LTC3703 datasheet(26/32 Pages) LINER

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LTC3703

3703f

higher input voltages (typically 20V or greater). Transition

losses can be estimated from the second term of the PMAIN

equation found in the Power MOSFET Selection section.

The transition losses can become very significant at the

high end of the LTC3703 operating voltage range. To

improve efficiency, one may consider lowering the fre-

quency and/or using MOSFETs with lower CRSS at the

expense of higher RDS(ON).

Other losses including CIN and COUT ESR dissipative

losses, Schottky conduction losses during dead-time, and

inductor core losses generally account for less than 2%

total additional loss.

Transient Response

Due to the high gain error amplifier and line feedforward

compensation of the LTC3703, the output accuracy due to

DC variations in input voltage and output load current will

be almost negligible. For the few cycles following a load

transient, however, the output deviation may be larger

while the feedback loop is responding. Consider a typical

48V input to 5V output application 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. Sud-

denly, an extra 4A (= 5A-1A) flows 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 step at the output.

The feedback loop will respond and will move at the band-

width allowed by the external compensation network

towards a new duty cycle. If the unity gain crossover fre-

quency 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

43V 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 10

µH, the peak di/dt will be 43V/10µH

or 4.3A/

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 cur-

rent will rise somewhat above the level of the output cur-

rent to replenish the charge lost from the output capacitor

during the load transient. With a properly compensated

loop, the entire recovery time will be inside of 10

µs.

Most loads care only about the maximum deviation from

ideal, which occurs somewhere in the first two cycles after

the load step hits. During this time, the output capacitor

does all the work until the inductor and control loop regain

control. The initial drop (or rise if the load steps down) is

entirely controlled by the ESR of the capacitor and amounts

to most of the total voltage drop. To minimize this drop,

choose a low ESR capacitor and/or parallel multiple ca-

pacitors at the output. The capacitance value accounts for

the rest of the voltage drop until the inductor current rises.

With most output capacitors, several devices paralleled to

get the ESR down will have so much capacitance that this

drop term is negligible. Ceramic capacitors are an excep-

tion; a small ceramic capacitor can have suitably low ESR

with relatively small values of capacitance, making this

second drop term more significant.

Optimizing Loop Compensation

Loop compensation has a fundamental impact on tran-

sient recovery time, the time it takes the LTC3703 to

recover after the output voltage has dropped due to a load

step. Optimizing loop compensation entails maintaining

the highest possible loop bandwidth while ensuring loop

stability. The feedback component selection section de-

scribes in detail the techniques used to design an opti-

mized Type 3 feedback loop, appropriate for most LTC3703

systems.

Measurement Techniques

Measuring transient response presents a challenge in two

respects: obtaining an accurate measurement and gener-

ating a suitable transient to test the circuit. Output mea-

surements should be taken with a scope probe directly

across the output capacitor. Proper high frequency prob-

ing techniques should be used. In particular, don’t use the

6" ground lead that comes with the probe! Use an adapter

that fits on the tip of the probe and has a short ground clip

to ensure that inductance in the ground path doesn’t cause

a bigger spike than the transient signal being measured.

Conveniently, the typical probe tip ground clip is spaced

just right to span the leads of a typical output capacitor.

Now that we know how to measure the signal, we need to

have something to measure. The ideal situation is to use

the actual load for the test and switch it on and off while

APPLICATIO S I FOR ATIO

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