The regulator for the walkway lights is based on the L5970D from STMicrolectronics. Several design elements were adopted from the application note AN2259 from STMicroelectronics. The fundamental principle of the L5970D is to control an internal switch at a high frequency (250kHz) by adjusting its on-time based on the voltage at the feedback (FB) pin. The L5970D will pass as much current as is needed until the voltage at the feedback pin reaches 1.235V.
Let’s first assume that the resistors R1 and R2 were not there and we want 350mA through the load. This would require a current sense resistor Rsense of 3.53 Ohms. The power lost in the sense resistor would be 0.43 Watts. While that does seem to be a lot of power, there is a trick to reduce it.
The L5973D has an internal reference voltage of 3.3V (at the pin REF). By using a voltage divider R1/R2, the voltage at the feedback pin is biased. This results in requiring a lesser contribution coming from the current sense resistor. Without going into the detailed calculations, selecting values for R1 and R2 as shown below, calls for a current sense resistor value of 1.15 Ohms cutting the power loss there by a factor of three.
The input capacitor C1 is there to allow the regulator to draw high current pulses from the supply side. The inductor L1, the so-called “free-wheeling diode” D1 and the output capacitor C6 smooth out the switch pulses. This is a typical design of a buck regulator where the output voltage is always lower than the input voltage. The capacitor C6 can even be omitted if one can accept a large percentage of ripple in the output current. The rest of the circuit deals with high frequency compensation and was taken directly from the application note.
If you look at the schematic below, you will see some different elements than have been described above. There are two models of the regulator IC and R1 is in series with a trim pot. Rsense is now .392 Ohms. The L5973AD regulator has a 2 Amp switching capacity vs. 1 Amp for the L5970D in the same SOP-8 package. The higher current capacity is achieved by an exposed pad on the chip which can be soldered to the circuit board for better heat dissipation. By making the bias voltage for the feedback pin adjustable by means of the trim pot, the regulated current can be adjusted between about 200 mA to more then 1,000 mA. This makes the design flexible for multiple applications, for example driving the new Luxeon L2 LEDs at 1,000 mA or even higher.