AT1314
Preliminary Product Information
High-Efficiency Step-Down
Current Regulator For LEDs
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
maximum load current.
Setting the Maximum LED Current
Resistor R
EF
sets the maximum current in the primary chains of LEDs:
I
LEDMAX
=
400
mV
R
FB
Where I
LEDMAX
is the maximum LED current.
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
)
×
V
O
∆
V
ripple
=
2
8
×
L
×
C
OUT
×
f
OSC
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Tel: 886-3-563-0878
Fax: 886-3-563-0879
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http://www.aimtron.com.tw
1/17/2006 REV:1.0
Email:
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GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
