LTC1703 datasheet(28/36 Pages) LINER

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LTC1703

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 LTC1703’s background quiescent level—about 3mA

per channel.

To maximize low load efficiency, make sure the LTC1703

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 LTC1703 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 LTC1703 initial DC output accuracy depends mainly

on internal reference accuracy, op amp offset and external

resistor accuracy (side 2 only). Two LTC1703 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 regula-

tion 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

LTC1703 itself add up to no more than 1.5% DC error at the

output.

At side 2, 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, equal to

that of all errors due to the LTC1703 combined. Using 0.1%

resistors in just those two positions can nearly halve the DC

output error for very little additional cost. Side 1 uses the

internal VID network to set the output voltage, and is

specified to be within

±1.5% of the values shown in

Table 1.

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 LTC1703

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 LTC1703. A

properly laid out LTC1703 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 LTC1703 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, 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)(CESR)voltage step at

the output; with a typical 0.015

Ω output capacitor ESR,

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

voltage).

APPLICATIO S I FOR ATIO

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