31
LTC1702
1702fa
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 LTC1702 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 I
LOAD
is low and will ride
low when I
LOAD
is high. Compared to a traditional regula-
tor, 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 LTC1702 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 LTC1702
circuits. Solder the MOSFET and the resistor(s) as close to
the output of the LTC1702 circuit as possible and set up
the signal generator to pulse at a 100Hz rate with a 5% duty
cycle. This pulses the LTC1702 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.
APPLICATIONS INFORMATION
WUU
U
Figure 18. Transient Load Generator
LTC1702
PULSE
GENERATOR
1702 F18
IRFZ44 OR
EQUIVALENT
50Ω
0V TO 10V
100Hz, 5%
DUTY CYCLE
V
OUT
R
LOAD
LOCATE CLOSE
TO THE OUTPUT
