LTC3637_15 Datasheet, PDF (14/26 Pages) Linear Technology – 76V, 1A Step-Down Regulator

LTC3637
APPLICATIONS INFORMATION
of ceramic capacitors in series with trace inductance can
also lead to significant input voltage ringing.
Input Voltage Steps
If the input voltage falls below the regulated output volt-
age, the body diode of the internal high side MOSFET will
conduct current from the output supply to the input sup-
ply. If the input voltage falls rapidly, the voltage across the
inductor will be significant and may saturate the inductor. A
large current will then flow through the high side MOSFET
body diode, resulting in excessive power dissipation that
may damage the part.
If rapid voltage steps are expected on the input supply, put
a small silicon or Schottky diode in series with the V_IN pin
to prevent reverse current and inductor saturation, shown
below as D2 in Figure 5. The diode should be sized for a
reverse voltage of greater than the input voltage, and to
withstand repetitive currents higher than the maximum
peak current of the LTC3637.
D2
INPUT
SUPPLY
LTC3637
V_IN
C_IN
SW
L
3637 F05
V_OUT
C_OUT
Figure 5. Preventing Current Flow to the Input
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
6 shows this circuit and the typical values required to
dampen the ringing.
Ceramic capacitors are also piezoelectric sensitive. The
LTC3637’s burst frequency depends on the load current,
and in some applications at light load the LTC3637 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.
L_IN
R=
L_IN
C_IN
4 • C_IN
LTC3637
V_IN
3637 F05
C_IN
Figure 6. Series RC to Reduce V_IN Ringing
Output Voltage Programming
The LTC3637 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 7. 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.
3637fa
14
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