12
LTC1624
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 10
µ
F capacitor would require a 250
µ
s 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 4 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 LTC1624 has a maximum input voltage of
APPLICATIO
N
S I
N
FOR
M
ATIO
N
W
U
U
U
36V, most applications will be limited to 30V by the
MOSFET BV
DSS
.
Modifying Burst Mode Operation
The LTC1624 automatically enters Burst Mode operation
at low output currents to boost efficiency. The point when
continuous mode operation changes to Burst Mode op-
eration scales as a function of maximum output current.
The output current when Burst Mode operation com-
mences is approximately 8mV/R
SENSE
(8% of maximum
output current).
With the additional circuitry shown in Figure 5 the LTC1624
can be forced to stay in continuous mode longer at low
output currents. Since the LTC1624 is not a fully synchro-
nous architecture, it will eventually start to skip cycles as
the load current drops low enough. The point when the
minimum on-time (450ns) is reached determines the load
current when cycle skipping begins at approximately 1%
of maximum output current. Using the circuit in Figure 5
the LTC1624 will begin to skip cycles but stays in regula-
tion when I
OUT
is less than I
OUT(MIN)
:
I
t
f
L
V
V
V
V
V
V
OUT MIN
ON MIN
IN
OUT
IN
D
OUT
D
( )
( )
=
−
(
)
+
+
2
2
where t
ON(MIN)
= 450ns, f = 200kHz.
The transistor Q1 in the circuit of Figure 5 operates as a
current source developing an 18mV offset across the
+
+
1000pF
100
Ω
18mV
R*
C
OUT
L1
R
SENSE
C
IN
V
OUT
D1
MBRS340T3
Q1
2N2222
V
IN
1624 F05
(V
OUT
– 0.7V)
180
µ
A
*R =
V
IN
LTC1624
SENSE
–
TG
SW
+
–
Figure 5. Modifying Burst Mode Operation
Figure 4. Plugging into the Cigarette Lighter
LTC1624
V
IN
50A I
PK
RATING
12V
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
1624 F04
