LTC3633A-3_15 Datasheet, PDF (17/28 Pages) Linear Technology – Dual Channel 3A, 20V Monolithic Synchronous Step-Down Regulator

LTC3633A-2/LTC3633A-3
APPLICATIONS INFORMATION
steady-state value. During this recovery time, V_OUT can
be monitored for overshoot or ringing that would indicate
a stability problem.
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, 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 effec-
tively put in parallel with C_OUT, causing a rapid drop in
V_OUT. No regulator 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 specifically for this purpose and
usually incorporates current limiting, short-circuit protec-
tion, and soft starting.
MODE/SYNC Operation
The MODE/SYNC pin is a multipurpose pin allowing both
mode selection and operating frequency synchronization.
Floating this pin or connecting it 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 tied to ground, forced continuous
mode operation is selected, creating the lowest fixed output
ripple at the expense of light load efficiency.
The LTC3633A-2 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 in-
coming clock. The presence of an external clock will place
both regulators into forced continuous mode operation.
Output Voltage Tracking and Soft-Start
The LTC3633A-2 allows the user to control the output
voltage ramp rate by means of the TRACKSS pin. From
0 to 0.6V, the TRACKSS voltage will override the internal
0.6V reference input to the error amplifier, thus regulating
the feedback voltage to that of the TRACKSS pin. When
TRACKSS is above 0.6V, tracking is disabled and the feed-
back voltage will regulate to the internal reference voltage.
The voltage at the TRACKSS pin may be driven from an
external source, or alternatively, the user may leverage
the internal 1.4µA pull-up current source to implement
a soft-start function by connecting an external capacitor
(C_SS) from the TRACKSS pin to ground. The relationship
between output rise time and TRACKSS capacitance is
given by:
t_SS = 430000Ω • C_SS
A default internal soft-start ramp forces a minimum soft-
start time of 400µs by overriding the TRACKSS pin input
during this time period. Hence, capacitance values less
than approximately 1000pF will not significantly affect
soft-start behavior.
When driving the TRACKSS pin from another source, each
channel’s output can be set up to either coincidentally or
ratiometrically track another supply’s output, as shown
in Figure 5. In the following discussions, V_OUT1 refers to
the LTC3633A-2 output 1 as a master channel and V_OUT2
refers to output 2 as a slave channel. In practice, either
channel can be used as the master.
To implement the coincident tracking in Figure 5a, con-
nect an additional resistive divider to V_OUT1 and connect
its midpoint to the TRACKSS pin of the slave channel.
The ratio of this divider should be the same as that of the
slave channel’s feedback divider shown in Figure 6a. In
this tracking mode, V_OUT1 must be set higher than V_OUT2.
To implement the ratiometric tracking, the feedback pin of
the master channel should connect to the TRACKSS pin of
the slave channel (as in Figure 6b). By selecting different
resistors, the LTC3633A-2 can achieve different modes of
tracking including the two in Figure 5.
For more information www.linear.com/LTC3633A-2
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