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LTC1705

output bypass capacitor until the feedback loop in the

LTC1705 can change the inductor current to match the

new load current value. This ESR step at the output is often

the single largest budget item in the load regulation

calculation. As an example, our hypothetical 1.6V, 15A

switcher with a 0.006

Ω ESR output capacitor would

experience a 90mV step at the output with a 0A to 15A load

step—a 5.6% output change!

Usually the solution is to parallel several capacitors at the

output. For example, to keep the transient response in side

of 3.5% with the previous design, we’d need an output

ESR better than 0.004

Ω. This can be met with six 0.025Ω,

180

µF special polymer capacitors in parallel.

Inductor

The inductor in a typical LTC1705 circuit is chosen prima-

rily for value and saturation current. The inductor value

sets the ripple current, which is commonly chosen be-

tween 20% to 40% of the anticipated full load current.

Ripple current is set by:

RIPPLE

ON QB

OUT

()()

In our 1.6V, 15A example, we’d set the ripple to 20% of

15A or 3A and the inductor value would be:

with t

kHz

ON QB

OUT

RIPPLE

ON QB

=µ

=−









=µ

()

)( .

)

067

550

1 2

The inductor must not saturate at the expected peak

current. In this case, if the current limit was set to 22.5A,

the inductor should be rated to withstand 22.5A + (0.5 •

IRIPPLE) or 24A without saturating.

FEEDBACK LOOP/COMPENSATION

Feedback Loop Types

In a typical LTC1705 circuit, the feedback loop consists of

the modulator, the external inductor, the output capacitor

and the feedback amplifier with its compensation network.

All of these components affect loop behavior and must be

accounted for in the loop compensation. The modulator

consists of the internal PWM generator, the output MOSFET

drivers and the external MOSFETs themselves. From a

feedback loop point of view, it looks like a linear voltage

transfer function from COMP to SW and has a gain roughly

equal to the input voltage. It has fairly benign AC behavior

at typical loop compensation frequencies with significant

phase shift appearing at half the switching frequency.

The external inductor/output capacitor combination makes

a more significant contribution to loop behavior. These

components cause a second order LC roll off at the output,

with the attendant 180

° phase shift. This rolloff is what

filters the PWM waveform, resulting in the desired DC

output voltage, but the phase shift complicates the loop

compensation if the gain is still higher than unity at the

pole frequency. Eventually (usually well above the LC pole

frequency), the reactance of the output capacitor will

approach its ESR and the rolloff due to the capacitor will

stop, leaving 6dB/octave and 90

° of phase shift (Figure 5).

APPLICATIO S I FOR ATIO

1705 F05

PHASE

–6dB/OCT

–12dB/OCT

GAIN

FREQ

–90

–180

–270

–360

Figure 5. Transfer Function of Buck Modulator

So far, the AC response of the loop is pretty well out of the

user’s control. The modulator is a fundamental piece of the

LTC1705 design and the external L and C are usually

chosen based on the regulation and load current require-

ments without considering the AC loop response. The

feedback amplifier, on the other hand, gives us a handle

with which to adjust the AC response. The goal is to have

180

° phase shift at DC (so the loop regulates) and some-

thing less than 360

° phase shift at the point that the loop

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