Datasheet

25
LTC1735
1735fc
APPLICATIO S I FOR ATIO
WUU
U
V
IN
= 12V
V
OUT
= 1.5V
1.5V
100mV/DIV
15A
0A
10A/DIV
OUTPUT
VOLTAGE
LOAD
CURRENT
50µs/DIV
1735 F09
Figure 9. Normal Transient Response (Without R1, R4)
V
IN
= 12V
V
OUT
= 1.5V
1.582V
1.5V
1.418V
100mV/DIV
15A
0A
10A/DIV
50µs/DIV
1735 F10
Figure 10. Transient Response with Active Voltage Positioning
OUTPUT
VOLTAGE
LOAD
CURRENT
Figure 11. Plugging into the Cigarette Lighter
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 power 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␣ 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
the MOSFET BV
DSS
.
V
IN
50A I
PK
RATING
1735 F11
LTC1735
12V
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
Design Example
As a design example, assume V
IN
= 12V(nominal),
V
IN
= 22V(max), V
OUT
= 1.8V, I
MAX
= 5A and f = 300kHz.
R
SENSE
and C
OSC
can immediately be calculated:
R
SENSE
= 50mV/5A = 0.01Ω
C
OSC
= 1.61(10
7
)/(300kHz) – 11pF = 43pF
Assume a 3.3µH inductor and check the actual value of the
ripple current. The following equation is used:
∆I
V
fL
V
V
L
OUT OUT
IN
=






()()
–1
The highest value of the ripple current occurs at the
maximum input voltage:
∆I
V
kHz H
V
V
A
L
=
µ






=
18
300 3 3
1
18
22
17
.
(. )
–
.
.
The maximum ripple current is 33% of maximum output
current, which is about right.
FIGURE 8 CIRCUIT
FIGURE 8 CIRCUIT