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LTC3633

3633f

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 INTVCC 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 LTC3633 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 both

regulators into forced continuous mode operation.

Output Voltage Tracking and Soft-Start

The LTC3633 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.

APPLICATIONS INFORMATION

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

(CSS) from the TRACKSS pin to ground. The relationship

between output rise time and TRACKSS capacitance is

given by:

tSS = 430000Ω • CSS

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, VOUT1 refers to the

LTC3633 output 1 as a master channel and VOUT2 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 VOUT1 and connect

its midpoint to the TRACKSS pin of the slave channel.

TIME

(4a) Coincident Tracking

VOUT1

VOUT2

3633 F04a

VOUT1

VOUT2

TIME

3633 F04b

(4b) Ratiometric Tracking

Figure 4. Two Different Modes of Output Voltage Tracking

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