22
LTC1436A
LTC1436-PLL-A/LTC1437A
14367afb
APPLICATIONS INFORMATION
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A second, more severe transient is caused by switching in
loads with large (>1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
OUT
, causing a rapid drop in V
OUT
. No regulator can
deliver enough current to prevent this problem if the load
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive so that
the load rise time is limited to approximately 25(C
LOAD
).
Thus a 10µF capacitor would require a 250µs rise time,
limiting the charging current to about 200mA.
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 automo-
bile 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 14 is the most straightfor-
ward 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 LTC1436A/LTC1437A have a maxi-
mum input voltage of 36V, most applications will be
limited to 30V by the MOSFET BV
DSS
.
Design Example
As a design example, assume V
IN
= 12V (nominal), V
IN
=
22V (max), V
OUT
= 1.6V, I
MAX
= 3A and f = 250kHz, R
SENSE
and C
OSC
can immediately be calculated:
R
mV
A
CpF
SENSE
OSC
==
=
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
=
100
3
0 033
13710
250
11 43
4
.
.( )
–
Ω
Refering to Figure 3, a 4.7µH inductor falls within the
recommended range. To 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:
CpF
Frequency kHz
OSC
()
=
()
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
13710
11
4
.( )
–
The lowest duty cycle also occurs at maximum input
voltage. The on-time during this condition should be
checked to make sure it doesn’t violate the LTC1436A/
LTC1437A’s minimum on-time and cause cycle skipping
to occur. The required on-time at V
IN(MAX)
is:
t
V
Vf
V
V kHz
ns
ON MIN
OUT
IN MAX
()
()
.
=
()
()
=
()( )
=
16
22 250
291
The ∆I
L
was previously calculated to be 1.3A, which is
43% of I
MAX
. From Figure 12, the LTC1436A/LTC1437A’s
minimum on-time at 43% ripple is about 235ns. There-
fore, the minimum on-time is sufficient and no cycle
skipping will occur.
Figure 14. Automotive Application Protection
1436 F14
50A I
PK
RATING
LTC1436A
LTC1437A
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
V
IN
12V
