31
LTC1703
1703fa
specified tolerance, the output voltage will ride high when
I
LOAD
is low and will ride low when I
LOAD
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 500µs 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 10µs, 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
50Ω
0V TO 10V
100Hz, 5%
DUTY CYCLE
V
OUT
R
LOAD
LOCATE CLOSE
TO THE OUTPUT
Figure 18. Transient Load Generator
APPLICATIO S I FOR ATIO
WUUU
