AT1799
2A Step-Down Voltage Regulator
Applications Information
External components of step-down converter can be designed by performing simple
calculations. It must be to follow regulation by the output voltage and the maximum
load current, as well as maximum and minimum input voltages. Begin by selecting an
inductor value. Once L is know, choose the diode and capacitors.
Inductor
Inductor selection depends on input voltage, output voltage, maximum current,
switching frequency and availability of inductor values. The following buck circuit
equations are useful in choosing the inductor values based on the application. Choose
an inductor that does not saturate under the maximum rating load conditions. The
magnitude of inductance is selected to maintain a peak to peak ripple current of 30%
of the maximum load current.
The peak inductor current is given by:
∆IL
2
ILpeak = ILAVG
+
Iin
ILAVG
=
D
VO
Vin
D =
D is the MOSFET turn on ratio
where:
△IL is the inductor peak-to-peak current ripple and is decided by:
Vin −VO
(Vin −VO )× D
∆IL × fOSC
D
∆IL =
×
⇒ L =
L
fOSC
fOSC is the switching frequency.
The inductor should be chosen to be able to handle this current and inductor saturation
current rating should be greater than IPEAK
.
Diode selection
When the power switch turns off, the current through the inductor continues to flow.
The path for this current is through the diode connected between the switch output
and ground. This forward biased diode must has a minimum voltage drop and
recovery times. Schottky diode is recommended and it should be able to handle those
current. As usual, the reverse voltage rating of the diode should be at least 1.3 times
greater than the maximum input voltage, and current rating is greater than the
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8/23/2006 REV:1.1
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