LM3464 Datasheet, PDF (13/33 Pages) National Semiconductor (TI) – 4 Channel LED Driver Evaluation Board

LM3464, LM3464A
www.ti.com
SNVS652F – APRIL 2010 – REVISED MAY 2013
V_VDHC is the voltage at the VDHC pin. This voltage decides the headroom voltage for the LM3464/64A driver
stage and equals to the minimum V_DRx 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 (V_DRx) 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 R_A and
R_B 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:
V_DHC
=
160
160 k: // R_B
k: // R_B + 1 M:
//R_A
x
V_CC
where
• 0.8V < V_VDHC < 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
(R_ISNS1-R_ISNS4) 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 (R_ISNS1-R_ISNS4)
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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