ltc1702a Linear Technology Corporation, ltc1702a Datasheet - Page 31

• Manufacturer Part Number: ltc1702a
• Description: Dual 550khz Synchronous 2-phase Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS
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 I
low and will ride low when I
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
U
INFORMATION
U
LOAD
is high. Compared to a
W
U
LOAD
is
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 500µs transients
10ms apart, adequate for viewing the entire transient
recovery time for both positive and negative transitions
while keeping the load resistor cool.
GENERATOR
LTC1702A
PULSE
Figure 18. Transient Load Generator
DUTY CYCLE
0V TO 10V
100Hz, 5%
50Ω
EQUIVALENT
IRFZ44 OR
R
LOAD
LOCATE CLOSE
TO THE OUTPUT
LTC1702A
V
OUT
1702A F18
31
1702afa