LTC4449 datasheet(29/38 Pages) LINER

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LTC3774

3774fc

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APPLICATIONS INFORMATION

Checking Transient Response

The regulator loop response can be checked by looking at

the load current transient response. Switching regulators

take several cycles to respond to a step in DC (resistive)

load current. When a load step occurs, VOUT shifts by an

amount equal to ∆ILOAD • ESR, where ESR is the effective

series resistance of COUT. ∆ILOAD also begins to charge or

discharge COUT generating the feedback error signal that

forces the regulator to adapt to the current change and

return VOUT to its steady-state value. During this recovery

time VOUT can be monitored for excessive overshoot or

ringing, which would indicate a stability problem. The

availability of the ITH pin not only allows optimization of

control loop behavior but also provides a DC-coupled and

AC-filtered closed-loop response test point. The DC step,

rise time and settling 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.Thebandwidthcanalsobeestimatedbyexaminingthe

rise time at the pin. The ITH external components shown

in the Typical Application circuit will provide an adequate

starting point for most applications. The ITH series RC-CC

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

capacitortypeandvaluehavebeendetermined.Theoutput

capacitors need to be selected because the various types

and values determine the loop gain and phase. An output

current pulse of 20% to 80% 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. Placing

a power MOSFET directly across the output capacitor and

driving the gate with an appropriate signal generator is a

practicalwaytoproducearealisticloadstepcondition. The

initial output voltage step resulting from the step change

in output current may not be within the bandwidth of the

feedback loop, so this signal cannot be used to determine

phase margin. This is why it is better to look at the ITH

pin signal which is in the feedback loop and is the filtered

and compensated control loop response. The gain of the

loop will be increased by increasing RC and the bandwidth

of the loop will be increased by decreasing CC. If RC is

increased by the same factor that CC is decreased, the

zero frequency will be kept the same, thereby keeping the

phase shift the same in the most critical frequency range

of the feedback loop. The output voltage settling behavior

is related to the stability of the closed-loop system and

will demonstrate the actual overall supply performance.

A second, more severe transient is caused by switching

in loads with large (>1µF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in parallel

with COUT, causing a rapid drop in VOUT. No regulator can

alter its delivery of current quickly enough to prevent this

sudden step change in output voltage if the load switch

resistance is low and it is driven quickly. If the ratio of

CLOAD to COUT is greater than 1:50, the switch rise time

should be controlled so that the load rise time is limited

to approximately 25 • CLOAD. Thus a 10µF capacitor would

require a 250µs rise time, limiting the charging current

to about 200mA.

PC Board Layout Checklist

When laying out the printed circuit board, the following

checklist should be used to ensure proper operation of the

IC. These items are also illustrated graphically in the layout

diagram of Figure 14. Check the following in the PC layout:

1. The INTVCC decoupling capacitor should be placed

immediately adjacent to the IC between the INTVCC pin

and GND plane. A 1µF ceramic capacitor of the X7R or

X5R type is small enough to fit very close to the IC. An

additional 4.7µF to 10µF of ceramic, tantalum or other

very low ESR capacitance is recommended in order to

keep the internal IC supply quiet.

LTC4449 Datenblatt(PDF) 29 Page - Linear Technology

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