LM3464MH National Semiconductor, LM3464MH Datasheet - Page 16

• Manufacturer Part Number: LM3464MH
• Description: IC, LED DRIVER, BUCK, 28TSSOP
• Manufacturer: National Semiconductor

Specifications of LM3464MH

• No. Of Outputs: 4
• Input Voltage: 12V To 80V
• Driver Case Style: TSSOP
• Svhc: No SVHC (15-Dec-2010)
• No. Of Pins: 28
• Operating Temperature Range: -40°C To +125°C
• Topology: Linear
• Efficiency: 98%
• Led Driver Application: Streetlights, Solid State Lighting Solutions
• Dimming Control Type: PWM / Analogue
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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value of the capacitor connects across the FAULT_CAP pin
and GND, C
delay time is suitable for most application. For those applica-
tions with slow response primary power supply, the value of
C
fault detection is governed by the following equation:
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
30mV by the current sensing resistor. As V
regulated level, the LM3464/64A increases the gate voltage
of the corresponding MOSFET (V
current regulation. Thus, the requirement for LED open circuit
is V
maximum (at V
open fault is fulfilled, the LM3464/64A begins to charge up the
C
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 dis-
abled 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 3.
SHORT CIRCUIT OF LED STRINGS
If any LED string experiences partially short circuit after nor-
mal system startup, the drain voltage (DRx) of the corre-
sponding 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
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
FLT
FLT
SEx
. When the voltage of the FAULT_CAP reaches 3V, and
may need to increase accordingly. The time delay for
below 30mV and internal gate voltage reaches its
FAULT-CAP-OVP
FLT
GDx
. Normally, a 2.2 nF C
about 5V). When the requirement of LED
) and results in fault detection time 4
SEx
pin is pulled down to below
GDx
) in order to maintain
FLT
that set a 264 us
SEx
falls below its
16
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 14 shows an
example application circuit that provides eight output chan-
nels.
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 synchro-
nization. In addition, the OutP pins of all the LM3464/64A
have to tie up though a diode and resistor R
feedback node of the primary power supply to accomplish
dynamic headroom control, as shown in Figure 14.
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 de-
vice 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 overrid-
den by the packaged synchronization signal delivered from
the master.
CONNECTION TO LED ARRAYS
When LEDs are connected to the LM3464/64A driver stage
through long cables, the parasitic components of the cable
harness and external MOSFETs may resonant and eventu-
ally lead to unstable system operation. In applications that the
cables between the LM3464/64A driver circuit and LED light
engine are longer than 1 meter, 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
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