Electronic Design

Differential Feedback Produces Two Regulated Outputs From One

The need often arises for a circuit with two regulated outputs from a single supply (controller). In some cases, the output won't require particularly tight regulation. Instead, only adequate regulation will be necessary for the parts to be powered. This circuit does just that (Fig. 1). It provides two quasi-regulated voltages controlled by one controller IC.

Implemented using a buck regulator with an overwinding, the device's main or "primary" output is 1.8 V (±2.5%) at 300 mA (maximum). Its auxiliary or "secondary" output is 3.3 V (±2.5%) at 150 mA (maximum). The overwinding is 180° out of phase with the primary winding so that the voltage developed across COUT2 is the main output voltage, 1.8 V, minus a diode drop. Adding this to the 1.8-V primary output generates the 3.3-V secondary output voltage.

Both outputs are fed back to the error amplifier via a couple of resistors. Since the error amplifier is configured as a summing-junction amplifier, both outputs are sensed by the error amplifier and driven accordingly. The feedback voltage is developed by the sum of the two feedback currents through the bottom resistor of the voltage divider. Therefore, the error amplifier controls both outputs and maintains 5% regulation on each output over all line and load variations.

All components, including the inductor/transformer, are easy-to-obtain, off-the-shelf items. For in-stance, the inductor/transformer is a Coiltronics CTX10-1P Econo-Pac. Since the LTC1878 controller used in this circuit operates on very low quiescent current, the circuit is very efficient. At a VIN of 5 V, the efficiency at full load is approximately 80%. Another feature is that the IC will run at 100% duty cycle. Therefore, when the input voltage drops to 3.3 V, the auxiliary output will still regulate at 3.3 V.

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