LTC3630A Datasheet, PDF (14/24 Pages) Linear Technology – High Efficiency, 76V 500mA Synchronous Step-Down Converter

LTC3630A
Applications Information
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.
Ceramic capacitors are also piezoelectric sensitive. The
LTC3630A’s burst frequency depends on the load current,
and in some applications at light load the LTC3630A 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 LTC3630A 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₊
R1^
R2 
The resistive divider allows the V_FB pin to sense a fraction
of the output voltage as shown in Figure 6. The output
voltage can range from 0.8V to V_IN. Be careful to keep the
divider resistors very close to the V_FB pin to minimize the
trace length and noise pick-up on the sensitive V_FB signal.
To minimize the no-load supply current, resistor values in
the megohm range may be used; however, large resistor
values should be used with caution. The feedback divider
is the only load current when in shutdown. If PCB leakage
current to the output node or switch node exceeds the load
current, the output voltage will be pulled up. In normal
operation, this is generally a minor concern since the load
current is much greater than the leakage.
To avoid excessively large values of R1 in high output volt-
age applications (V_OUT ≥ 10V), a combination of external
and internal resistors can be used to set the output volt-
age. This has an additional benefit of increasing the noise
immunity on the V_FB pin. Figure 7 shows the LTC3630A
with the V_FB pin configured for a 5V fixed output with an
external divider to generate a higher output voltage. The
internal 5M resistance appears in parallel with R2, and
the value of R2 must be adjusted accordingly. R2 should
be chosen to be less than 200k to keep the output volt-
age variation less than 1% due to the tolerance of the
LTC3630A’s internal resistor.
L_IN
R = ^LIN
C_IN
4 • C_IN
LTC3630A
V_IN
C_IN
3630a F05
V_OUT
0.8V
V_FB
LTC3630A
V_PRG1
V_PRG2
R1
R2
3630a F06
14
Figure 5. Series RC to Reduce V_IN Ringing
Figure 6. Setting the Output Voltage with External Resistors
3630af
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