LTC1705 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1705
Linear Technology
'LTC1705' PDF : 28 Pages View PDF
LTC1705
TYPICAL APPLICATIO S
Most loads care only about the maximum deviation from
ideal, which occurs somewhere in the first two cycles after
the load step hits. During this time, the output capacitor
does all the work until the inductor and control loop regain
control. The initial drop (or rise if the load steps down) is
entirely controlled by the ESR of the capacitor and amounts
to most of the total voltage drop. To minimize this drop,
reduce the ESR as much as possible by choosing low ESR
capacitors and/or paralleling multiple capacitors at the
output. The capacitance value accounts for the rest of the
voltage drop until the inductor current rises. With most
output capacitors, several devices paralleled to get the
ESR down will have so much capacitance that this drop
term is negligible. Ceramic capacitors are an exception; a
small ceramic capacitor can have suitably low ESR with
relatively small values of capacitance, making this second
drop term significant.
Optimizing Loop Compensation
Loop compensation has a fundamental impact on tran-
sient recovery time, the time it takes the LTC1705 to
recover after the output voltage has dropped due to output
capacitor ESR. Optimizing loop compensation entails
maintaining the highest possible loop bandwidth while
ensuring loop stability. The feedback component selection
section describes in detail the techniques used to design
an optimized Type 3 feedback loop, appropriate for most
LTC1705 systems.
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 di-
rectly across the output capacitor. Proper high frequency
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.
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 LTC1705 and the transient generator
must be minimized.
Figure 11 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 LTC1705
circuits. Solder the MOSFET and the resistor(s) as close to
the output of the LTC1705 circuit as possible and set up
the signal generator to pulse at a 100Hz rate with a 5% duty
cycle. This pulses the LTC1705 with 500µs transients10ms
apart, adequate for viewing the entire transient recovery
time for both positive and negative transitions while keep-
ing the load resistor cool.
LTC1705
VOUT
RLOAD
PULSE
GENERATOR
IRFZ44 OR
EQUIVALENT
50Ω
0V TO 10V
100Hz, 5%
DUTY CYCLE
LOCATE CLOSE TO THE OUTPUT
1705 F11
Figure 11. Transient Load Generator
26
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