3A Synchronous Step-Down Converter
Setting the Output Voltage
The output voltage is set using a resistive
volt-age divider from the output voltage to FB
(see Typical Application circuit on page 1).
The volt - age divider divides the output
voltage down by the ratio:
Where VFB is the feedback voltage and VOUT is
the output voltage.
Thus the output voltage is:
R2 can be as high as 100kΩ, but a typical
value is 10kΩ.Using the typical value for R2,
R1 is determined by:
For example, for a 3.3V output voltage, R2 is
10kΩ, and R1 is 26.1kΩ. Table 3 lists recom-
mended resistance values of R1 and R2 for
standard output voltages.
input voltage,fS is the switching frequency,
and ΔIL is the peak-to-peak inductor ripple
Choose an inductor that will not saturate
under the maximum inductor peak current.
The peak inductor current can be calculated
Where ILOAD is the load current.
The choice of which style inductor to use
mainly de- pends on the price vs. size
requirements and any EMI requirements.
Optional Schottky Diode
During the transition between high-side switch
and low-side switch, the body diode of the
lowside power MOSFET conducts the
inductor current. The forward voltage of this
body diode is high. An optional Schot- tky
diode may be paralleled between the SW pin
and GND pin to improve overall efficiency.
Table 4 lists example Schottky diodes and
The inductor is required to supply constant
cur- rent to the output load while being driven
by the switched input voltage.A larger value
inductor will result in less ripple current that
will result in lower output ripple voltage.
However, the larger value inductor will have a
larger physical size, higher series resistance,
and/or lower saturation current. A good rule
for determining the induc- tance to use is to
allow the peak-to-peak ripple current in the
inductor to be approximately 30% of the
maximum switch current limit.Also, make sure
that the peak inductor current is below the
maximum switch current limit. The inductance
value can be calculated by:
Where VOUT is the output voltage, VIN is the
The input current to the step-down converter
is discontinuous , therefore a capacitor is
required to supply the AC current to the
step-down converter while maintaining the DC
input voltage. Use low ESR ca- pacitors for
the best performance. Ceramic capacitors are
preferred, but tantalum or low-ESR
electrolytic capacitors may also suffice.
Choose X5R or X7R dielectrics when using
ceramic capacitors. Since the input capacitor
absorbs the input switching current it requires
an adequate ripple current rating. The RMS
current in the input capacitor can be
The worst-case condition occurs at VIN =
2VOUT, where ICIN = ILOAD/2. For simplification,
choose the input capacitor whose RMS