LM3463 datasheet(21/43 Pages) TI2

21 / 43 page

VLED-MAX-COLD, the highest forward

voltage of LED strings under low

temperature

Identify VLED-MAX-COLD

Identify VLED-MIN-HOT

Adjust the nominal output voltage of the

primary power supply,

VRAIL(nom) = VLED-MIN-HOT - 5V

Set the peak output voltage of the primary

power supply (at VOutP = 0.15V),

VRAIL(peak) = VLED-MAX-COLD + 10V

Set the LED turn on voltage,

VDHC_READY = VLED-MAX-COLD + 5V

Begin design

End of design

VLED-MIN-HOT, the lowest forward voltage

of LED strings under high temperature

VRAIL(nom), the nominal output voltage

of the primary power supply.

e.g. Assume VFB = 2.5V,

REMARKS

PROCEDURES

VRAIL(peak), the output voltage of the

primary power supply when the OutP

pin is pulling to its minimum.

VFB

VFB - 0.15 - 0.6

RDHC

VFB

VRAIL(peak) =

R1 + R2

2.5V

VRAIL(nom) =

VDHC_READY, the LED turn on voltage is

defined by RFB1 and RFB2 connected to

the VLedFB pin.

RFB1 + RFB2

RFB2

2.5V

VDHC_READY =

LM3463

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SNVS807A – MAY 2012 – REVISED MAY 2013

Figure 26. Procedure of defining startup parameters

Setting the RDHC and VRAIL

Prior to defining the parameters for the operations in steady state, the value of the RDHC and different levels of

the supply rail voltage (VRAIL) during system startup must be determined. Figure 26 illustrates the procedures of

determining the value of the RDHC and voltage levels of the VRAIL(nom), VRAIL(peak) and VDHC_READY.

In Figure 26, the VLED-MAX-COLD and VLED-MIN-HOT are the maximum and minimum forward voltages of the LED

strings under the required lowest and highest operation temperatures respectively. In order to ensure all the LED

string are supplied with adequate forward current when turning on the LEDs, the VDHC_READY must be set higher

than the VLED-MAX-COLD. For most applications, the VDHC_READY can be set 5 V higher than the VLED-MAX-COLD.

In order to reserve voltage headroom to perform DHC under high operation temperature, the nominal output

voltage of the primary power supply must be set lower than the VLED-MIN-HOT. For most applications, the VRAIL(nom)

can be set 5 V lower than the VLED-MIN-HOT. Figure 27 shows an example connection diagram of interfacing the

LM3463 to a power supply of 2.5V feedback reference voltage.

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