SNVS553C January   2008  – November 2016 LM3407

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Pin Configuration and Functions
  6. Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics
  7. Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 Floating Buck Switching Converter
      2. 7.3.2 Pulse Level Modulation (PLM)
      3. 7.3.3 Internal VCC Regulator
      4. 7.3.4 Clock Generator
      5. 7.3.5 PWM Dimming of LED String
      6. 7.3.6 Input Under-Voltage Lock-Out (UVLO)
    4. 7.4 Device Functional Modes
      1. 7.4.1 Low-Power Shutdown Mode
  8. Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Switching Frequency Selection
      2. 8.1.2 LED Current Setting
      3. 8.1.3 Input and Output Capacitors
      4. 8.1.4 Selection of Inductor
      5. 8.1.5 Free-Wheeling Diode
    2. 8.2 Typical Applications
      1. 8.2.1 LM3407 Design Example
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1 Calculate RISNS
          2. 8.2.1.2.2 Calculate RFS
          3. 8.2.1.2.3 Choose L
          4. 8.2.1.2.4 Choose CIN and CVCC
        3. 8.2.1.3 Application Curve
      2. 8.2.2 Typical Application for Driving 6 LEDs
      3. 8.2.3 Typical Application for Driving 1 LED
  9. Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Community Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

パッケージ・オプション

メカニカル・データ(パッケージ|ピン)
サーマルパッド・メカニカル・データ
発注情報

Detailed Description

Overview

The LM3407 is a high power floating buck LED driver with a wide input voltage range. The device requires no loop compensation network. The integrated power N-MOSFET enables high-output power with up to 350-mA output current. The combination of Pulse Width Modulation (PWM), control architecture, and the proprietary Pulse Level Modulation (PLM) ensures accurate current regulation, good EMI performance, and provides high flexibility on inductor selection. High-speed dimming control input allows precision and high resolution brightness control for applications require fine brightness adjustment.

Functional Block Diagram

LM3407 30046618.gif

Feature Description

Floating Buck Switching Converter

The LM3407 is designed for floating buck configuration. Different from conventional buck converters, a low-side power N-MOSFET is used. The floating buck configuration simplifies the driver stage design and reduces the die size of the power MOSFET. Additionally, the connections of the power diode, inductor and output capacitor are switched to ground with a ground referenced power switch, Q1. The extraction of inductor current information can be easily realized by a simple current sensing resistor. These benefits combine to provide a high efficiency, low cost, and reliable solution for LED lighting applications.

The operation of the LM3407 constant current output floating buck converter is explained below. With the internal switch Q1 turned ON, current flows through the inductor L1 and the LED array. Energy is also stored in the magnetic field of the inductor during the ON cycle. The current flowing through RISNS during the ON cycle is monitored by the Average Current Sensing block. The switch will remain ON until the average inductor current equals 198 mV / RISNS. When the switch is turned OFF, the magnetic field starts to collapse and the polarity of the inductor voltage reverses. At the same time, the diode is forward biased and current flows through the LED, releasing the energy stored in the inductor to the output. True average output current is achieved as the switching cycle continuously repeats and the Average Current Sensing block controls the ON duty cycle. A constant current output floating buck converter only works in Continuous Conduction Mode (CCM); if the converter enters Discontinuous Conduction Mode (DCM) operation, the current regulation will deteriorate and the accuracy of LED current cannot be maintained. The operating waveforms for the typical application circuit are shown in Figure 18.

LM3407 30046619.gif Figure 18. Operating Waveforms of a Floating Buck Converter

Pulse Level Modulation (PLM)

The LM3407 incorporates the innovative Pulse Level Modulation technique. With an external 1% thick film resistor connected to the ISNS pin, the converter output voltage can adjust automatically as needed to deliver constant current within 10% accuracy to a serially connected LED string of different number and type. Pulse Level Modulation is a novel method to provide precise constant current control with high efficiency. It allows the use of low side current sensing and facilitates true average output current regulation regardless of the input voltage and inductor value. Pulse Level Modulation can be treated as a process that transforms a trapezoidal pulse chain into a square pulse chain with an amplitude equal to the center of inductor current ramp. Figure 19 shows the waveform of the converter in steady state. In the figure, IL1 is the inductor current and ILX is the switch current into the LX pin. VISNS is the voltage drop across the current sensing resistor RISNS. VMSL is the center of the inductor current ramp and is a reference pulse that is synchronized and has an identical pulse width to VISNS.

LM3407 30046620.gif Figure 19. LM3407 Switching Waveforms

The switching frequency and duty ratio of the converter equal:

Equation 1. LM3407 30046621.gif

By comparing the area of VISNS and VRP over the ON period, an error signal is generated. Such a comparison is functionally equivalent to comparing the middle level of ISNS to VRP during the ON-period of a switching cycle. The error signal is fed to a PWM comparator circuit to produce the PWM control pulse to drive the internal power N-MOSFET. Figure 20 shows the implementation of the PWM switching signal. The error signal is fed to a PWM comparator circuit to produce the PWM control pulse to drive the internal power N-MOSFET. Figure 20 shows the implementation of the PWM switching signal.

In closed-loop operation, the difference between VMSL and VRP is reflected in the changes of the switching duty cycle of the power switch. This behavior is independent of the inductance of the inductor and input voltage because for the same set of IOUT * RISNS, ON time, and switching period, there exists only one VMSL. Figure 21 shows two sets of current sense signals named VISNS1 and VISNS2 that have identical frequencies and duty cycles but different shapes of trapezoidal waveforms, each generating identical PWM signals.

LM3407 30046623.gif Figure 20. Pulse-Level Transformation

When VMSL is higher than VREF, the peak value of VRP, the switching duty cycle of the power switch will be reduced to lower VMSL. When VMSL is lower than the peak value of VRP, the switching duty cycle of the power switch will be increased to raise VMSL. For example, when IOUT is decreased, VMSL will become lower than VREF. In order to maintain output current regulation, the switching duty cycle of the power switch will be increased and eventually push up VMSL until VMSL equals VREF. Because in typical floating buck regulators VMSL is equal to IOUT × RISNS, true average output current regulation can be achieved by regulating VMSL. Figure 22 shows the waveforms of VISNS and VRP under closed loop operation.

LM3407 30046622.gif Figure 21. Implementation of the PWM Switching Signal
LM3407 30046624.gif Figure 22. Waveforms of VISNS and VRP Under Closed-Loop Operation

Internal VCC Regulator

The LM3407 has an internal 4.5 V 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 (VIN) can be connected directly to an input voltage up to 30 V. The VCC pin provides voltage regulated at 4.5 V for VIN ≤ 6 V. For 4.5 V ≤ VIN ≤ 6 V, VIN pin will be connected to VCC pin directly by an internal bypassing switch. For stability reason, an external capacitor CVCC 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, fSW. An external resistor 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 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.

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 (VIN) falls below the Lock-Out Low threshold, 3.4 V 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, 3.6 V typical, the internal circuits are then enabled and normal operation begins.

Device Functional Modes

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 50 µA. The EN pin is internally pulled high by a 5-µA 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.