LM3464 Datasheet, PDF(12/33 Page) National Semiconductor (TI)

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LM3464 Datasheet, PDF (12/33 Pages) National Semiconductor (TI) – 4 Channel LED Driver Evaluation Board

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LM3464, LM3464A

SNVS652F â APRIL 2010 â REVISED MAY 2013

APPLICATION INFORMATION

www.ti.com

SETTING (VRAIL(nom))

The nominal rail voltage VRAIL(nom) is the nominal output voltage of the primary power supply (AC/DC) prior to

DHC begins. The selection of VRAIL(nom) is primarily depend on the forward voltages of the LED arrays and should

follow the equation shows below:

VRAIL(nom) â¤ Vf(all_temp) + VVDHC

(1)

In the equation, Vf(all_temp) is the lowest forward voltage among all the LED strings under all possible temperature.

And VVDHC is the voltage headroom which equals to the voltage at the VDHC pin. Normally, the forward voltage

of an LED drops as the ambient temperature increases. This could create large variation of total forward voltage

of a LED sting under different temperature. In order to ensure proper system startup, the variation of LED

forward voltage against temperature must be considered in calculations.

SETTING VDHC_READY AND VRAIL(peak)

DHC begins when the voltage at VLedFB pin reaches 2.5V, which is defined by the values of RFB1 and RFB2:

2.5V

VDHC_READY

RFB2

RFB1 + RFB2

(2)

Where

VDHC_READY < VRAIL(peak)

(3)

At this stage, the current of the LED strings are regulated and the rail voltage decreases in order to maintain

minimum voltage drop and power dissipation on the MOSFETs.

In case the OutP pin is accidentally shorten to ground, the rail voltage will increase and end up exceeds

VDHC_READY. To avoid damaging the AC/DC converter, the possible peak output voltage, VRAIL(peak) can be roughly

defined by the forward voltage of the LED strings and must set below the rated voltage of the components at the

output of the AC/DC converter. In order to limit the power dissipation on the external MOSFETs, VRAIL(peak) is set

to to no more than 10VDC higher than the forward voltage of the LED string. The following equations define the

maximum output voltage of the AC/DC converter that can be pushed up by the LM3464/64A:

VRAIL(peak) = VR1 + VREF(AC/DC) = (R1 x IR1) + VREF(AC/DC)

(4)

for VREF(AC/DC) = 2.5V

VRAIL(peak) - 2.5V

IR1 =

(5)

also since

IR1

VREF(AC/DC)

RD2

VREF(AC/DC) - VD1

RDHC

VoutP(min)

2.5V +

RD2

2.5V - 0.5V - 0.3V

RDHC

(6)

1.7V

RDHC =

IR1 -

2.5V

(7)

As the system enters steady state, the rail voltage VRAIL decreases and finally settles to an optimal level that

maintains the maximum power efficiency of the system. The voltage level of VRAIL under steady state can be

calculated following this equation:

VRAIL = Vf(highest) + VVDHC

(8)

In the equation, VRAIL is the rail voltage in steady state and Vf(highest) is the total forward voltage of the LED string

which carry the highest forward voltage among the LED stings.

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