Texas Instruments AN-2255, User Manual, Page: 11 / 26

V
LED-MAX-COLD
, the highest forward
voltage of LED strings under low
temperature
Identify V
LED-MAX-COLD
Identify V
LED-MIN-HOT
Adjust the nominal output voltage of the
primary power supply,
V
RAIL(nom)
= V
LED-MIN-HOT
- 5V
Set the peak output voltage of the primary
power supply (at V
OutP
= 0.15V),
V
RAIL(peak)
= V
LED-MAX-COLD
+ 10V
Set the LED turn on voltage,
V
DHC_READY
= V
LED-MAX-COLD
+ 5V
Begin of design
End of design
V
LED-MIN-HOT
, the lowest forward voltage
of LED strings under high temperature
V
RAIL(nom)
, the nominal output voltage
of the primary power supply.
e.g. Assume V
FB
= 2.5V,
REMARKS PROCEDURES
V
RAIL(peak)
, the output voltage of the
primary power supply when the OutP
pin is pulling to its minimum.
R
1
+ R
2
R
2
2.5V x
V
RAIL(nom)
=
V
DHC_READY
, the LED turn on voltage is
defined by R
FB1
and R
FB2
connected to
the VLedFB pin.
R
FB1
+ R
FB2
R
FB2
2.5V + V
DHC_READY
=
V
FB
R
2
V
FB
- 0.15 - 0.6
R
DHC
+ V
FB
R
1
x V
RAIL(peak)
=
+
www.ti.com
Primary Power Supply
Figure 6. Procedures of setting the rail voltage levels.
Because the V
RAIL(peak)
is the possible highest output voltage of the primary power supply with the LEDs
turned on, the primary power supply must be able to deliver an output power no less than the total LED
current multiplied by the V
RAIL(peak)
. The flow chart in Figure 5 shows the recommended procedure of
selecting a power supply for the LM3463 evaluation board. If the power supply is an off-the-shelf product,
the output voltage and value of the output voltage feedback resistor divider may need to be changed to
allow DHC.
11 SNVA642 – May 2012 AN-2255 LM3463 Evaluation Board
Submit Documentation Feedback
Copyright © 2012, Texas Instruments Incorporated