ltc3728legn-1-tr Linear Technology Corporation, ltc3728legn-1-tr Datasheet - Page 24

• Manufacturer Part Number: ltc3728legn-1-tr
• Description: Dual, 550khz, 2-phase Synchronous Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTC3728L-1
at this test point truly refl ects the closed loop response.
Assuming a predominantly 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
circuit will provide an adequate starting point for most
applications.
The I
loop compensation. The values can be modifi ed slightly
(from 0.5 to 2 times their suggested values) to optimize
transient response once the fi nal 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 1μs to 10μs will
produce output voltage and I
give a sense of the overall loop stability without break-
ing 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
the feedback loop and is the fi ltered and compensated
control loop response. The gain of the loop will be in-
creased by increasing R
will be increased by decreasing C
the same factor that C
will be kept 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 (>1μF) 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
24
TH
OUT
series R
, causing a rapid drop in V
TH
external components shown in the Figure 1
C
-C
C
fi lter sets the dominant pole-zero
C
is decreased, the zero frequency
C
and the bandwidth of the loop
TH
TH
pin waveforms that will
C
. If R
pin signal which is in
OUT
. No regulator can
C
is increased by
C
should be controlled so that the load rise time is limited
to approximately 25 • C
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 opera-
tion. But before you connect, be advised: you are plug-
ging 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-battery.
Load-dump is the result of a loose battery cable. When the
cable breaks connection, the fi eld collapse in the alterna-
tor 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 fi nding 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 fl owing 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-1 has a maximum input voltage
of 30V, most applications will also be limited to 30V by
the MOSFET BVD
LOAD
to C
Figure 9. Automotive Application Protection
12V
TRANSIENT VOLTAGE
SUPPRESSOR
GENERAL INSTRUMENT
1.5KA24A
OUT
is greater than 1:50, the switch rise time
50A I
SS
PK
.
RATING
LOAD
. Thus a 10μF capacitor would
V
IN
LTC3728L-1
3728L1 F09
3728l1fc