LM3407_09 Datasheet, PDF(10/16 Page) National Semiconductor (TI) – 350 mA, Constant Current Output Floating Buck Switching Converter for High Power LEDs

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LM3407_09 Datasheet, PDF (10/16 Pages) National Semiconductor (TI) – 350 mA, Constant Current Output Floating Buck Switching Converter for High Power LEDs

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FIGURE 4. Implementation of the PWM Switching Signal

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FIGURE 5. Waveforms of VISNS and VRP Under Closed Loop Operation

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INTERNAL VCC REGULATOR

The LM3407 has an internal 4.5V linear regulator. This regu-

lated voltage is used for powering the internal circuitry only

and any external loading at the VCC pin is not recommended.

The supply input (VIN) can be connected directly to an input

voltage up to 30V. The VCC pin provides voltage regulated at

4.5V for VIN â¤ 6V. For 4.5V â¤ VIN â¤ 6V, VIN pin will be con-

nected to VCC pin directly by an internal bypassing switch.

For stability reason, an external capacitor CVCC with at least

680 nF (1 ÂµF recommended) must be connected to the VCC

pin.

CLOCK GENERATOR

The LM3407 features an integrated clock generator to control

the switching frequency of the converter, fSW. An external re-

sistor RFS, connected to the FS pin and ground, determines

the switching frequency. The oscillator frequency can be set

in the range of 300 kHz to 1 MHz. The relationship between

the frequency setting resistance and the oscillator frequency

is described in the Application Information Section.

PWM DIMMING OF LED STRING

Dimming of LED brightness is achieved by Pulse Width Mod-

ulation (PWM) control of the LED current. Pulse Width Mod-

ulation control allows LED brightness to be adjusted while still

maintaining accurate LED color temperature. The LM3407

accepts an external PWM dimming signal at the DIM pin. The

signal is buffered before being applied to the internal switch

control block responsible for controlling the ON/OFF of the

power switch, Q1. The DIM pin is internally pulled low by a

resistor and no LED current will be available when the DIM

pin is floating or shorted to ground. Functionally, the DIM pin

can also be used as an external device disable control. Device

switching will be disabled if the DIM pin is not connected or

tied to ground.

LOW POWER SHUTDOWN MODE

The LM3407 comes with a dedicated device enable pin, EN,

for low power shutdown of the device. By putting the device

in shutdown mode, most of the internal circuits will be disabled

and the input current will reduced to below typically 50ÂµA. The

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