LTC3355
14
3355fb
For more information www.linear.com/LTC3355
APPLICATIONS INFORMATION
Buck Input Capacitor
Bypass V
IN
and V
INS
with a ceramic capacitor of X7R or
X5R type. A 10µF to 22µF ceramic capacitor is adequate for
bypassing. Note that a larger V
INS
bypass capacitor may
be required if the input power supply source impedance
is high or there is significant inductance due to long wires
or cables. This can be provided with a lower
performance
electrolytic capacitor in parallel with the ceramic capacitor.
Buck regulators draw current from the input supply in
pulses with very fast rise and fall times. The input capacitors
are required to reduce the resulting voltage ripple at V
INS
and V
IN
and to force this very high frequency switching
into a tight local loop, minimizing EMI. The capacitors
must be placed close to the LTC3355
pins.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by
the buck regulator to produce the DC output. In this role
it determines the output ripple, and low impedance at the
switching frequency is important. The second function is to
store energy in order to satisfy transient loads and stabilize
the buck regulator control
loop. Ceramic capacitors have
very low equivalent series resistance (ESR) and provide
the best ripple performance. A good starting value is:
C
OUT
= f
SW
100
V
OUT
⎛
⎝
⎜
⎞
⎠
⎟
where f
SW
is in MHz and C
OUT
is the recommended output
capacitance in µF. Use X5R or X7R types. This choice will
provide low output ripple and good transient response.
When choosing a capacitor look carefully through the
data sheet to find out what the actual capacitance is under
operating conditions (applied voltage and temperature).
A physically larger capacitor, or one with a higher
voltage
rating, may be required. High performance tantalum or
electrolytic capacitors can be used for the output capacitor.
Low ESR is important, so choose one that is intended for
use in switching regulators. The ESR should be specified
by the supplier, and should be 0.05Ω or less. Table 2 lists
several capacitor vendors.
Table 2. Capacitor Vendors
VENDOR URL PART SERIES COMMANDS
Panasonic www.panasonic.com Ceramic, Polymer,
Tantalum
EEF Series,
POSCAP
Kemet www.kemet.com Ceramic, Tantalum T494, T495
Murata www.murata.com Ceramic
AVX www.avxcorp.com Ceramic, Tantalum TPS Series
Taiyo Yuden www.taiyo-yuden.com Ceramic
Buck Catch Diode Selection
The catch diode (D1 in the Block Diagram) conducts cur-
rent only during the switch-off time. The average forward
current in normal operation can be calculated from:
I
D(AVG)
= I
OUT
(1 – DC)
where DC is the duty cycle. The only reason to consider
a diode with a larger current rating than necessary for
nominal operation is for the case of shorted or overloaded
output conditions. For the worst case of shorted output
the diode average current will then increase to a value that
depends on the switch current limit.
If operating at high temperatures
select a Schottky diode
with low reverse leakage current.
