LTC3642 Linear Technology, LTC3642 Datasheet - Page 12

• Manufacturer Part Number: LTC3642
• Description: High Voltage 50mA Synchronous Step-Down Converter
• Manufacturer: Linear Technology

w w w . D a t a S h e e t 4 U . c o m
APPLICATIONS INFORMATION
LTC3642
This formula has a maximum at V
I
monly used for design because even signifi cant deviations
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.
The output capacitor, C
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:
where the lower limit of V
feedback comparator hysteresis.
The value of the output capacitor must be large enough
to accept the energy stored in the inductor without a large
change in output voltage. Setting this voltage step equal to
1% of the output voltage, the output capacitor must be:
Typically, a capacitor that satisfi es the ESR requirement is
adequate to fi lter the inductor ripple. To avoid overheating,
the output capacitor must also be sized to handle the ripple
current generated by the inductor. The worst-case ripple
current in the output capacitor is given by I
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
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
12
RMS
C
V
160
OUT
OUT
= I
OUT
>
<
50
Δ
/2. This simple worst-case condition is com-
V
• •
OUT
L
⎛
⎜
⎝
≤
I
V
I
PEAK
PEAK
OUT
OUT
⎞
⎟
⎠
•
2
, fi lters the inductor’s ripple
ESR
OUT
/160 is due to the 5mV
IN
= 2V
RMS
OUT
= I
, where
PEAK
/2.
electrolytic capacitors have signifi cantly higher ESR but
can be used in cost-sensitive applications provided that
consideration is given to ripple current ratings and long-
term reliability. Ceramic capacitors have excellent low ESR
characteristics but can have high voltage coeffi cient and
audible piezoelectric effects. The high quality factor (Q)
of ceramic capacitors in series with trace inductance can
also lead to signifi cant ringing.
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
the power is supplied by a wall adapter through long wires,
a load step at the output can induce ringing at the input,
V
mistaken as loop instability. At worst, a sudden inrush
of current through the long wires can potentially cause a
voltage spike at V
For applications with inductive source impedance, such
as a long wire, a series RC network may be required in
parallel with C
Figure 5 shows this circuit and the typical values required
to dampen the ringing.
Output Voltage Programming
For the adjustable version, the output voltage is set by
an external resistive divider according to the following
equation:
IN
V
. At best, this ringing can couple to the output and be
OUT
=
Figure 5. Series RC to Reduce V
0 8
.
IN
V
to dampen the ringing of the input supply.
L
•
IN
IN
⎛
⎝ ⎜
1
large enough to damage the part.
+
R =
R
R
4 • C
2
1
⎞
⎠ ⎟
IN
L
C
IN
IN
C
IN
V
IN
LTC3642
IN
Ringing
3642 F05
3642f