LM3414 datasheet(13/20 Pages) NSC

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FIGURE 3. Waveforms of a Floating Buck LED Driver with PLM

INDUCTOR SELECTION

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 1.8

Ω maximum.

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, C

VCC 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 I

LED

2 x R

ISNS, where ILED is the average output current and

ISNS is the resistance of the current sensing resistor. In the

LM3414/LM3414HV, because of the incorporation of PLM,

power dissipates on the R

ISNS only in ON period of the internal

power switch. The power loss on R

ISNS becomes ILED

2 x

ISNS 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 R

ISNS with PLM is half of those with conven-

tional output current sensing.

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