LTC3630 Datasheet, PDF (14/26 Pages) Linear Technology – High Efficiency, 65V 500mA Synchronous

LTC3630
APPLICATIONS INFORMATION
by I_RMS = I_PEAK/2. 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
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-
term reliability. Ceramic capacitors have excellent low ESR
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 input voltage ringing.
Ceramic Capacitors and Audible Noise
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_IN. At best, this ringing can couple to the output and be
mistaken as loop instability. At worst, a sudden inrush
of current through the long wires can potentially cause
a voltage spike at V_IN large enough to damage the part.
For application with inductive source impedance, such as
a long wire, an electrolytic capacitor or a ceramic capacitor
with a series resistor may be required in parallel with C_IN
to dampen the ringing of the input supply. Figure 5 shows
this circuit and the typical values required to dampen the
ringing.
L_IN
R " ^LIN
C_IN

t
$_IN
-5$
V_IN
3630 F05
$_IN
Figure 5. Series RC to Reduce V_IN Ringing
Ceramic capacitors are also piezoelectric sensitive. The
LTC3630’s burst frequency depends on the load current,
and in some applications at light load the LTC3630 can
excite the ceramic capacitor at audio frequencies, gen-
erating audible noise. If the noise is unacceptable, use
a high performance tantalum or electrolytic capacitor at
the output.
Output Voltage Programming
The LTC3630 has three fixed output voltage modes that
can be selected with the V_PRG1 and V_PRG2 pins and an
adjustable mode. The fixed output modes use an internal
feedback divider which enables higher efficiency, higher
noise immunity, and lower output voltage ripple for 5V,
3.3V and 1.8V applications. To select the fixed 5V output
voltage, connect V_PRG1 to SS and V_PRG2 to GND. For 3.3V,
connect V_PRG1 to GND and V_PRG2 to SS. For 1.8V, connect
both V_PRG1 and V_PRG2 to SS. For any of the fixed output
voltage options, directly connect the V_FB pin to V_OUT.
For the adjustable output mode (V_PRG1 = 0V, V_PRG2 = 0V),
the output voltage is set by an external resistive divider
according to the following equation:
V_OUT = 0.8V • ^{⎛⎝⎜}1+ _RR21^{⎞⎠⎟}
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