LTC1702AIGN datasheet(31/36 Pages) LINER

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LTC1702A

1702afa

it goes out of spec. Similarly, at full load, the output current

can only decrease, causing a positive shift in the output

voltage; the initial low value allows it to rise further before

the spec is exceeded. The primary benefit of voltage

positioning is it increases the allowable ESR of the output

capacitors, saving cost. An additional bonus is that at

maximum load, the output voltage is near the minimum

allowable, decreasing the power dissipated in the load.

Implementing voltage positioning is as simple as creating

an intentional resistance in the output path to generate the

required voltage drop. This resistance can be a low value

resistor, a length of PCB trace, or even the parasitic

resistance of the inductor if an appropriate filter is used. If

the LTC1702A senses the output voltage upstream from

the resistance (Figure 17), the output voltage will move

with load as I • R, where I is the load current and R is the

value of the resistance. If the feedback network is then

reset to regulate near the upper edge of the specified

tolerance, the output voltage will ride high when ILOAD is

low and will ride low when ILOAD is high. Compared to a

traditional regulator, a voltage positioning regulator can

theoretically stand as much as twice the ESR drop across

the output capacitor while maintaining output voltage

regulation. This means smaller, cheaper output capacitors

can be used while keeping the output voltage within

acceptable limits.

Measurement Techniques

Measuring transient response presents a challenge in

two respects: obtaining an accurate measurement and

generating a suitable transient to use to test the circuit.

Output measurements should be taken with a scope

probedirectly across the output capacitor. Proper high

frequency probing techniques should be used. In particu-

lar, 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 tran-

sient signal being measured. Conveniently, the typical

probe tip ground clip is spaced just right to span the leads

of a typical output capacitor. In general, it is best to take

this measurement with the 20MHz bandwidth limit on the

oscilloscope turned on to limit high frequency noise. Note

that microprocessor manufacturers typically specify ripple

≤20MHz, as energy above 20MHz is generally radiated

and not conducted and will not affect the load even if it

appears at the 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

watching the output. If this isn’t convenient, a current step

generator is needed. This generator needs to be able to

turn on and off in nanoseconds to simulate a typical

switching logic load, so stray inductance and long clip

leads between the LTC1702A and the transient generator

must be minimized.

Figure 18 shows an example of a simple transient genera-

tor. Be sure to use a noninductive resistor as the load

element—many power resistors use an inductive spiral

pattern and are not suitable for use here. A simple solution

is to take ten 1/4W film resistors and wire them in parallel

to get the desired value. This gives a noninductive resistive

load which can dissipate 2.5W continuously or 50W if

pulsed with a 5% duty cycle, enough for most LTC1702A

circuits. Solder the MOSFET and the resistor(s) as close to

the output of the LTC1702A circuit as possible and set up

the signal generator to pulse at a 100Hz rate with a 5% duty

cycle. This pulses the LTC1702A with 500

µs transients

10ms apart, adequate for viewing the entire transient

recovery time for both positive and negative transitions

while keeping the load resistor cool.

APPLICATIONS INFORMATION

Figure 18. Transient Load Generator

LTC1702A

PULSE

GENERATOR

1702A F18

IRFZ44 OR

EQUIVALENT

Ω

0V TO 10V

100Hz, 5%

DUTY CYCLE

VOUT

RLOAD

LOCATE CLOSE

TO THE OUTPUT

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