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LTC1703

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 gener-

ating a suitable transient to use to test the circuit. Output

measurements should be taken with a scope probe

directly across the output capacitor. Proper high fre-

quency probing 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. Make sure the bandwidth limit on the scope is

turned off, since a significant portion of the transient

energy occurs above the 20MHz cutoff.

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 LTC1703 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 LTC1703

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

the output of the LTC1703 circuit as possible and set up

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

cycle. This pulses the LTC1703 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.

Changing the Output Voltage on the Fly

The voltage at side 1 of the LTC1703 can be changed on the

fly by changing the VID code while the output is enabled,

but care must be taken to avoid tripping the overvoltage

fault circuit. Stepping the voltage upwards abruptly is safe,

but stepping down quickly by more than 15% can leave the

system in a state where the output voltage is still at the old

higher level, but the feedback node is set to expect a new,

substantially lower voltage. If this condition persists for

more than 10

µs, the overvoltage fault circuitry will fire and

latch off the LTC1703.

The simplest solution is to disable the fault circuit by

grounding the FAULT pin. Systems that must keep the

fault circuit active should ensure that the output voltage is

never programmed to step down by more than 15% in any

single step. The safest strategy is to step the output down

by 10% or less at a time and wait for the output to settle

to the new value before taking subsequent steps.

LTC1703

PULSE

GENERATOR

1703 F18

IRFZ44 OR

EQUIVALENT

Ω

0V TO 10V

100Hz, 5%

DUTY CYCLE

VOUT

RLOAD

LOCATE CLOSE

TO THE OUTPUT

Figure 18. Transient Load Generator

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