ltc1702a Linear Technology Corporation, ltc1702a Datasheet - Page 28

• Manufacturer Part Number: ltc1702a
• Description: Dual 550khz Synchronous 2-phase Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS
LTC1702A
Maximizing Low Load Current Efficiency
Low load current efficiency depends strongly on proper
operation in discontinuous and Burst Mode operations. In
an ideally optimized system, discontinuous mode reduces
conduction losses but not switching losses, since each
power MOSFET still switches on and off once per cycle. In
a typical system, there is additional loss in discontinuous
mode due to a small amount of residual current left in the
inductor when QB turns off. This current gets dissipated
across the body diode of either QT or QB. Some LTC1702A
systems lose as much to body diode conduction as they
save in MOSFET conduction. The real efficiency benefit of
discontinuous mode happens when Burst Mode operation
is invoked. At typical power levels, when Burst Mode
operation is activated, gate drive is the dominant loss
term. Burst Mode operation turns off all output switching
for several clock cycles in a row, significantly cutting gate
drive losses. As the load current in Burst Mode operation
falls toward zero, the current drawn by the circuit falls to
the LTC1702A’s background quiescent level—about 3mA
per channel.
To maximize low load efficiency, make sure the LTC1702A
is allowed to enter discontinuous and Burst Mode opera-
tion as cleanly as possible. FCB must be above its 0.8V
threshold. Minimize ringing at the SW node so that the
discontinuous comparator leaves as little residual current
in the inductor as possible when QB turns off. It helps to
connect the SW pin of the LTC1702A as close to the drain
of QB as possible. An RC snubber network can also be
added from SW to PGND.
REGULATION OVER COMPONENT TOLERANCE/
TEMPERATURE
DC Regulation Accuracy
The LTC1702A initial DC output accuracy depends mainly
on internal reference accuracy, op amp offset and external
resistor accuracy. Two LTC1702A specs come into play:
feedback voltage and feedback voltage line regulation. The
feedback voltage spec is 800mV ± 8mV over the full
temperature range, and is specified at the FB pin, which
encompasses both reference accuracy and any op amp
28
U
INFORMATION
U
W
U
offset. This accounts for 1% error at the output with a 5V
input supply. The feedback voltage line regulation spec
adds an additional 0.05%/V term that accounts for change
in reference output with change in input supply voltage.
With a 5V supply, the errors contributed by the LTC1702A
itself add up to no more than 1% DC error at the output.
The output voltage setting resistors (R1 and R
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, im-
proving accuracy. Still, using 1% resistors for R1 and R
will add 1% to the total output error budget, equal to that
of all errors due to the LTC1702A combined. 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 voltage, feedback
amplifier gain and external ground drops in the feedback
path. Feedback voltage is covered above and is within 1%
over temperature. 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 10µV shift at
FB, negligible compared to the reference accuracy terms.
External ground drops aren’t so negligible. The LTC1702A
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 LTC1702A.
See the Layout/Troubleshooting section for more infor-
mation. A properly laid out LTC1702A 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 LTC1702A can keep the DC output voltage
constant to within 1% when averaged over hundreds of
1702afa
B
in
B