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LTC3633A-2/LTC3633A-3

3633a23f

steady-state value. During this recovery time, VOUT can

be monitored for overshoot or ringing that would indicate

a stability problem.

When observing the response of VOUT 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 COUT, causing a rapid drop in

VOUT. 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 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 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.

APPLICATIONS INFORMATION

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

(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 5. In the following discussions, VOUT1 refers to

the LTC3633A-2 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 5a, con-

nect an additional resistive divider to VOUT1 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, VOUT1 must be set higher than VOUT2.

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.

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