LTC3839 Datasheet, PDF (32/50 Pages) Linear Technology – Fast, Accurate, 2-Phase, Single-Output Step-Down DC/DC Controller

LTC3839
APPLICATIONS INFORMATION
response test point. The DC step, rise time and settling
at this test point truly reflects the closed-loop response.
Assuming a predominantly 2nd order system, phase
margin and/or damping factor can be estimated using the
percentage of overshoot seen at this pin.
The external series R_ITH-C_ITH1 filter at the ITH pin sets the
dominant pole-zero loop compensation. The values can
be adjusted to optimize transient response once the final
PCB layout is done and the particular output capacitor type
and value have been determined. The output capacitors
need to be selected first because their various types and
values determine the loop feedback factor gain and phase.
An additional small capacitor, C_ITH2, can be placed from
the ITH pin to SGND to attenuate high frequency noise.
Note this C_ITH2 contributes an additional pole in the loop
gain therefore can affect system stability if too large. It
should be chosen so that the added pole is higher than
the loop bandwidth by a significant margin.
The regulator loop response can also be checked by
looking at the load transient response. An output current
pulse of 20% to 100% of full-load current having a rise
time of 1μs to 10μs will produce V_OUT and ITH voltage
transient-response waveforms that can give a sense of the
overall loop stability without breaking the feedback loop.
For a detailed explanation of OPTI-LOOP compensation,
refer to Application Note 76.
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 ∆I_LOAD • ESR,
where ESR is the effective series resistance of C_OUT. ∆I_LOAD
also begins to charge or discharge C_OUT, generating 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.
Connecting a resistive load in series with a power MOSFET,
then placing the two directly across the output capacitor
and driving the gate with an appropriate signal generator
is a practical way to produce a realistic load step condi-
tion. The initial output voltage step resulting from the step
change in load current may not be within the bandwidth
of the feedback loop, so it cannot be used to determine
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phase margin. The output voltage settling behavior is more
related to the stability of the closed-loop system. However,
it is better to look at the filtered and compensated feedback
loop response at the ITH pin.
The gain of the loop increases with the R_ITH and the band-
width of the loop increases with decreasing C_ITH1. If R_ITH
is increased by the same factor that C_ITH1 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. In addition, a feedforward capacitor, C_FF,
can be added to improve the high frequency response, as
shown in Figure 1. Capacitor C_FF provides phase lead by
creating a high frequency zero with R_FB2 which improves
the phase margin.
A more severe transient can be caused by switching in
loads with large supply bypass capacitors. The discharged
bypass capacitors of the load are effectively put in parallel
with the converter’s C_OUT, causing a rapid drop in V_OUT.
No regulator can deliver current quick enough to prevent
this sudden step change in output voltage, if the switch
connecting the C_OUT to the load has low resistance and is
driven quickly. The solution is to limit the turn-on speed of
the load switch driver. Hot Swap™ controllers are designed
specifically for this purpose and usually incorporate current
limiting, short-circuit protection and soft starting.
Load-Release Transient Detection
As the output voltage requirement of step-down switching
regulators becomes lower, V_IN to V_OUT step-down ratio
increases, and load transients become faster, a major
challenge is to limit the overshoot in V_OUT during a fast
load current drop, or “load-release” transient.
Inductor current slew rate di_L/dt = V_L/L is proportional
to voltage across the inductor V_L = V_SW – V_OUT. When
the top MOSFET is turned on, V_L = V_IN – V_OUT, inductor
current ramps up. When bottom MOSFET turns on, V_L =
V_SW – V_OUT = –V_OUT, inductor current ramps down. At
very low V_OUT, the low differential voltage, V_L, across the
inductor during the ramp down makes the slew rate of the
inductor current much slower than needed to follow the
load current change. The excess inductor current charges
up the output capacitor, which causes overshoot at V_OUT.
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