Datasheet
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 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,
REMARKSPROCEDURES
V
RAIL(peak)
, the output voltage of the
primary power supply when the OutP
pin is pulling to its minimum.
V
FB
R
2
V
FB
- 0.15 - 0.6
R
DHC
V
FB
R
1
V
RAIL(peak)
=
R
1
+ R
2
R
2
2.5V
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
=
LM3463
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SNVS807A –MAY 2012–REVISED MAY 2013
Figure 26. Procedure of defining startup parameters
Setting the R
DHC
and V
RAIL
Prior to defining the parameters for the operations in steady state, the value of the R
DHC
and different levels of
the supply rail voltage (V
RAIL
) during system startup must be determined. Figure 26 illustrates the procedures of
determining the value of the R
DHC
and voltage levels of the V
RAIL(nom)
, V
RAIL(peak)
and V
DHC_READY
.
In Figure 26, the V
LED-MAX-COLD
and V
LED-MIN-HOT
are the maximum and minimum forward voltages of the LED
strings under the required lowest and highest operation temperatures respectively. In order to ensure all the LED
string are supplied with adequate forward current when turning on the LEDs, the V
DHC_READY
must be set higher
than the V
LED-MAX-COLD
. For most applications, the V
DHC_READY
can be set 5 V higher than the V
LED-MAX-COLD
.
In order to reserve voltage headroom to perform DHC under high operation temperature, the nominal output
voltage of the primary power supply must be set lower than the V
LED-MIN-HOT
. For most applications, the V
RAIL(nom)
can be set 5 V lower than the V
LED-MIN-HOT
. Figure 27 shows an example connection diagram of interfacing the
LM3463 to a power supply of 2.5V feedback reference voltage.
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