LM3464AMHSLASHNOPB datasheet(13/33 Pages) TI1

13 / 33 page

VDHC =

160 k: // RB

160 k: // RB + 1 M: //RA

x VCC

LM3464, LM3464A

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SNVS652F – APRIL 2010 – REVISED MAY 2013

VVDHC is the voltage at the VDHC pin. This voltage decides the headroom voltage for the LM3464/64A driver

stage and equals to the minimum VDRx among the drain voltages of the MOSFETs under steady state. The

VDHC pin is internally biased to 0.9V which also set the default voltage headroom to 0.9V. In applications that

the output of the AC/DC converter contains more than 0.9V peak-to-peak ripple voltage, the voltage headroom

can be increased by applying external bias to the VDHC pin.

DEFINING VOLTAGE HEADROOM

The voltage headroom is the rail voltage margin that reserve for precision linear current regulation under steady

state. Under steady state, the voltage headroom is always minimized by the LM3464/64A to reduce power losses

on the MOSFETs till one of the drain voltage (VDRx) of the MOSFETs equals the voltage on VDHC pin (0.9V

typical).

With external bias, the voltage of the VDHC pin can be adjusted up or down to adapt to different types of primary

power supply. Figure 17 shows a simple resistor based biasing circuit that derives biasing voltage from the

output of the internal voltage regulator, the VCC pin.

Figure 17. Adjusting Voltage Headroom with Resistors

With the additional resistors, the VDHC pin voltage is adjustable in between 0.8V and 2V. The values of RA and

RB should be at least 10 times lower than the typical values of the internal resistor divider of the VDHC pin (see

Figure 17). However, it is recommended not to set the voltage headroom too low because the ripple voltage of

the primary power supply output may cause visible flicker due to insufficient voltage headroom. Thus the voltage

headroom follows this equation:

where

•

0.8V < VVDHC < 2V

(9)

SETTING LED CURRENT

The LED current regulating mechanism of the LM3464/64A driver stage contains four individual LED current

regulators. Every LED current regulator is composed of an external MOSFET (Q1-Q4), a current sensing resistor

(RISNS1-RISNS4) and an amplifier inside the LM3464/64A that monitors the feedback voltage from the current

sensing resistor. The integrated amplifier compares the voltage across current sensing resistors (RISNS1-RISNS4)

to a 200mV typical reference voltage and controls the gate voltage of the MOSFETs (Q1-Q4) to realize linear

current regulations. Figure 18 shows the simplified circuit of the linear LED current regulators.

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