The power supply shown in Figure 1 is implemented using a simplebuck topology that allows for two output voltages without the needfor a transformer. Typical applications include metering power supplies,where a 3.3 V supply is required for metrology and a 12 Vsupply is required to switch on and off relays or power a radio forremote meter reading.Resistor RF1 is a fusible fl ame proof type, which acts as a fuse inthe event of a catastrophic failure and provides differential surgefi ltering. Diode D1 provides AC input rectifi cation while C1 and C2Figure 1. 1.25 W Dual Output Supply Using LinkSwitch-TN.provide smoothing and, together with L1, act as a π-fi lter. EMI isfurther reduced by the integrated frequency jitter feature of the Link-Switch family of devices (see Figure 3).During the MOSFET off-time, capacitor C4 is charged to the outputvoltage via D3 (the voltage drops of D2 and D3 cancel). This voltageis used to provide feedback to U1 via the resistor divider formedby R1 and R2. The FB pin is sampled during each switching cycle.If the current that fl ows into the feedback pin of U1 is less than 49μA, the subsequent cycle is enabled. During each enabled switchingcycle, U1’s internal MOSFET is enabled, causing a linear rampin current through L2 and C5. Once the internal current limit isreached, the MOSFET turns off, and the inductor current can freewheelvia diode D2. Regulation is maintained by adjusting the ratioof enabled to disabled cycles.