AT1799
2A Step-Down Voltage Regulator
maximum load current.
Feedback Resistor Network
An external resistor divider is required to divide the output voltage down to the
nominal reference voltage. Current drawn by the resistor network should be limited to
maintain the overall converter efficiency. The maximum value of the resistor network
is limited by the feedback input bias current and the potential for noise being coupled
into the feedback pin. A resistor network in the order of 100kΩ is recommended. The
buck converter output voltage is determined by the following relationship:
⎛
⎞
R3
R2
⎜
⎟
⎟
VO = VREF × 1+
⎜
⎝
⎠
where VREF =1.215V as specified.
Input capacitor
An input capacitor helps to provide additional current to the power supply as well as
smooth input voltage variations in high current switching regulators. When selecting
an input capacitor, a low ESR capacitor is required to keep the noise at the IC to a
minimum. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic
capacitors may also suffice. Choose an input capacitor who’s maximum voltage rating
is 1.3 times greater than the maximum input voltage, and who’s RMS current rating is
equal to one-half of the maximum dc load current. It may be necessary in some
designs to add a small valued ceramic type capacitor in parallel with the input
capacitor to prevent any ring.
Output Capacitor
The output capacitor acts to smooth the dc output voltage and also provides energy
storage. Selection of an output capacitor, with an associated equivalent series
resistance (ESR), impacts both the amount of output ripple voltage and stability of the
control loop. Low ESR capacitors are preferred to keep the output voltage ripple low.
The output voltage ripple is estimated to be:
(1− D)×VO
8× L×COUT × fOSC
∆Vripple
=
2
For low ESR ceramic capacitors, the output ripple is dominated by the charging or
2F, No.10, Prosperity RD. II, Science-Based Industrial Park, Hsinchu 300,Taiwan, R.O.C.
Tel: 886-3-563-0878
8/23/2006 REV:1.1
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