LTC3728LX Linear Technology, LTC3728LX Datasheet - Page 24

• Manufacturer Part Number: LTC3728LX
• Description: 2-Phase Synchronous Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3728L/LTC3728LX
truly reflects the closed loop response . Assuming a pre-
dominantly second order system, phase margin and/or
damping factor can be estimated using the percentage of
overshoot seen at this pin. The bandwidth can also be
estimated by examining the rise time at the pin. The I
external components shown in the Figure 1 circuit will
provide an adequate starting point for most applications.
The I
loop compensation. The values can be modified slightly
(from 0.5 to 2 times their suggested values) to optimize
transient response once the final PC layout is done and the
particular output capacitor type and value have been
determined. The output capacitors need to be selected
because the various types and values determine the loop
gain and phase. An output current pulse of 20% to 80% of
full-load current having a rise time of 1 s to 10 s will
produce output voltage and I
give a sense of the overall loop stability without breaking
the feedback loop. Placing a power MOSFET directly
across the output capacitor and driving the gate with an
appropriate signal generator is a practical way to produce
a realistic load step condition. 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 I
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 shift 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
overall 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
24
LOAD
TH
OUT
to C
series R
, causing a rapid drop in V
OUT
C
C
is decreased, the zero frequency will be kept
and the bandwidth of the loop will be
is greater than1:50, the switch rise time
C
-C
U
C
filter sets the dominant pole-zero
U
C
. If R
TH
TH
C
pin signal which is in the
pin waveforms that will
is increased by the same
W
OUT
. No regulator can
U
TH
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.
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-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 9 is the most straightforward
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 LTC3728L/LTC3728LX have a maximum
input voltage of 30V, most applications will also be limited
to 30V by the MOSFET BVD
Figure 9. Automotive Application Protection
12V
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
50A I
PK
LOAD
RATING
. Thus a 10 F capacitor would
SS
V
.
IN
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