LTC3633 LINER [Linear Technology], LTC3633 Datasheet - Page 16

• Manufacturer Part Number: LTC3633
• Description: Dual Channel 3A, 15V Monolithic Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

LTC3633
Suggested compensation component values are shown in
Figure 3. For a 2MHz application, an R-C network of 220pF
and 13kΩ provides a good starting point. The bandwidth
of the loop increases with decreasing C. If R is increased
by the same factor that C 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.
A 10pF bypass capacitor on the ITH pin is recommended
for the purposes of fi ltering out high frequency coupling
from stray board capacitance. In addition, a feedforward
capacitor C
response, as previously shown in Figure 2. Capacitor C
provides phase lead by creating a high frequency zero
with R2 which improves the phase margin.
Checking Transient Response
The regulator loop response can be checked by observing
the response of the system to a load step. When confi gured
for external compensation, the availability of the ITH pin
not only allows optimization of the control loop behavior
but also provides a DC-coupled and AC fi ltered closed loop
response test point. The DC step, rise time, and settling
behavior at this test point refl ect 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 Figure 3 circuit
will provide an adequate starting point for most applica-
tions. The series R-C fi lter sets the dominant pole-zero
loop compensation. The values can be modifi ed slightly
APPLICATIONS INFORMATION
16
F
Figure 3. Compensation Component
can be added to improve the high frequency
LTC3633
SGND
ITH
3633 F03
R
13k
COMP
C
220pF
COMP
F
(from 0.5 to 2 times their suggested values) to optimize
transient response once the fi nal 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 gain and phase. An output current pulse of 20% to
100% of full load current having a rise time of ~1μs 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
mediately shifts by an amount equal to ΔI
ESR is the effective series resistance of C
begins to charge or discharge C
error signal used by the regulator to return V
steady-state value. During this recovery time, V
be monitored for overshoot or ringing that would indicate
a stability problem.
When observing the response of V
initial output voltage step may not be within the bandwidth
of the feedback loop, so the standard second order over-
shoot/DC ratio cannot be used to determine phase margin.
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) input capacitors.
The discharged input capacitors are effectively put in paral-
lel with C
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 specifi cally
for this purpose and usually incorporates current limiting,
short-circuit protection, and soft starting.
OUT
, causing a rapid drop in V
OUT
OUT
generating a feedback
to a load step, the
LOAD
OUT
OUT
. No regulator
. ΔI
• ESR, where
OUT
LOAD
OUT
OUT
to its
also
can
im-
3633f