Linear Technology DC193 Demo Manual page 6

DEMO MANUAL DC193
NO DESIGN SWITCHER
OPERATIO
Any unshielded lead, such as a ground lead on a scope
probe, acts as an antenna for the switching noise in the
supply. Therefore, any use of ground lead will invalidate
the measurement.
Further, be extremely careful to ensure that other sources
of noise do not invalidate the measurement. Noise from
the 60Hz power line that feeds the bench supply powering
the LTC1626 demonstration board can cause errors in the
measurement. This noise (especially spikes) can propa-
gate through the supply and appear on the ground of the
demonstration unit. If this is a problem, a battery can be
used to power the unit for ripple tests.
Also, be wary of ground loops. The DC supply should float,
and the only ground should be that of the scope probe.
Never float the oscilloscope, as it may present a safety
hazard.
An alternative technique is to take a 50Ω or 75Ω piece of
coax and solder the leads directly to the output capacitor.
Keep the shield over the center conductor for as great a
distance as possible. The center conductor can pick up
stray radiation when not shielded, so minimize the length
of exposed center conductor. The other end of the coax
should have a BNC connector for attaching to the
oscilloscope.
CHECKING TRANSIENT RESPONSE
Switching regulators take several cycles to respond to a
DC (resistive) load current. When a load step occurs, V
shifts by an amount equal to ∆I
the effective series resistance of C
to charge or discharge C
to the current change and returns V
value. During this recovery time, V
for overshoot or ringing, which would indicate a stability
problem. The external components shown in Figure 1 will
prove adequate for most applications.
A second, more severe, transient is caused by switching in
loads with large (>1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C , causing a rapid drop in V
OUT
deliver enough current to prevent this problem if the load
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U
• ESR, where ESR is
LOAD
. ∆I
OUT LOAD
until the regulator loop adapts
OUT
to its steady-state
OUT
can be monitored
OUT
. No regulator can
OUT
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive so that
the load rise time is limited to approximately 25 • C
Thus, a 10µF capacitor would require a 250µs rise time
limiting the charging current to about 200mA.
COMPONENTS
Component selection can be very critical in switching
power supply applications. The LTC1626 data sheet de-
tails more specific selection criteria for most of the exter-
nal components surrounding the IC. Be sure to refer to the
data sheet if changes to this demonstration board are
anticipated.
Capacitors
In the circuit shown in Figure 1, C
high ripple-current tantalum capacitors specifically de-
signed for use in switching power supplies. ESR (equiva-
lent series resistance) is the parasitic series resistance in
the capacitor. Very often this resistance is the limiting
factor in reducing ripple at the output or input of the
supply. Standard electrolytic capacitors may cause the
feedback loop to be unstable (that is, your power supply
may become an oscillator). Also, they may cause poor
transient response or have limited operating life. Standard
capacitors generally do not have an ESR specification at
high frequencies, for example at 100kHz. So, although you
may find a capacitor that works to your satisfaction in a
prototype, the same part may not work consistently in
OUT
production .
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Standard tantalum capacitors (most notably, the low cost
ones) are not recommended for use in LTC1626 applica-
tions as they do not have the ability to take the large peak
currents that are required for the application. Tantalum
capacitors have a failure mechanism whereby they be-
come a low value resistor or short.
One alternative to tantalum and electrolytic capacitors is
organic semiconductor type capacitors (OSCONs) that are
specifically made for power supply applications. They
exhibit very low ESR and are roughly one-half the size of
an equivalent electrolytic capacitor.
.
LOAD
and C are low ESR,
IN OUT