AT1309
Preliminary Product Information
Micro-Power Step up DC-DC Converter
Application Information
External components of boost converter can be designed by performing simple
calculations.
Boost inductor
Inductor selection depends on input voltage, output voltage, maximum output current,
switching frequency and availability of inductor values. The following boost circuit
equations are useful in choosing the inductor values based on the application. They
allow the trading of peak current and inductor value while allowing for consideration
of component availability and cost.
The peak inductor current is given by:
∆IL
2
ILpeak = ILAVG
+
IO
ILAVG
=
1− D
where:
△IL is the inductor peak-to-peak current ripple and is decided by:
Vin
L
D
∆IL =
×
fOSC
D is the MOSFET turn on ratio and is decided by:
VO −V in
D =
VO
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
The output diode has average current of IO, and peak current the same as the
inductor’s peak current and a voltage rating at least 1.5 times the output voltage.
Schottky diode is recommended and it should be able to handle those current.
Output Capacitor
The AT1308 is specially compensated to be stable with capacitors which have a worst-
case minimum value of 1uF at the particular VOUT being set. Output ripple voltage
requirements also determine the minimum value and type of capacitors. Output ripple
voltage consists of two components the voltage drop caused by the switching current
through the ESR of the output capacitor and the charging and discharging of the
output capacitor:
VO −Vin
IO
VRIPPLE = ILPEAK × ESR +
×
VO
COUT × fOSC
For low ESR ceramic capacitors, the output ripple is dominated by the charging or
discharging of the output capacitor.
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3/2/2006 REV:1.1
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