LM3430_07 NSC [National Semiconductor], LM3430_07 Datasheet - Page 5

• Manufacturer Part Number: LM3430_07
• Description: LED Backlighting Solution with LM3430 and LM3432
• Manufacturer: NSC [National Semiconductor]

Connect Power Supply to the
Demonstration Board
Input supply voltage, V
solder pads of the demonstration board. V
to 21V. Once the power is applied, all LEDs on the LED panel
will be lighted up. The default setting of the demonstration
board is in analog dimming mode, by adjusting the poten-
tiometer, VR1; the LED brightness can be adjusted accord-
ingly.
For some cases, due to the incorrect power-up sequence tim-
ing of the LM3430 and the LM3432, some strings may have
problem at cold start-up and the LM3432's fault detect may
be falsely triggered. The normal operation can be recovered
by shorting JP2 for few mini-seconds to reset the LM3432's
fault detect circuit. This condition only happens when the
board is powered up with LEDs in maximum brightness. This
condition is not common in most backlighting applications.
Typical applications normally have some sort of power-up se-
quence control to make sure the high voltage supply from the
LM3430 is available before the LM3432 is being enabled.
Synchronization of the Multiple
Boost Converters
In certain applications, more than one LED rail voltages are
required in a single system. For example, a RGB backlighting
system requires at least two different LED rail voltages, one
for RED and one for BLUE and GREEN. Multiple switching
converters running at slightly different switching frequency
can introduce complex EMI situation. In order to accommo-
date this problem, the LM3430 can be synchronized to an
external master clock that drives all boost converters switch-
ing at one single frequency. The external clock must running
at a higher frequency than the preset free running oscillator
frequency, i.e. > 1 MHz with this demonstration board. This
feature can be enabled by applying external synchronization
signal to CN3; the device will pass the control to the external
synchronization signal automatically.
IN
is connected to the V
IN
ranges from 8V
IN
and GND
5
Dynamic Headroom Control
With the LM3432's VDHC output pin connected to LM3430's
VDHC input pin through a gain setting resistor (R19), the Dy-
namic Headroom Control (DHC) function will be in operation.
The LM3432's DHC function will interact with the LM3430 to
adjust the boost converter output voltage, VLED to LED
strings just enough to keep all LED strings current in regula-
tion. This minimum headroom voltage across the LED strings
guarantees the best achievable system efficiency.
LED OPEN and SHORT Fault
Indication
The LM3432 has the ability to detect both OPEN and SHORT
faults of LEDs string by string. Whenever a fault is detected,
the respective LED string will be latched OFF to protect the
device from any possible damage and a signal will be issued
to communicate with system micro-controller for appropriate
actions. The SHORT fault detect threshold is 7.9V (typical)
across IOUT pin to ground and OPEN fault check for zero sink
current in any channel for more than 50 µs during ON duty.
The fault output, FAULTb pin is connected to an indicator LED
in the demonstration board.
Over-Temperature Monitor and
Shutdown
If the on-die temperature is over 125°C, the OTMb pin will be
pulled to ground to inform the system micro-controller for im-
mediate attention. Corrective action is expected to lower the
die temperature, for example, the system can reduce the
PWM duty factor to lower the average current into the IOUTs.
If the on-die temperature rises further to 165°C, the LM3432
will shutdown all channels to prevent any potential damage to
the device. When the device is cooled down to about 145°C,
normal operation will resume. Again, in the demonstration
board, the over-temperature signal output, OTMb pin is con-
nected to an indicator LED.
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