Datasheet

LTC3838
35
3838fa
APPLICATIONS INFORMATION
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
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
.
If the bottom MOSFET could be turned off during the load-
release transient, the inductor current would flow through
the body diode of the bottom MOSFET, and the equation
can be modified to include the bottom MOSFET body
diode drop to become V
L
= –(V
OUT
+ V
BD
). Obviously the
benefit increases as the output voltage gets lower, since
V
BD
would increase the sum significantly, compared to a
single V
OUT
only.
The load-release overshoot at V
OUT
causes the error ampli-
fier output, ITH, to drop quickly. ITH voltage is proportional