LTC1704EGN#TRPBF【PDF 23/28 页】IC REG DL BUCK/LINEAR 16SSOP

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LTC1704/LTC1704B

1704bfa

APPLICATIO S I FOR ATIO

External ground drops arent so negligible. The LTC1704

can sense the positive end of the output voltage by

attaching the feedback resistor directly at the load, but it

cannot do the same with the ground lead. Just 0.001& of

resistance in the ground lead at 10A load will cause a 10mV

error in the output voltageas much as all the other DC

errors put together. Proper layout becomes essential to

achieving optimum load regulation from the LTC1704. A

properly laid out LTC1704 circuit should move less than a

millivolt at the output from zero to full load.

Transient Response

Transient response is the other half of the regulation

equation. The LTC1704 can keep the DC output voltage

constant to within 1% when averaged over hundreds of

cycles. Over just a few cycles, however, the external

components conspire to limit the speed that the output

can move. Consider a typical 5V to 1.5V circuit, subjected

to a 1A to 5A load transient. Initially, the loop is in

regulation and the DC current in the output capacitor is

zero. Suddenly, an extra 4A start flowing out of the output

capacitor while the inductor is still supplying only 1A. This

sudden change will generate a (4A)(R

ESR

) voltage step at

the output; with a typical 0.015& output capacitor ESR,

this is a 60mV step at the output, or 4% (for a 1.5V output

voltage.)

Very quickly, the feedback loop will realize that something

has changed and will move at the bandwidth allowed by

the external compensation network towards a new duty

cycle. If the bandwidth is set to 50kHz, the COMP pin will

get to 60% of the way to 90% duty cycle in 3祍. Now the

inductor is seeing 3.5V across itself for a large portion of

the cycle, and its current will increase from 1A at a rate set

by di/dt = V/L. If the inductor value is 0.5礖, the di/dt will

be 3.5V/0.5礖 or 7A/祍. Sometime in the next few micro-

seconds after the switch cycle begins, the inductor current

will have risen to the 5A level of the load current and the

output voltage will stop dropping. At this point, the induc-

tor current will rise somewhat above the level of the output

current to replenish the charge lost from the output

capacitor during the load transient. During the next couple

of cycles, the MIN comparator may trip on and off,

preventing the output from falling below its 5% thresh-

old until the time constant of the compensation loop runs

out and the main feedback amplifier regains control. With

a properly compensated loop, the entire recovery time will

be inside of 10祍.

Most loads care only about the maximum deviation from

ideal, which occurs somewhere in the first two cycles after

the load step hits. During this time, the output capacitor

does all the work until the inductor and control loop regain

control. The initial drop (or rise if the load steps down) is

entirely controlled by the ESR of the capacitor and amounts

to most of the total voltage drop. To minimize this drop,

reduce the ESR as much as possible by choosing low ESR

capacitors and/or paralleling multiple capacitors at the

output. The capacitance value accounts for the rest of the

voltage drop until the inductor current rises. With most

output capacitors, several devices paralleled to get the

ESR down will have so much capacitance that this drop

term is negligible. Ceramic capacitors are an exception; a

small ceramic capacitor can have suitably low ESR with

relatively small values of capacitance, making this second

drop term significant.

Optimizing Loop Compensation

Loop compensation has a fundamental impact on tran-

sient recovery time, the time it takes the LTC1704 to

recover after the output voltage has dropped due to output

capacitor ESR. Optimizing loop compensation entails

maintaining the highest possible loop bandwidth while

ensuring loop stability. The Feedback Component Selec-

tion section describes in detail the techniques used to

design an optimized Type 3 feedback loop, appropriate for

most LTC1704 systems.

Measurement Techniques

Measuring transient response presents a challenge in two

respects: obtaining an accurate measurement and gener-

ating a suitable transient to use to test the circuit. Output

measurements should be taken with a scope probe di-

rectly across the output capacitor. Proper high frequency

probing techniques should be used. In particular, dont

use the 6" ground lead that comes with the probe! Use an

adapter that fits on the tip of the probe and has a short

ground clip to ensure that inductance in the ground path

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