Texas Instruments AN-2255, User Manual, Page: 10 / 26

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V
DHC_READY
V
RAIL(nom)
V
RAIL(steady)
V
RAIL(peak)
V
RAIL
Time
Pushed up
by LM3463
Initiated by
primary power
supply
DHC activated
Primary Power Supply
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Generally the required range of the V
RAIL
is determined by the highest and lowest possible forward
voltages of the LED strings (respectively, V
LED-MAX-COLD
and V
LED-MIN-HOT
). Since the forward voltage of the
LED strings varies according to the changing of the ambient temperature, the voltage for turning the LEDs
on at system startup must be set higher than the V
LED-MAX-COLD
. Figure 3 shows the different voltage level of
V
RAIL
at system startup.
Figure 5. Different voltage levels of the V
RAIL
at system startup
In Figure 3, the V
RAIL
is the output voltage of the primary power supply under the control of the LM3463.
V
RAIL(peak)
is the highest level of V
RAIL
when the voltage of the OutP pin of the LM3463 equals 0V. V
DHC_READY
is the voltage level that the LM3463 turns all output channels on. V
RAIL(nom)
is the nominal output voltage of
the primary power supply when the OutP pin voltage is higher than V
FB
+0.6V (i.e. prior to DHC starting)
In order to secure sufficient rail voltage to maintain regulated LED currents when enabling the output
channels, the V
RAIL(peak)
and V
DHC_READY
must be set higher than V
LED-MAX-COLD
(the highest forward voltage of
the LED strings under low ambient temperature). The following settings are suggested to ensure correct
system startup sequence:
1. V
RAIL(nom)
= V
LED-MIN-HOT
- 5V
2. V
DHC_READY
= V
LED-MAX-COLD
+ 5V
3. V
RAIL(peak)
= V
DHC_READY
+ 5V
Figure 4 shows a suggested procedure to determine the V
RAIL(nom)
, V
DHC_READY
and V
RAIL(peak)
.
10 AN-2255 LM3463 Evaluation Board SNVA642 – May 2012
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