Texas Instruments LM3464 Application Note Download Page 6

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Structure of the System

A LM3464A LED lighting system is basically consist of three
main parts, the LM3464A evaluation board, an AC/DC power
supply and an LED array containing four LED strings. In gen-
eral, the LM3464A evaluation board can be regarded as four
independent current sources that the dropout voltages on the
current sources are being monitored by an internal circuit that
generates the DHC signal. The LM3464A evaluation board is
designed to drive 4 LED strings of 12 LEDs in series. With
350mA driving current for every LED string, the default total
output power of the LM3464A evaluation board is around
60W. In order to ensure proper operation, the AC/DC power
supply and LED array should be selected following the steps
presented in this document.

Selection of AC/DC Power Supply

The LM3464A evaluation board can be powered by an AC/
DC power supply through the banana-plug type connectors
on the board as shown in figure 2. Assuming the nominal for-
ward voltage of one LED is 3.5V, the total forward voltage of
a LED string containing 12 LED is about 42V. In order to re-
serve extra voltage headroom to compensate the variations
of the LED forward voltages due to changes of operation tem-
perature, the LED turn ON voltage of this evaluation board is
set to 48V. As this evaluation board is designed to deliver
350mA for each output channel, which is about 60W output
power at 48V rail voltage, the AC/DC power supply must be
able to supply no less than 60W continuous output power at
48V. Therefore, a 60W AC/DC power supply with 48V output
voltage is needed.
In order to facilitate Dynamic Headroom Control (DHC), the
output voltage of the AC/DC power supply is adjusted by the
LM3464A. The LM3464A adjusts the output voltage of the AC/
DC power supply by sinking current from the output voltage
feedback node of the AC/DC converter through a resistor
RDHC into the OutP pin according to the dropout voltage of
the linear current regulators. The OutP pin of the LM3464A is
a open drain pin that can only sink current from the voltage
feedback node of the AC/DC power supply, thus the
LM3464A evaluation board is only able to increase the output
voltage of the AC/DC power supply to acquire wider voltage
headroom.
Since the output voltage of the AC/DC converter will be in-
creased by the LM3464A to allow dynamic head room control
(DHC), the nominal output voltage of the AC/DC power supply
must be reduced prior to connecting to the LM3464A evalu-
ation board to reserve voltage headroom for DHC to take
place. This is achieved by modifying the resistance of the
output voltage sensing resistors of the AC/DC power supply.
To adapt the AC/DC power supply to the LM3464A evaluation
board, the nominal output voltage of the AC/DC power supply
is recommended to be reduced from 48V to 36V. Usually, the
nominal output voltage of the AC/DC power supply can be
reduced by changing the resistance of the resistor divider for

output voltage feedback. Figure 2 shows the voltage feed-
back circuit using LM431 which has been widely used in
typical AC/DC power supplies as an example.
To reduce the output voltage of the AC/DC power supply from
48V to 36V, the resistance of R

is increased without changing

the value of R

. The output voltage and value of R

are related

by the following equations:

(1)

For V

REF(AC/DC)

= 2.5V

And V

RAIL(nom)

= 36V:

(2)

In the above equations, V

REF(AC/DC)

is the reference voltage

of the AC/DC converter for output voltage feedback. V

RAIL

(nom)

is the objective rail voltage level being adjusted to. In this

example, reducing of the rail voltage is achieved by increasing
the value of R

. With the rail voltage is reduced to 36V, the

LED strings are unable to be driven at 350mA due to insuffi-
cient voltage headroom until the DHC loop functions. In order
to ensure the LED strings an regulated driving current at the
time that the LED stings being turned on, the LM3464A in-
creases the output voltage of the AC/DC power supply
(V

RAIL

) from 36V to 48V (V

DHC_READY

) prior to turning on the

LED strings. The level of V

DHC_READY

is defined by the value

of the resistors, RFB1 and RFB2. Figure 3 shows the changes
of V

RAIL

upon the AC/DC power supply is powered until the

system enters steady state operation.
As the output voltage of the AC/DC power supply is depend-
ing on the current being sunk from the output voltage feed-
back node of the AC/DC power supply, the output voltage
could increase to exceed the rated output voltage of the AC/
DC power supply and damage the system if the resistance of
the RDHC is too low and the OutP pin of the LM3464A is ac-
cidentally shortened to GND (V

OutP

= 0V). To avoid this, the

value of the RDHC must be selected appropriately following
the equations below. In the equations, V

RAIL(peak)

is the max-

imum voltage that V

RAIL

can reach if the OutP pin is shortened

to GND. R

and R

are the resistors of the output voltage

feedback resistor divider of the AC/DC power supply. When
designing the values of the RDHC, it is essential to ensure
that the V