LTC3646-1_15 Datasheet, PDF (16/28 Pages) Linear Technology – 40V, 1A Synchronous Step-Down Converter

LTC3646/LTC3646-1
APPLICATIONS INFORMATION
Once R_COMP is determined, C_COMP can be chosen to set
the zero frequency (f_Z):
f_Z
=
2π
1
• C_COMP
• R_COMP
For maximum phase margin, f_Z should be chosen to be
less than one-tenth of f_C.
Since the ITH node is sensitive to noise coupling, a small
bypass capacitor (C_BYP) may be used to filter out board
noise. However, this cap contributes a pole at f_P and may
introduce some phase loss at the crossover frequency:
f_P
=
2π
•
1
C_BYP• R_COMP
For best results, f_P should be set high enough such that
phase margin is not significantly affected.
If necessary, a capacitor C_F (as shown in Figure 2) may
be used to add some phase lead.
Though better load transient response can generally be
achieved with external compensation, at switching fre-
quencies above 1MHz, component count can be reduced
by connecting the ITH pin to INTV_CC enabling internal
compensation. When using internal compensation, a
reasonable starting point for the minimum amount of
output capacitance necessary for stability can be found
as the greater of 15µF or C_OUT defined by the equation:
C_OUT > 3 • 10^–5/V_OUT
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 system’s closed-
loop response. Assuming a predominantly second order
system, the phase margin and/or damping factor can be
estimated by observing the percentage of overshoot seen
at this pin. Use a high impedance, low capacitance probe
(>50MΩ, <5pF). The ITH external components shown in
Figure 3 will provide an adequate starting point for most
applications. The series R-C filter sets the pole-zero loop
compensation. The values can be modified from their
suggested values once the final PC layout is done, and the
particular switching frequency, output capacitor type and
value have been chosen. The output capacitors need to be
selected because their various types and values determine
the loop feedback factor gain and phase. An output cur-
rent pulse of 20% to 100% of full load current with a rise
time of 1μs to 10μs will produce output voltage and ITH
pin waveforms that will give a sense of the overall loop
stability without breaking the feedback loop.
When observing the response of V_OUT to a load step, the
initial output voltage step may not be within the bandwidth
of the feedback loop. As a result, the standard second
order overshoot/DC ratio cannot be used to estimate
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 Application Note 76. As shown in Figure 2 a feed-
forward capacitor, C_F, may be added across feedback
resistor R1 to improve the high frequency response of
the system. Capacitor C_F provides phase lead by creating
a high frequency zero with R1.
In some applications severe transients can be caused by
switching in loads with large (>10μF) input capacitors. The
discharged input capacitors are effectively put in parallel
with C_OUT, causing a rapid drop in V_OUT. No regulator can
deliver enough current to prevent this output droop 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
specifically for this purpose and usually incorporates cur-
rent limit, short-circuit protection and soft-start functions.
MODE/SYNC Operation
The MODE/SYNC pin is a multipurpose pin allowing both
mode selection and operating frequency synchronization.
Connecting this pin to INTV_CC enables Burst Mode operation
for superior efficiency at low load currents at the expense
of slightly higher output voltage ripple. When the MODE/
SYNC pin is pulled to ground, forced continuous mode
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