LM3464MHX National Semiconductor Corporation, LM3464MHX Datasheet - Page 15

• Manufacturer Part Number: LM3464MHX
• Description: Lm3464 Led Driver With Dynamic Headroom Control And Thermal Control Interfaces
• Manufacturer: National Semiconductor Corporation

feedback node of the primary power stage and causes the rail
voltage fall back to V
maintain operation to supply power and bias for external cir-
cuitries. When the EN pin is released (floating), the LM3464
exits low power standby mode and the startup sequence be-
gins as described in Figure 3.
FAULT HANDLING and INDICATION
The LM3464 features a complete mechanism for fault han-
dling and indication. The LM3464 detects LED failures like
open and short circuits of LED strings, insufficient supply volt-
age etc. In order to avoid false triggering the fault detection
circuitry, the LM3464 features a timer for fault recognition.
When a fault condition fulfils and sustains longer than certain
preset delay time, a fault is confirmed and the Faultb pin will
be pulled low as an indication. The delay time for fault detec-
tion is defined by the value of the capacitor connects across
the FAULT_CAP pin and GND, C
C
tion. For those applications with slow response primary power
supply, the value of C
The time delay for fault detection is governed by the following
equation:
OPEN CIRCUIT OF LED STRINGS
Detection of LED open circuit is facilitated by detecting both
the V
internal MOSFET gate driver. When a LED string is open cir-
cuit, the V
current sensing resistor. As V
el, LM3464 will then push up the gate voltage of the corre-
sponding MOSFET (V
regulation. Thus, a fault of open circuit will be detected when
V
maximum (V
When fault happens and the fault recognition time defined by
C
those cut-off channel(s), the EN pin should be pulled to GND
for soft reset or repowering the whole system for system reset,
then the startup sequence shows in Figure 3 starts-over.
SHORT CIRCUIT OF LED STRINGS
In the situations that the system has been normally started up
and the LEDs in a LED string are shortened, the correspond-
ing drain voltage (DRx) will increase so as to maintain correct
current regulation. When drain voltage of that particular chan-
nel rises up to 8.4V higher than those of other channels, the
channel with LEDs shortened will be latched off and isolated
from the system to avoid further damages. Once a situation
is recognized as fault, no matter it is due to inappropriate op-
eration, failure of source power or shortening / disconnecting
of LED strings, the Faultb pin will be pulled low. The Faultb
pin is an active low output (normal high) that can be use as
an acknowledgement pin of the LM3464 to report failure to
the supporting control circuitries such as hosting micro-con-
troller.
DRIVING LESS THAN FOUR LED STRINGS
The LM3464 allows users to disable the unused output chan-
nels. Any output channel without a LED string connected or
SEx
FLT
FLT
expires, the failed channel(s) will be cut-off. To reactivate
that set a 264 us delay time is suitable for most applica-
falls below 30mV and internal gate voltage reaches its
SEx
and the internal gate signal before feeding to the
SEx
GDx
pin will be pulled down to below 30mV by the
is about 5V).
RAIL(nom)
FLT
GDx
may need to increase accordingly.
) and try to maintain current
SEx
slowly. The V
falls below its regulated lev-
FLT
. Normally, a 2.2 nF
CC
regulator will
15
with DRX and SEx pins floating will be disabled at system
startup. A disabled channel will be taken out from the DHC
loop and will not contribute headroom control signal to the
LM3464. This function is applicable to both single LM3464
and cascade operation modes.
EXPANDING NUMBER OF OUTPUT CHANNEL
The LM3464 can be cascaded to expand the number of output
channel. Bases on the master-slave architecture, one of the
LM3464 in the system must be set to master mode and the
rest must be set to slave mode. Figure 13 shows an example
application circuit that features eight output channels.
To enable cascade operation, the SYNC pin of the master
LM3464 should connect to the DIM pin of the first slave device
and similarly the SYNC pin of such slave device should con-
nect to its down stream slave device for startup synchroniza-
tion. In addition, the OutP pins of all the LM3464 have to tie
up though a diode and resistor R
node of the primary power supply to accomplish dynamic
headroom control.
The slave devices can only be commanded by the master
LM3464. With the connections between master and slave de-
vices, the information of startup synchronization, thermal fold-
back and PWM dimming are gathered by the master device
and then distribute stage by stage through the SYNC pin.
To set a LM3464 in master mode, the voltage of the VLedFB
pin must be 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 control are overridden by the
packaged synchronization signal delivered from the master.
CONNECTION TO LED ARRAYS
When LEDs are connected to the LM3464 driver stage
through long cables, the parasitic components of the cable
harness and MOSFETS may resonant and eventually lead to
unstable system operation. In applications with over 1 meter
cables between the LM3464 driver stage and LED light en-
gine, a 4.7kΩ resistor should be added across the GDx pins
to GND as shown in Figure 12.
FIGURE 12. Additional Resistor Across GDx and SEx for
Cable Harness Over 1m Long
DHC
to the voltage feedback
30115043
www.national.com