LTC3633A-1_15 Datasheet, PDF (17/30 Pages) Linear Technology – Dual Channel 3A, 20V Monolithic Synchronous Step-Down Regulator

LTC3633A/LTC3633A-1
APPLICATIONS INFORMATION
for this purpose and usually incorporates current limiting,
short-circuit protection, 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 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 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 feedback 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 4. In the following discussions, V_OUT1 refers to the
LTC3633A 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 4a, 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 5a. In
V_OUT1
V_OUT1
V_OUT2
V_OUT2
TIME
3633a F04a
(4a) Coincident Tracking
TIME
3633a F04b
(4b) Ratiometric Tracking
Figure 4. Two Different Modes of Output Voltage Tracking
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