LTC3633A-2/LTC3633A-3
16
3633a23fb
For more information www.linear.com/LTC3633A-2
APPLICATIONS INFORMATION
Conversely, the minimum on-time is the smallest dura-
tion of time in which the top power MOSFET can be in
its “on” state. This time is typically 20ns. In continuous
mode operation, the minimum on-time limit imposes a
minimum duty cycle of:
DC
(MIN)
f • t
ON(MIN)
where t
ON(MIN)
is the minimum on-time. As the equation
shows, reducing the operating frequency will alleviate the
minimum duty cycle constraint.
In the rare cases where the minimum duty cycle is
surpassed, the output voltage will still remain in regula
-
tion, but the switching frequency will decrease from its
programmed value. This constraint may not be of critical
importance in most cases, so high switching frequencies
may be used in the design without any fear of severe
consequences. As the sections on Inductor and Capacitor
selection show
, high switching frequencies allow the use
of
smaller board components, thus reducing the footprint
of the application circuit.
Internal/External Loop Compensation
The LTC3633A-2 provides the option to use a fixed internal
loop compensation network to reduce both the required
external component count and design time. The internal
loop compensation network can be selected by connect
-
ing the ITH pin to the INTV
CC
pin. To ensure stability it is
recommended that internal compensation only be used with
applications with f
SW
> 1MHz. Alternatively, the user may
choose specific external loop compensation components
to optimize the main control loop transient response as
desired. External loop compensation is chosen by simply
connecting the desired network to the ITH pin.
Suggested compensation component values are shown in
Figure 4. 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 filtering out high frequency coupling
from stray board capacitance. In addition, a feedforward
capacitor C
F
can be added to improve the high frequency
response, as previously shown in Figure 3. Capacitor C
F
provides phase lead by creating a high frequency zero
with R2 which improves the phase margin.
Figure 4. Compensation Component
ITH
R
COMP
13k
C
220pF
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SGND
LTC3633A-2
Checking Transient Response
The regulator loop response can be checked by observing
the response of the system to a load step. When configured
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 filtered closed loop
response test point. The DC step, rise time, and settling
behavior at this test point reflect 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 4 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 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
OUT
im-
mediately 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
