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LTC1702A

1702afa

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

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 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, im-

proving accuracy. Still, using 1% resistors for R1 and RB

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 10

µV 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 0.001

Ω 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

APPLICATIONS INFORMATION

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