Datasheet

V
DRn
V
SEn
V
DRx
= V
DRVLIM
x 4
LM3463
www.ti.com SNVS807A –MAY 2012–REVISED MAY 2013
Choosing the proper Primary Power Supply
If the primary power supply is an off-the-shelf power converter, it is essential to make certain that the power
converter is able to withstand the V
RAIL(peak)
. In order to allow DHC, the nominal output voltage of the primary
power supply needs to be adjusted to below V
LED-MIN-HOT
as well. The suggested procedures for selecting the
proper power supply are as shown in Figure 28.
Selection of External MOSFET
The selection of external MOSFET is dependent on the highest current and the highest voltage that could be
applied to the drain terminal of the MOSFET. Generally, the Drain-to-Source breakdown voltage (V
DSS
) and the
continuous drain current (I
D
) of the external MOSFET must be higher than the defined peak supply rail voltage
(V
RAIL(peak)
) and the maximum output LED current (I
OUTn
) respectively.
Testing LEDs at System Startup
As V
RAIL
increases to V
DHC_READY
, the voltage at the VLedFB pin equals 2.5V. When the voltage at the VLedFB
pin rises to 2.5V, the LM3463 sinks 100 µA through every LED strings from the supply rail into the DRn pins for
certain period of time to determine the status of the LED strings. The time for checking LED strings is defined by
the value of the external capacitor, C
FLT
and is governed by the following equation:
(11)
If the voltage at any DRn pin is detected lower than 350 mV in the LED test period, that particular output channel
will be disabled and excluded from the DHC loop. All disabled output channels will remain in OFF state until a
system restarting is undertaken. The LED test performs only once after the voltage at VLedFB pin hits 2.5V. The
disabled channel can be re-enabled by pulling the EN pin to GND for 10 ms (issuing a system restart) or re-
powering the entire system.
MOSFET Power Dissipation Limit
In order to protect the MOSFETs from thermal break down when a short circuit of the LED sting(s) is
encountered, the LM3463 reduces the output current according to the increment of the drain voltage of the
MOSFET (V
DRn
) when the drain voltage exceeds a certain preset threshold voltage to limit the power dissipation
on the MOSFETs. This threshold voltage is defined by the voltage being applied to the DRVLIM pin, V
DRVLIM
and
is roughly four times the voltage of the V
DRVLIM
. For example, if the desired drain threshold voltage to perform
output current reduction is 16V, the DRVLIM pin voltage should be biased to 4V. Figure 29 shows the relation
between V
SEn
, V
DRn
and V
DRVLIM
.
Figure 29. V
SEn
reduces as V
DRn
exceeds V
DRVLIM
x4
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