LM3407 Datasheet, PDF (12/22 Pages) National Semiconductor (TI) – 350 mA, Constant Current Output Floating Buck Switching Converter for High Power LEDs

LM3407
SNVS553B – JANUARY 2008 – REVISED MAY 2013
V_RP
T
t_ON
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I_OUT * R_ISNS
V_ISNS
V_MSL < V_REF
0
V_MSL = V_REF
Figure 23. Waveforms of V_ISNS and V_RP Under Closed Loop Operation
INTERNAL VCC REGULATOR
The LM3407 has an internal 4.5V linear regulator. This regulated voltage is used for powering the internal
circuitry only and any external loading at the VCC pin is not recommended. The supply input (V_IN) can be
connected directly to an input voltage up to 30V. The VCC pin provides voltage regulated at 4.5V for V_IN ≤ 6V.
For 4.5V ≤ V_IN ≤ 6V, VIN pin will be connected to VCC pin directly by an internal bypassing switch. For stability
reason, an external capacitor C_VCC with at least 680 nF (1 µF 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, f_SW. An
external resistor R_FS, 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 Modulation (PWM) control of the LED current. Pulse
Width Modulation 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 50µA. The EN pin is internally pulled high by a 5µA current source. Connecting the EN pin to
ground will force the device to enter low power shutdown mode. To resume normal operation, leave the EN pin
open or drive with a logic high voltage.
INPUT UNDER-VOLTAGE LOCK-OUT (UVLO)
The LM3407 incorporates an input Under-Voltage Lock-Out (UVLO) circuit with hysteresis to keep the device
disabled when the input voltage (V_IN) falls below the Lock-Out Low threshold, 3.4V typical. During the device
power-up, internal circuits are held inactive and the UVLO comparator monitors the voltage level at the VIN pin
continuously. When the VIN pin voltage exceeds the UVLO threshold, 3.6V typical, the internal circuits are then
enabled and normal operation begins.
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