LTC3604 datasheet(14/24 Pages) LINER

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LTC3604

3604f

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 VOUT 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, CF, may be added

across feedback resistor R1 to improve the high frequency

response of the system. Capacitor CF provides phase lead

by creating a high frequency zero with R1.

APPLICATIONS INFORMATION

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 COUT, causing a rapid drop in VOUT. 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 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 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

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