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LM3414HVSD View Datasheet(PDF) - National ->Texas Instruments

Part NameDescriptionManufacturer
LM3414HVSD 1A 60W* Common Anode Capable Constant Current Buck LED Driver Requires No External Current Sensing Resistor National-Semiconductor
National ->Texas Instruments National-Semiconductor
LM3414HVSD Datasheet PDF : 20 Pages
First Prev 11 12 13 14 15 16 17 18 19 20
FIGURE 3. Waveforms of a Floating Buck LED Driver with PLM
30124823
INDUCTOR SELECTION
To ensure proper output current regulation, the
LM3414/14HV must operate in Continuous Conduction Mode
(CCM). With the incorporation of PLM, the peak-to-peak in-
ductor current ripple can be set as high as ±60% of the defined
average output current. The minimum inductance of the in-
ductor is decided by the defined average LED current and
allowable inductor current ripple. The minimum inductance
can be found by the equations shown in below:
Since:
Thus:
The LM3414/14HV can maintain LED current regulation with-
out output filter capacitor. This is because the inductor of the
floating buck structure provides continuous current to the LED
throughout the entire switching cycle. When LEDs are driven
without filter capacitor, the LED peak current must not set ex-
ceeding the rated current of the LED. The peak LED current
is governed by the following equation:
INTERNAL N_MOS POWER SWITCH
The LM3414/14HV features an integrated N-channel power
MOSFET that connects between the LX and GND pins for
power switching. With the switch turned ON, the resistance
across the LX and GND pins is 1.8maximum.
INTERNAL VCC REGULATOR
The LM3414/14HV features a 5.4V internal voltage regulator
that connects between the VIN and VCC pins for powering
internal circuitry and provide biases to external components.
The VCC pin must be bypassed to the GND pin with a 1µF
ceramic capacitor, CVCC that connected to the pins as close
as possible. When the input voltage falls below 6V, the VCC
voltage will drop below 5.4V and decrease proportionally as
Vin decreases. The device will shutdown as the VCC voltage
falls below 3.9V. When the internal regulator is used to pro-
vide bias to external circuitry, it is essential to ensure the
current sinks from VCC pin does not exceed 2mA to maintain
correct voltage regulation.
CONTROL SCHEME
The main control circuitry of the LM3414/14HV is generally a
Pulse-Width-Modulated (PWM) controller with the incorpora-
tion of the Pulse-Level-Modulation (PLM) technology. PLM is
a technology that facilitates true output average current con-
trol without the need to sense the output current directly. In
the LM3414/LM3414HV, the PLM circuit senses the current
of the internal switch through an integrated current sensing
circuitry to realize average output current control. The use of
PLM reduces the power losses on current sensor as it needs
current information only when the switch is turned ON.
In general, the LED drivers with current sensing resistor at the
output, the power dissipation on the current sensing resistor
is ILED2 x RISNS, where ILED is the average output current and
RISNS is the resistance of the current sensing resistor. In the
LM3414/LM3414HV, because of the incorporation of PLM,
power dissipates on the RISNS only in ON period of the internal
power switch. The power loss on RISNS becomes ILED2 x
RISNS x D, where D is the switching duty cycle. For example,
when the switching duty cycle, D of a converter is 0.5, the
power loss on RISNS with PLM is half of those with conven-
tional output current sensing.
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