LM3464, LM3464A

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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 VSEx pin is pulled

down to below 30mV by the current sensing resistor. As VSEx falls below its regulated level, the LM3464/64A

increases the gate voltage of the corresponding MOSFET (VGDx) in order to maintain current regulation. Thus,

the requirement for LED open circuit is VSEx below 30mV and internal gate voltage reaches its maximum (at VGDx

about 5V). When the requirement of LED open fault is fulfilled, the LM3464/64A begins to charge up the CFLT.

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 (IFAULT-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 RDHC 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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