LTC3565 datasheet(14/22 Pages) LINER

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LTC3565

3565fb

Checking Transient Response

The OPTI-LOOP

® compensation allows the transient re-

sponse to be optimized for a wide range of loads and output

capacitors. 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 time at this test

point truly reﬂects the closed loop response. Assuming a

predominantly second order system, phase margin and/or

damping factor can be estimated using the percentage of

overshoot seen at this pin. The bandwidth can also be

estimated by examining the rise time at the pin.

The ITH external components shown in the circuit on page 1

of this data sheet will provide an adequate starting point for

most applications. The series R-C ﬁlter sets the dominant

pole-zero loop compensation. The values can be modiﬁed

slightly (from 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 the various types and values determine

the loop feedback factor gain and phase. An output current

pulse of 20% to 100% of full load current having 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.

Switching regulators take several cycles to respond to

a step in load current. When a load step occurs, VOUT

immediately shifts by an amount equal to ΔILOAD • ESR,

where ESR is the effective series resistance of COUT.

ΔILOAD also begins to charge or discharge COUT generat-

ing a feedback error signal used by the regulator to return

VOUT to its steady-state value. During this recovery time,

VOUT can be monitored for overshoot or ringing that would

indicate a stability problem.

The initial output voltage step may not be within the

bandwidth of the feedback loop, so the standard second

order overshoot/DC ratio cannot be used to determine

phase margin. The gain of the loop increases with R and

the bandwidth of the loop increases with decreasing C.

If R is increased by the same factor that C is decreased,

the zero frequency will be kept the same, thereby keeping

the phase the same in the most critical frequency range

of the feedback loop. In addition, a feedforward capacitor

CF can be added to improve the high frequency response,

as shown in Figure 2. Capacitor CF provides phase lead by

creating a high frequency zero with R2 which improves

the 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.

APPLICATIONS INFORMATION

PVIN

LTC3565

PGOOD

SVIN

SYNC/MODE

RUN

VFB

ITH

OPTIONAL

VIN

GND

3565 F04

CITH

VOUT

CIN

COUT

Figure 2. LTC3565 General Schematic

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