V

OutP is accidentally shorted to GND. The V

exceed the rated output voltage of the AC/DC converter even

if the OutP pin is shorted to GND (V

V

culated following this equation:

(3)

where

(4)

In this equation, V

voltage feedback of the AC/DC power supply. R

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

exceed the rated output voltage of the AC/DC power supply,

otherwise the AC/DC power supply could be damaged.

Setting of V

DHC_READY

As V

RFB2, the voltage at the VLedFB pin of the LM3464 should

equal to 2.5V. As the VLedFB pin voltage reaches 2.5V, the

LM3464 performs a test no long than 400uS to identify the

output channels that are not in use (No LED connected). If the

voltage of any current sensing input pins of the LM3464 (SE1

— SE4) is below 30mV because of open circuit of LED string

(s), that particular output channel will be disabled (latched off)

and taken out from DHC control loop by the LM3464 to allow

the functional LED strings to operate. Similarly, if any drain

voltage of the MOSFETS (DR1 — DRx) is 8.4V exceeding the

drain voltage of any other channel due to short circuit of LEDs,

that particular channel will be disabled to avoid permanent

damage. After the detection is completed, LED strings turn on

and current regulation starts. The level of V

fined by the values of RFB1 and RFB2 on the evaluation

board and can be adjusted to any level below 80V as desired.

By default, the V

must set no more than 20V higher than the forward voltages

of any LED string connected to the system under possible

temperatures, otherwise a ‘short fault’ may arise and results

in immediate output channel latch-off to protect the MOSFETs

from over-heat. The V

to the equation (5):

(5)

Adjusting Voltage Headroom

The voltage headroom of the LM3464 evaluation board can

be increased or decreased by adjusting the voltage at the

VDHC pin (V

with low rail voltage ripple, the voltage headroom can be de-

creased to reduce power dissipation on the MOSFETs. By

default, the VDHC pin is biased internally to 0.9V as shown

in figure 4.

30127104

FIGURE 4. Adjusting the VDHC Pin Voltage

To adjust V

tor divider (R

pad and VCC or AGND terminals on the board. It is recom-

mended that the resistances of R

100k

Ω and 16kΩ respectively to ensure the accuracy of the

headroom voltage under steady state. The V

by the following equation:

(6)

where

(7)

Connecting the LED Strings

The LM3464 evaluation board is designed to drive 4 common

anode LEDs strings of 12 serial LEDs per string. The board

includes four turret connectors, LED1, LED2, LED3 and LED4

for cathode connections of the LED strings. The anode of the

LED strings should connect to the positive power output ter-

minal. By default, the output current for every output channel

is set at 350mA. The driving currents can be adjusted by

changing the value of the resistors RISNS1, RISNS2,

RISNS3 and RISNS4 respectively. The LED driving current is

governed by the following equation:

(8)

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