Datasheet

14
LTC1435
APPLICATIONS INFORMATION
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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 7 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 LT1435 has a maximum input
voltage of 36V, most applications will be limited to 30V
by the MOSFET BV
DSS
.
MOSFET and the synchronous MOSFET. If the two
MOSFETs have approximately the same R
DS(ON)
, then
the resistance of one MOSFET can simply be summed
with the resistances of L and R
SENSE
to obtain I
2
R
losses. For example, if each R
DS(ON)
= 0.05Ω,
R
L
= 0.15Ω, and R
SENSE
= 0.05Ω, then the total
resistance is 0.25Ω. This results in losses ranging
from 3% to 10% as the output current increases from
0.5A to 2A. I
2
R losses cause the efficiency to drop at
high output currents.
4. Transition losses apply only to the topside MOSFET(s),
and only when operating at high input voltages (typi-
cally 20V or greater). Transition losses can be esti-
mated from:
Transition Loss = 2.5 (V
IN
)
1.85
(I
MAX
)(C
RSS
)(f)
Other losses, including C
IN
and C
OUT
ESR dissipative
losses, Schottky conduction losses during dead-time,
and inductor core losses, generally account for less
than 2% total additional loss.
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in DC (resistive) load
current. When a load step occurs, V
OUT
immediately shifts
by an amount equal to (∆I
LOAD
)(ESR), where ESR is the
effective series resistance of C
OUT
. ∆I
LOAD
also begins to
charge or discharge C
OUT
which generates a feedback
error signal. The regulator loop then acts to return V
OUT
to
its steady-state value. During this recovery time V
OUT
can
be monitored for overshoot or ringing which would indi-
cate a stability problem. The I
TH
external components
shown in the Figure 1 circuit will provide adequate com-
pensation for most applications.
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 paral-
lel 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
Figure 7. Automotive Application Protection
1435 F07
50A I
PK
RATING
LTC1435
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
V
IN
12V