LTC1775 LINER [Linear Technology], LTC1775 Datasheet - Page 18

• Manufacturer Part Number: LTC1775
• Description: High Power No RSENSE TM Current Mode Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

LTC1775
APPLICATIO S I FOR ATIO
18
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 plug-
ging 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 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 10 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 LTC1775 has a maximum input
voltage of 36V, most applications will be limited to 30V by
the MOSFET V
Figure 10. Automotive Application Protection
LTC1775
(BR)DSS
PGND
V
IN
U
.
U
50A I
RATING
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
PK
W
1775 F10
U
12V
Design Example
As a design example, take a supply with the following
specifications: V
I
estimated:
A 0.019
30 C/W) is close to this value.
For 40% ripple current at maximum V
should be:
Choosing a Magnetics 55380-A2 core with 8 turns of 15
gauge wire yields a 6 H inductor. The resulting maximum
ripple current will be:
Next, check that the minimum value of the current limit is
acceptable. Assume a junction temperature about 20 C
above the 70 C ambient with
Now double-check the assumed T
Since (93 C)
O(MAX)
T
I
R
L
P
LIMIT
J
TOP
DS ON
I
L MAX
= 70 C + (0.77W)(30 C/W) = 93 C
(
(
(
= 10A. The required R
150
)
0 56
22
)
( .
5
1 7 22
Siliconix SUD50N03-10 MOSFET (
kHz
.
0 019
V
.
V
(
10
(
300
W
240
)( . )(
150
5
IN
10
V
A
0 4 10
(90 C), the solution is self-consistent.
= 6V to 22V (15V nominal), V
)( . )
mV
A
mV
V
kHz
0 21
)( . )
1 3
5
.
2
2
1 3
V
)(
1 3 0 019
10
W
.
A
6
)
A
–
0 018
H
.
1
0 77
)
DS(ON)
170
.
2
1
–
.
90 C
1
22
4 3
5
–
mW
J
pF
.
V
:
V
22
= 1.3.
5
A
can immediately be
V
150
V
10
IN
6 4
kHz
.
A
the inductor
4 3
.
H
OUT
A
= 5V,
JA
=