Datasheet

LTC3411A
14
3411afc
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pin. This prevents the output from discharging to below
the regulation point when soft-starting.
Mode Selection and Frequency Synchronization
The SYNC/MODE pin is a multipurpose pin which provides
mode selection and frequency synchronization. Connect
-
ing this pin to V
IN
enables Burst Mode operation, which
provides the best low current efficiency at the cost of a
higher output voltage ripple. When this pin is connected to
ground, pulse skipping operation is selected which provides
the lowest output voltage and current ripple at the cost
of low current efficiency. Applying a voltage that is half
the value of the input voltage results in forced continuous
mode, which creates a fixed output ripple and is capable of
sinking up to 0.4A. Since the switching noise is constant
in this mode, it is also the easiest to filter out.
The LTC3411A can also be synchronized to an external
clock signal by the SYNC/MODE pin. The internal oscilla
-
tor frequency should be set to ±20% of the external clock
frequency to ensure adequate slope compensation, since
slope compensation is derived from the internal oscillator.
During synchronization, the mode is set to pulse skipping
and the top switch turn on is synchronized to the falling
edge of the external clock.
Checking Transient Response
The OPTI-LOOP
®
compensation allows the transient re-
sponse to be optimized for a wide range of loads and output
capacitors. The availability of the I
TH
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 time at this test
point truly reflects 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 bandwidth can also be
estimated by examining the rise time at the pin.
The I
TH
external components shown in the circuit on the
front page of this data sheet will provide an adequate starting
point for most applications. 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 the various types and values determine
the loop feedback factor gain and phase. An output current
pulse of 20% to 100% of full load current having a rise
time of 1µs to 10µs will produce output voltage and I
TH
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
imme-
diately shifts by an amount equal to ΔI
LOAD
• ESR, where
ESR is the effective series resistance of C
OUT
. ΔI
LOAD
also
begins to charge or discharge C
OUT
generating a feedback
error signal used by the regulator to return V
OUT
to its
applicaTions inForMaTion
PV
IN
LTC3411A
PGOOD
PGOOD
SW
SV
IN
SYNC/MODE
V
FB
I
TH
SHDN/R
T
L1
D1
OPTIONAL
V
IN
SGND PGND
R5
C
F
R
T
R
C
R1
R2
3411A F05
C
C
C
ITH
C5
V
OUT
C
IN
+
+
C6
PGND
SGND
PGND
SGND SGND SGND SGNDGND
PGND PGND
C
OUT
R6
C8
SGND
Figure 5. LTC3411A General Schematic