LM3464 Datasheet, PDF (19/33 Pages) National Semiconductor (TI) – 4 Channel LED Driver Evaluation Board

LM3464, LM3464A
www.ti.com
SNVS652F – APRIL 2010 – REVISED MAY 2013
OPEN CIRCUIT OF LED STRINGS
Detection of LED open circuit is achieved by detecting the voltages of the SEx pin and the internal gate control
signal being fed to the internal MOSFET gate driver. When a LED string is open circuit, the V_SEx pin is pulled
down to below 30mV by the current sensing resistor. As V_SEx falls below its regulated level, the LM3464/64A
increases the gate voltage of the corresponding MOSFET (V_GDx) in order to maintain current regulation. Thus,
the requirement for LED open circuit is V_SEx below 30mV and internal gate voltage reaches its maximum (at V_GDx
about 5V). When the requirement of LED open fault is fulfilled, the LM3464/64A begins to charge up the C_FLT.
When the voltage of the FAULT_CAP reaches 3V, and the condition of open fault retains, an open fault is
confirmed.
After an open fault is confirmed, the failed channel(s) will be disabled and excluded from DHC loop. To reactivate
the disabled channel(s), the EN pin can be pulled to GND for a soft reset or re-power the primary power supply
for a system reset. Either reset methods results in a system restart with startup sequence shows in Figure 16.
SHORT CIRCUIT OF LED STRINGS
If any LED string experiences partially short circuit after normal system startup, the drain voltage (DRx) of the
corresponding channel(s) will increase so as to maintain correct current regulation. When drain voltage increases
up to 8.4V higher than the drain voltages of any other channels, the shortened channel will be latched off and
excluded from the DHC loop to avoid further damages. Once a short fault is confirmed, the Faultb pin will be
pulled low no matter it is due to failure of the power source or shortening of LED strings. When a short circuit of
LED sting is confirmed, the failed channel(s) will be disabled and excluded from DHC loop. The disabled
channels can be reactivated by either pulling the EN pin to GND or system re-powering.
DRx PIN OVER-VOLTAGE PROTECTION
The LM3464/64A features a over-voltage protection function that prevents damaging of the external MOSFETs
due to short circuit of LED string(s). When the voltage of any DRx pin reaches 19V typical, the fault detection
timer is triggered with the output current of the FAULT_CAP pin increases by 4 times (I_{FAULT-CAP-OVP}) and results
in fault detection time 4 times shorter. If a over-voltage of any DRx pin is confirmed, the particular channel will be
latched off and excluded from DHC loop until the EN pin is pulled low (soft reset) or system re-powering is
undertaken.
DRIVING LESS THAN FOUR LED STRINGS
The LM3464/64A allows users to disable the unused output channels. Any output channel without a LED string
connected or with DRx and SEx pins floating will be disabled at system startup. A disabled channel will be
excluded from the DHC loop and will not contribute headroom control signal to the LM3464/64A. This function is
applicable to both single LM3464/64A and cascade operation modes.
EXPANDING NUMBER OF OUTPUT CHANNEL
The LM3464/64A can be cascaded to expand the number of output channel. Bases on the master-slave
architecture, one of the LM3464/64A in the system must be set to master mode and the rest must be set to slave
mode. Figure 27 shows an example application circuit that provides eight output channels.
To enable cascade operation, the SYNC pin of the master LM3464/64A should connect to the DIM pin of the first
slave device and similarly the SYNC pin of such slave device should connect to its down stream slave device for
startup synchronization. In addition, the OutP pins of all the LM3464/64A have to tie up though a diode and
resistor R_DHC to the voltage feedback node of the primary power supply to accomplish dynamic headroom
control, as shown in Figure 27.
The slave devices can only be commanded by the master LM3464/64A. With the master and slave devices
linked up, the information of startup synchronization, thermal foldback and PWM dimming controls are gathered
by the master device and distribute stage by stage through the SYNC pin.
To set a LM3464/64A in master mode, the voltage of the VLedFB pin must be set below 3.25V. When the
VLedFB pin is connected to VCC, the device is in slave mode. In slave mode, local thermal foldback and PWM
controls are overridden by the packaged synchronization signal delivered from the master.
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