LTC1704/LTC1704B
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APPLICATIO S I FOR ATIO
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External ground drops aren’t 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 0.001Ω of
resistance in the ground lead at 10A load will cause a 10mV
error in the output voltage—as 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 0.015Ω 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 3µs. 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 0.5µH, the di/dt will
be 3.5V/0.5µH or 7A/µs. 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 10µs.
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, don’t
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
