LTC3600_15 Datasheet, PDF (14/28 Pages) Linear Technology – 15V, 1.5A Synchronous Rail-to-Rail Single Resistor Step-Down Regulator

LTC3600
Applications Information
Once the value for L is known, the type of inductor must be
selected. Ferrite designs have very low core losses and are
preferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing satura-
tion. Ferrite core material saturates “hard”, which means
that inductance collapses abruptly when the peak design
current is exceeded. This results in an abrupt increase in
inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and do not radiate much energy, but generally cost
more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price versus size requirements
and any radiated field/EMI requirements. New designs for
surface mount inductors are available from Toko, Vishay,
NEC/Tokin, Cooper, TDK, and Würth Elektronik. Refer to
Table 1 for more details.
Checking Transient Response
The OPTI-LOOP compensation allows the transient re-
sponse to be optimized for a wide range of loads and
output capacitors. The availability of the ITH pin not only
allows optimization of the 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 ITH external components shown in the Figure 1 circuit
will provide an adequate starting point for most applica-
tions. The series R-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 their various types and values determine the
loop feedback factor gain and phase. An output current
pulse of 20% to 100% of full load current having a rise
time of 1µs to 10µs will produce output voltage and ITH
pin waveforms that will give a sense of the overall loop
stability without breaking the feedback loop.
Switching regulators take several cycles to respond to
a step in load current. When a load step occurs, V_OUT
immediately 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 generat-
ing a feedback error signal used by the regulator to return
V_OUT to its steady-state value. During this recovery time,
V_OUT can be monitored for overshoot or ringing that would
indicate a stability problem.
The initial output voltage step may not be within the band-
width of the feedback loop, so the standard second order
overshoot/DC ratio cannot be used to determine phase
margin. The gain of the loop increases with the R_ITH and
the bandwidth of the loop increases with decreasing C_ITH. If
R_ITH is increased by the same factor that C_ITH is decreased,
the zero frequency will be kept 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
overall supply performance. For a detailed explanation of
optimizing the compensation components, including a
review of control loop theory, refer to Linear Technology
Application Note 76.
In some applications, a more severe transient can be caused
by switching in loads with large (>10µF) load capacitors.
The discharged load 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
switch connecting the load has low resistance and is driven
quickly. The solution is to limit the turn-on speed of the
load switch driver. A Hot Swap^™ controller is designed
specifically for this purpose and usually incorporates cur-
rent limit, short-circuit protection, and soft-start.
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