SNVS574G July 2008 – July 2019 LM3421 , LM3423
PRODUCTION DATA.
Current regulators can create three basic topologies: buck, boost, or buck-boost. All three topologies in their most basic form contain a main switching MOSFET, a recirculating diode, an inductor and capacitors. The controller is designed to drive a ground referenced N-channel FET which is perfect for a standard boost regulator. However, buck and buck-boost regulators usually have a high-side switch. When driving an LED load, a ground referenced load is often not necessary, therefore a ground referenced switch drives a floating load instead. The controller can then be used to drive all three basic topologies as shown in the Basic Topology Schematics section. Other topologies such as the SEPIC and flyback converter (both derivatives of the buck-boost) can be implemented as well.
Looking at the buck-boost design, the basic operation of a current regulator can be analyzed. During the time that the N-channel FET (Q1) is turned on (tON), the input voltage source stores energy in the inductor (L1) while the output capacitor (CO) provides energy to the LED load. When Q1 is turned off (tOFF), the re-circulating diode (D1) becomes forward biased and L1 provides energy to both CO and the LED load. Figure 12 shows the inductor current (iL(t)) waveform for a regulator operating in CCM.
The average output LED current (ILED) is proportional to the average inductor current (IL) , therefore if IL is tightly controlled, ILED is well regulated. As the system changes input voltage or output voltage, the ideal duty cycle (D) is varied to regulate IL and ultimately ILED. For any current regulator, D is a function of the conversion ratio:
Buck
Boost
Buck-boost