LM3464 Datasheet, PDF (10/33 Pages) National Semiconductor (TI) – 4 Channel LED Driver Evaluation Board

LM3464, LM3464A
SNVS652F – APRIL 2010 – REVISED MAY 2013
OVERVIEW
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The LM3464/64A is a four channel linear current regulator designed for LED lighting systems with wide input
voltage range, high speed PWM and thermal foldback dimming control interface. The LM3464/64A incorporates a
Dynamic Headroom Control (DHC) technology which maximizes overall efficiency of the lighting system by
adjusting the output voltage of the primary power source dynamically. Linear current regulation secures high
accuracy output current, LED and system reliability. High speed PWM dimming provides the flexibility of
brightness control while maintaining constant color temperature of the light. The thermal foldback feature enables
the LM3464/64A to manage the temperature of the LED heat sink or system chassis with a simple NTC/PTC
temperature sensor. The thermal foldback input can also be used as an analog dimming control input to adapt to
other sensors easily, such as ambient light sensor.
Dynamic Headroom Control (DHC)
Operation Principles of DHC
Dynamic Headroom Control is a technology that aims at maximizing the overall system efficiency by altering the
supply voltage to the LED(s) dynamically in respect to the characteristics of the LED(s). In the LM3464/64A, DHC
is facilitated by connecting a resistor in between the OutP pin of the LM3464/64A and the voltage feedback node
of the primary power supply (AC/DC) as shown in Figure 15.
Figure 15. Circuitry of the DHC Mechanism
For example, in steady state, when all the output channels are in regulation and the forward voltage of any LED
string decreases due to temperature raise, the drain voltage of the corresponding channel (DRx) increases to
exceed the default 0.9V typical headroom voltage in order to maintain constant output current. As the drain
voltage increases, the voltage of CDHC increases and the current sink into the OutP pin decreases. This will
finally result in decrease of rail voltage (V_RAIL) until the corresponding DRx voltage returns to minimum level.
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