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LTC1705 Datasheet(PDF) 25 Page - Linear Technology

Part No. LTC1705
Description  Dual 550kHz Synchronous Switching Regulator Controller with 5-Bit VID and 150mA LDO
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC1705 Datasheet(HTML) 25 Page - Linear Technology

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LTC1705
25
APPLICATIO S I FOR ATIO
REGULATION OVER COMPONENT TOLERANCE/
TEMPERATURE
DC Regulation Accuracy
The LTC1705 initial DC output accuracy depends mainly
on internal reference accuracy, op amp offset and internal
or external resistor accuracy. Two LTC1705 specs come
into play: VSENSEC voltage and feedback voltage line regu-
lation. The VSENSEC voltage spec is within ±1.25% for all
VID codes over the full temperature range, which encom-
passes reference accuracy, error amplifier offset and the
input resistor divider mismatch. The feedback voltage line
regulation spec adds an additional 0.1%/V term that
accounts for change in reference output with change in
input supply voltage. With a 5V supply, the errors contrib-
uted by the LTC1705 itself add up to less than 1.5% DC
error at the output.
At the I/O side, the output voltage setting resistors (R1 and
RB in Figure 3) are the other major contributor to DC error.
At a typical 1.xV output voltage, the resistors are of
roughly the same value, which tends to halve their error
terms, improving accuracy. Still, using 1% resistors for
R1 and RB will add 1% to the total output error budget.
Using 0.1% resistors in just those two positions can nearly
halve the DC output error for very little additional cost.
Load Regulation
Load regulation is affected by feedback amplifier gain and
external ground drops in the feedback path. A full-range
load step might require a 10% duty cycle change to keep
the output constant, requiring the COMP pin to move
about 100mV. With amplifier gain at 85dB, this adds up to
only a 10
µV shift at FB, negligible compared to the refer-
ence accuracy terms.
External ground drops aren’t so negligible. The LTC1705
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
Ω of
resistance in the ground lead at 15A load will cause a 15mV
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 LTC1705. A
properly laid out LTC1705 circuit should move not more
than one to two millivolts at the output from zero to full
load.
Transient Response
Transient response is the other half of the regulation
equation. The LTC1705 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 our typical 5V to 1.5V circuit, sub-
jected 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)(RESR )voltage step at
the output; with a typical 0.015
Ω output capacitor ESR,
this is a 60mV or 4% (for a 1.5V output voltage) step at the
output!
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
µs. 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
µH, the peak di/dt
will be 3.5V/0.5
µH 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
inductor 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% threshold
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
µs.


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