The power supply shown in Figure 1 is implemented using a simple
buck topology that allows for two output voltages without the need
for a transformer. Typical applications include metering power supplies,
where a 3.3 V supply is required for metrology and a 12 V
supply is required to switch on and off relays or power a radio for
remote meter reading.
Resistor RF1 is a fusible fl ame proof type, which acts as a fuse in
the event of a catastrophic failure and provides differential surge
fi ltering. Diode D1 provides AC input rectifi cation while C1 and C2
Figure 1. 1.25 W Dual Output Supply Using LinkSwitch-TN.
provide smoothing and, together with L1, act as a π-fi lter. EMI is
further 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 output
voltage via D3 (the voltage drops of D2 and D3 cancel). This voltage
is used to provide feedback to U1 via the resistor divider formed
by 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 49
μA, the subsequent cycle is enabled. During each enabled switching
cycle, U1’s internal MOSFET is enabled, causing a linear ramp
in current through L2 and C5. Once the internal current limit is
reached, the MOSFET turns off, and the inductor current can freewheel
via diode D2. Regulation is maintained by adjusting the ratio
of enabled to disabled cycles.