of a loose battery cable. When the
cablebreaksconnection,thefieldcollapseinthealternator
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.
1.8V
300kHz(3.3µH)
1.8V
22V
∆IL =
1–
= 1.7A
The maximum ripple current is 33% of maximum output
current, which is about right.
1735fc
25
Linear [ Linear ]
