7.3.1 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 R_ISNS 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 / R_ISNS. 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.

Figure 18. Operating Waveforms of a Floating Buck Converter

7.3.2 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, I_L1 is the inductor current and I_LX is the switch current into the LX pin. V_ISNS is the voltage drop across the current sensing resistor R_ISNS. V_MSL is the center of the inductor current ramp and is a reference pulse that is synchronized and has an identical pulse width to V_ISNS.

Figure 19. LM3407 Switching Waveforms

The switching frequency and duty ratio of the converter equal:

Equation 1.

By comparing the area of V_ISNS and V_RP over the ON period, an error signal is generated. Such a comparison is functionally equivalent to comparing the middle level of I_SNS to V_RP 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 V_MSL and V_RP 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 I_OUT * R_ISNS, ON time, and switching period, there exists only one V_MSL. Figure 21 shows two sets of current sense signals named V_ISNS1 and V_ISNS2 that have identical frequencies and duty cycles but different shapes of trapezoidal waveforms, each generating identical PWM signals.

Figure 20. Pulse-Level Transformation

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

Figure 21. Implementation of the PWM Switching Signal

Figure 22. Waveforms of V_ISNS and V_RP Under Closed-Loop Operation

7.3.3 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 (V_IN) can be connected directly to an input voltage up to 30 V. The VCC pin provides voltage regulated at 4.5 V for V_IN ≤ 6 V. For 4.5 V ≤ V_IN ≤ 6 V, 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.

7.3.4 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.

7.3.5 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.

7.3.6 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, 3.4 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.

7.4.1 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.