LTC3828_15 Datasheet, PDF (23/32 Pages) Linear Technology – Dual, 2-Phase Step-Down Controller with Tracking

LTC3828
APPLICATIONS INFORMATION
Checking Transient Response
The regulator loop response can be checked by looking at
the load current transient response. Switching regulators
take several cycles to respond to a step in DC (resistive)
load current. When a load step occurs, V_OUT shifts by an
amount equal to ΔI_LOAD (ESR), where ESR is the effective
series resistance of C_OUT. ΔI_LOAD also begins to charge or
discharge C_OUT generating the feedback error signal that
forces the regulator to adapt to the current change and
return V_OUT to its steady-state value. During this recovery
time V_OUT can be monitored for excessive overshoot or ring-
ing, which would indicate a stability problem. OPTI-LOOP
compensation allows the transient response to be optimized
over a wide range of output capacitance and ESR values.
The availability of the I_TH pin not only allows optimization
of control loop behavior but also provides a DC coupled
and AC filtered closed-loop response test point. The DC
step, rise time and settling at this test point truly reflects 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_TH external components shown
in the Figure 15 circuit will provide an adequate starting
point for most applications.
The I_TH series R_C-C_C filter sets the dominant pole-zero
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 1μs to 10μs will
produce output voltage and I_TH pin waveforms that will
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_TH pin signal which is in
the feedback loop and is the filtered and compensated
control loop response. The gain of the loop will be in-
creased by increasing R_C and the bandwidth of the loop
will be increased by decreasing C_C. If R_C is increased by
the same factor that C_C is decreased, the zero frequency
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_OUT, causing a rapid drop in V_OUT. No regulator can
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_LOAD to C_OUT is greater than1:50, the switch rise time
should be controlled 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 and Low V_IN Considerations
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, in-
cluding 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 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 finding that a 24V jump start cranks
cold engines faster than 12V.
The network shown in Figure 11a is the most straight-
forward 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
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