The circuit described here supplies 0 V to −40 V for those applications that require a variable programmable power supply, such as in automated testing systems, battery-discharge-curve simulation, and when driving heating elements to various temperatures under closed-loop computer control. The circuit uses a negative boost Cuk-configured switching regulator (U1) controlled by a 12-bit DAC (U3) (Fig. 1). Output currents up to 1 A are provided and an output voltage adjustable in 10-mV steps offers superior efficiency.
U1’s FB pin makes possible the control and programming of the regulator’s output. This pin acts as a summing node. The current in this node is:
When the DAC’s output voltage is increased under external control, the op amp’s output follows U1’s decreasing output. Simultaneously, the feedback loop will force the FB current to remain constant, satisfying the equation above. As the DAC’s output varies from 4.095 Volts to 0 Volts, the regulator’s output varies from 0 Volts to −40 Volts.
Similar programmable supplies designed around linear regulators suffer from high power loss. The power loss of this switching regulator topology is considerably lower over most of the operating range when compared to an equivalent circuit using a linear regulator. Figure 2 illustrates the comparison between the circuit in Figure 1 and an equivalent linear regulator. Each curve has been generated with the output current held constant at 1 A.
