7.3.1 Theory of Operation

For an image of the LM3444 along with basic external circuitry, see Figure 8.

Figure 8. LM3444 Schematic

7.3.2 Valley-Fill Circuit

V_BUCK supplies the power which drives the LED string. Diode D3 allows V_BUCK to remain high while V+ cycles on and off. V_BUCK has a relatively small hold capacitor C10 which reduces the voltage ripple when the valley fill capacitors are being charged. However, the network of diodes and capacitors shown between D3 and C10 make up a valley-fill circuit. The valley-fill circuit can be configured with two or three stages. The most common configuration is two stages. Figure 9 illustrates a two- and three-stage valley-fill circuit.

Figure 9. Two and Three Stage Valley Fill Circuit

The valley-fill circuit allows the buck regulator to draw power throughout a larger portion of the AC line. This allows the capacitance needed at V_BUCK to be lower than if there were no valley-fill circuit, and adds passive power factor correction (PFC) to the application.

7.3.3 Valley-Fill Operation

When the input line is high, power is derived directly through D3. The term input line is high is explained as follows. The valley-fill circuit charges capacitors C7 and C9 in series (Figure 10) when the input line is high.

Figure 10. Two Stage Valley-Fill Circuit When AC Line is High

The peak voltage of a two-stage valley-fill capacitor is given by Equation 1.

Equation 1.

As the AC line decreases from its peak value every cycle, there is a point where the voltage magnitude of the AC line is equal to the voltage that each capacitor is charged. At this point, diode D3 becomes reversed biased, and the capacitors are placed in parallel to each other (Figure 11), and V_BUCK equals the capacitor voltage.

Figure 11. Two Stage Valley-Fill Circuit when AC Line is Low

A three stage valley-fill circuit performs exactly the same as two-stage valley-fill circuit, except now three capacitors are charged in series when the line voltage decreases, as shown in Equation 2:

Equation 2.

Diode D3 is reverse-biased and three capacitors are in parallel to each other.

The valley-fill circuit can be optimized for power factor, voltage hold-up, and overall application size and cost. The LM3444 operates with a single-stage or a three-stage valley-fill circuit as well. Resistor R8 functions as a current limiting resistor during start-up, and during the transition from series to parallel connection. Resistors R6 and R7 are 1-MΩ bleeder resistors, and may or may not be necessary for each application.

7.3.4 Buck Converter

The LM3444 is a buck controller that uses a proprietary constant off-time method to maintain constant current through a string of LEDs. While transistor Q2 is on, current ramps up through the inductor and LED string. A resistor R3 senses this current and this voltage is compared to the reference voltage at FILTER. When this sensed voltage is equal to the reference voltage, transistor Q2 is turned off and diode D10 conducts the current through the inductor and LEDs. Capacitor C12 eliminates most of the ripple current seen in the inductor. Resistor R4, capacitor C11, and transistor Q3 provide a linear current ramp that sets the constant off-time for a given output voltage.

Figure 12. LM3444 Buck Regulation Circuit

7.3.5 Overview Of Constant Off-Time Control

The conversion ratio of a buck converter is defined as given by Equation 3.

Equation 3.

Constant off-time control architecture operates by simply defining the off-time and allowing the on-time, and therefore the switching frequency, to vary as either V_IN or V_O changes. The output voltage is equal to the LED string voltage (V_LED), and should not change significantly for a given application. The input voltage or V_BUCK in this analysis varies as the input line varies. The length of the on-time is determined by the sensed inductor current through a resistor to a voltage reference at a comparator. During the on-time, denoted by t_ON, MOSFET switch Q2 is on causing the inductor current to increase. During the on-time, current flows from V_BUCK, through the LEDs, through L2, Q2, and finally through R3 to ground. At some point in time, the inductor current reaches a maximum (I_L2-PK) determined by the voltage sensed at R3 and the ISNS pin. This sensed voltage across R3 is compared against the voltage of FILTER, at which point Q2 is turned off by the controller.

Figure 13. Inductor Current Waveform in CCM

During the off-period denoted by t_OFF, the current through L2 continues to flow through the LEDs through D10.

7.3.6 Thermal Shutdown

Thermal shutdown limits total power dissipation by turning off the output switch when the IC junction temperature exceeds 165°C. After thermal shutdown occurs, the output switch does not turn on until the junction temperature drops to approximately 145°C.