LTC3855 LINER [Linear Technology], LTC3855 Datasheet - Page 35

• Manufacturer Part Number: LTC3855
• Description: Dual, Fast, Accurate Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

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
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
immediately shifts by an amount equal to ∆I
where ESR is the effective series resistance of C
also begins to charge or discharge C
feedback error signal used by the regulator to return V
to its steady-state value. During this recovery time, V
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
width of the loop increases with decreasing C
is increased by the same factor that C
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
can be added to improve the high frequency response, as
shown in Figure 1. Capacitor C
creating a high frequency zero with R
the phase margin.
FF
provides phase lead by
OUT
ITH1
FB2
OUT
ITH
is decreased, the
and ITH voltage
which improves
, generating a
and the band-
LOAD
ITH1
OUT
. ∆I
. If R
• ESR,
LOAD
OUT
OUT
OUT
ITH
FF
,
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
No regulator can deliver current quick enough to prevent
this sudden step change in output voltage, if the switch
connecting the C
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
increases, and load transients become faster, a major
challenge is to limit the overshoot in V
load current drop, or “load-release” transient.
Inductor current slew rate di
to voltage across the inductor V
the top MOSFET is turned on, V
current ramps up. When bottom MOSFET turns on, V
V
very low V
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
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
benefit increases as the output voltage gets lower, since
V
single V
The load-release overshoot at V
fier output, ITH, to drop quickly. ITH voltage is proportional
SW
BD
would increase the sum significantly, compared to a
– V
OUT
OUT
OUT
only.
= –V
, the low differential voltage, V
OUT
OUT
to the load has low resistance and is
, inductor current ramps down. At
OUT
L
, causing a rapid drop in V
= –(V
IN
L
OUT
/dt = V
OUT
to V
L
L
= V
causes the error ampli-
= V
+ V
OUT
IN
L
SW
BD
/L is proportional
LTC3838
OUT
– V
step-down ratio
). Obviously the
– V
OUT
during a fast
L
, across the
OUT
, inductor
35
. When
OUT
OUT
3838fa
L
=
.
.