Datasheet

LTC3775
25
3775fa
ripple, and this could interfere with the operation of the
LTC3775. A few inches of PC trace or wire (L ≅ 100nH)
between C
IN
of the LTC3775 and the actual source V
IN
should be suffi cient to prevent input noise interference
problems.
8. The top current limit programming resistor, R
ILIMT
,
should be placed close to the LTC3775 and the other
end of R
ILIMT
should run parallel to the SENSE trace
to the Kelvin sense connection underneath the sense
resistor.
9. The bottom current limit programming resistor, R
ILIMB
,
should be placed close to the LTC3775 and the other
end of R
ILIMB
should connect to SGND.
10. The SW pin should be connected to the drain of the
bottom MOSFET.
Checking Transient Response
For all new LTC3775 PCB circuits, transient tests need to
be performed to verify the proper feedback loop operation.
The regulator loop response can be checked by looking at
the load current transient response. Switching regulators
take several cycles to respond to a step in DC (resistive)
load current. When a load step occurs, V
OUT
shifts by an
amount equal to ΔI
LOAD
• (ESR), where ESR is the effective
series resistance of C
OUT
. ΔI
LOAD
also begins to charge or
discharge C
OUT
generating the feedback error signal that
forces the regulator to adapt to the current change and
return V
OUT
to its steady-state value. During this recovery
time, V
OUT
can be monitored for excessive overshoot or
ringing which would indicate a stability problem.
Measuring transient response presents a challenge in two
respects: obtaining an accurate measurement and gen-
erating a suitable transient for testing the circuit. Output
measurements should be taken with a scope probe directly
across the output capacitor. Proper high frequency prob-
ing techniques should be used. Do not use the 6" ground
lead that comes with the probe! Use an adapter that fi ts
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.
The typical probe tip ground shield is spaced just right to
APPLICATIONS INFORMATION
PULSE
GENERATOR
0V TO 10V
100Hz, 1%
DUTY CYCLE
LTC3775
LOCATE CLOSE TO THE OUTPUT
V
OUT
10k
507
IRFZ44 OR
EQUIVALENT
R
LOAD
3775 F18
Figure 18. Transient Load Generator
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 gener-
ally radiated (and not conducted) and does 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, switching 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 LTC3775 and the transient generator
must be minimized.
Figure 18 shows an example of a simple transient generator.
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 fi lm resistors and wire them in parallel to get
the desired value. This gives a noninductive resistive load
which can dissipate 2.5W continuously or 250W if pulsed
with a 1% duty cycle, enough for most LTC3775 circuits.
Solder the MOSFET and the resistor(s) as close to the
output of the LTC3775 circuit as possible and set up the
signal generator to pulse at a 100Hz rate with a 1% duty
cycle. This pulses the LTC3775 with 100µs transients
10ms apart, adequate for viewing the entire transient
recovery time for both positive and negative transitions
while keeping the load resistor cool.