Selection of AC/DC Power Supply

The LM3464 evaluation can be powered by an AC/DC power

supply through the banana-plug type connectors on the board

as shown in figure 2. The component values of this evaluation

board is designed to drive 4 LED strings of 12 High Brightness

LEDs (HBLEDs) (V

As this evaluation board is set to drive 48 pieces of HBLEDs

at 350mA, which is around 50W power consumption, the pri-

mary AC/DC power supply must be able to deliver no less

than 50W continuous output power. On the other hand, as

every LED strings includes 12 serial LEDs with total forward

voltage around 40V, the AC/DC power supply should be able

to supply no less than 41V (with around 1V current regulator

dropout) to power up the LEDs. In order to cover variations of

LED forward voltage under different ambient temperatures, a

50W AC/DC power supply with 48V nominal output voltage is

recommended.

In order to facilitate Dynamic Headroom Control (DHC), the

output voltage of the AC/DC power supply is pushed up to

48V under control of the LM3464 without violating the rated

output voltage of the AC/DC converter. The required voltage

headroom is obtained by reducing the nominal 48V output

voltage of a AC/DC converter. For example: adjusting the

nominal output voltage of a AC/DC converter from 48V

(nom.)

to 36V

Prior to connect the AC/DC power supply to the LM3464 eval-

uation board, the nominal output voltage of the AC/DC power

supply has to be adjusted down to allow DHC to take place.

To reduce the nominal output voltage of the AC/DC converter,

the output voltage feedback circuit of the selected AC/DC

converter has to be located and the reference voltage for out-

put voltage sensing should be identified. Figure 2 shows the

voltage feedback circuit with LM431 that has been widely

used in typical AC/DC power supplies as an example.

Assume the output voltage of the AC/DC power supply is 48V

and is going to be adjusted down to 36V, the resistance of

R

output voltage and value of R

equation:

(1)

For V

And V

(2)

After the modification, the AC/DC power supply will have a

36V nominal output voltage (V

AC/DC converter is connected to the power rail to supply

power to the LEDs, the rail voltage (V

LM3464 up to certain level (V

drive 12 serial LEDs with current regulation before turning on

the LEDs. In this evaluation board, V

which is adequate for most situations with 12 serial LEDs

driving at 350mA per channel. Figure 3 shows the changes of

V

tem enters steady state operation.

30127103

FIGURE 3. Changes of Rail Voltage Upon Power Up

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