LTC3604 Datasheet, PDF (14/24 Pages) Linear Technology – 2.5A, 15V Monolithic Synchronous Step-Down Regulator

LTC3604
APPLICATIONS INFORMATION
Checking Transient Response
The regulator loop response can be checked by observing
the response of the system to a load step. When config-
ured 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. 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 slightly, from
approximately 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 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 Linear Technology Application Note 76. As shown
in Figure 2 a feedforward 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 specifi-
cally for this purpose and usually incorporates current 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 synchroniza-
tion. 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 operation is selected creating the lowest fixed output
ripple at the expense of light load efficiency.
The LTC3604 will detect the presence of the external clock
signal on the MODE/SYNC pin and synchronize the internal
oscillator to the phase and frequency of the incoming clock.
The presence of an external clock will place the LTC3604
into forced continuous mode operation.
Output Voltage Tracking and Soft-Start
The LTC3604 allows the user to control the output voltage
ramp rate by means of the TRACK/SS pin. From 0V to 0.6V
the TRACK/SS pin will override the internal reference input
to the error amplifier forcing regulation of the feedback
voltage to that seen at the TRACK/SS pin. When the voltage
at the TRACK/SS pin rises above 0.6V, tracking is disabled
and the feedback voltage will be regulated to the internal
reference voltage.
The voltage at the TRACK/SS pin may be driven from an
external source, or alternatively, the user may leverage the
internal 1.4μA pull-up current on TRACK/SS to implement
3604f
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