LTC1929-PG LINER [Linear Technology], LTC1929-PG Datasheet - Page 21

• Manufacturer Part Number: LTC1929-PG
• Description: 2-Phase, High Efficiency,Synchronous Step-Down Switching Regulators
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
that will give a sense of the overall loop stability without
breaking the feedback loop. The initial output voltage step
resulting from the step change in output current may not
be within the bandwidth of the feedback loop, so this signal
cannot be used to determine phase margin. This is why it
is better to look at the Ith pin signal which is in the feedback
loop and is the filtered and compensated control loop
response. The gain of the loop will be increased by
increasing R
increased by decreasing C
factor that C
the same, thereby keeping the phase the same in the most
critical frequency range of the feedback loop. The output
voltage settling behavior is related to the stability of the
closed-loop system and will demonstrate the actual over-
all supply performance.
A second, more severe transient is caused by switching in
loads with large (>1 F) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
alter its delivery of current quickly enough to prevent this
sudden step change in output voltage if the load switch
resistance is low and it is driven quickly. If the ratio of
C
should be controlled so that the load rise time is limited to
approximately 25 • C
require a 250 s rise time, limiting the charging current to
about 200mA.
LOAD
OUT
to C
, causing a rapid drop in V
OUT
C
C
is decreased, the zero frequency will be kept
is greater than 1:50, the switch rise time
and the bandwidth of the loop will be
U
LOAD
C
U
. Thus a 10 F capacitor would
. If R
C
is increased by the same
W
OUT
. No regulator can
12V
GENERAL INSTRUMENT
Figure 8. Automotive Application Protection
TRANSIENT VOLTAGE
SUPPRESSOR
U
50A I
1.5KA24A
PK
RATING
V
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-bat-
tery.
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 8 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 LT1929 has a maximum input voltage of 36V,
most applications will be limited to 30V by the MOSFET
BV
IN
DSS
.
LTC1929
LTC1929/LTC1929-PG
1929 F08
21