LTC3642
APPLICATIONS INFORMATION
electrolytic capacitors have significantly higher ESR but
can be used in cost-sensitive applications provided that
consideration is given to ripple current ratings and long-
termreliability.CeramiccapacitorshaveexcellentlowESR
characteristics but can have high voltage coefficient and
audible piezoelectric effects. The high quality factor (Q)
of ceramic capacitors in series with trace inductance can
also lead to significant ringing.
This formula has a maximum at V = 2V , where
IN
OUT
I
= I /2. This simple worst-case condition is com-
RMS
OUT
monlyusedfordesignbecauseevensignificantdeviations
do not offer much relief. Note that ripple current ratings
from capacitor manufacturers are often based only on
2000 hours of life which makes it advisable to further
derate the capacitor, or choose a capacitor rated at a
higher temperature than required. Several capacitors may
also be paralleled to meet size or height requirements in
the design.
Using Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now be-
coming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
output. When a ceramic capacitor is used at the input and
thepowerissuppliedbyawalladapterthroughlongwires,
a load step at the output can induce ringing at the input,
The output capacitor, C , filters the inductor’s ripple
OUT
current and stores energy to satisfy the load current
when the LTC3642 is in sleep. The output voltage ripple
during a burst cycle is dominated by the output capacitor
equivalent series resistance (ESR) and can be estimated
by the following equation:
VOUT
160
< ΔVOUT ≤IPEAK •ESR
V . At best, this ringing can couple to the output and be
IN
mistaken as loop instability. At worst, a sudden inrush
where the lower limit of V /160 is due to the 5mV
OUT
of current through the long wires can potentially cause a
feedback comparator hysteresis.
voltage spike at V large enough to damage the part.
IN
The value of the output capacitor must be large enough
to accept the energy stored in the inductor without a large
changeinoutputvoltage. Settingthisvoltagestepequalto
1% of the output voltage, the output capacitor must be:
For applications with inductive source impedance, such
as a long wire, a series RC network may be required in
parallel with C to dampen the ringing of the input supply.
IN
Figure 5 shows this circuit and the typical values required
to dampen the ringing.
2
⎛
⎞
IPEAK
COUT > 50 •L •
⎜
⎟
V
⎝
⎠
LTC3642
OUT
L
IN
V
IN
Typically, a capacitor that satisfies the ESR requirement is
adequatetofiltertheinductorripple. Toavoidoverheating,
theoutputcapacitormustalsobesizedtohandletheripple
current generated by the inductor. The worst-case ripple
L
C
IN
R =
4 • C
IN
3642 F05
C
IN
IN
current in the output capacitor is given by I
= I
/2.
RMS PEAK
Figure 5. Series RC to Reduce VIN Ringing
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Output Voltage Programming
Dry tantalum, special polymer, aluminum electrolytic,
and ceramic capacitors are all available in surface mount
packages. Special polymer capacitors offer very low ESR
but have lower capacitance density than other types.
Tantalum capacitors have the highest capacitance density
but it is important only to use types that have been surge
tested for use in switching power supplies. Aluminum
For the adjustable version, the output voltage is set by
an external resistive divider according to the following
equation:
⎛
⎞
R1
R2
VOUT = 0.8V • 1+
⎜
⎝
⎟
⎠
3642f
12
Linear [ Linear ]
