摘要

This application note illustrates how to use the MAX668 boost circuit and the MAX1846 inverting circuit to implement a switch-mode power supply that provides a ±12V or ±15V at 0.5A output from a 4.5V to 12V input.

Overview

The switch-mode power-supply circuits illustrated in this application note provide a ±12V or ±15V at 0.5A output from a 4.5V to 12V input. This wide input-voltage range allows the device to be powered from a regulated DC voltage or even an unregulated DC voltage, such as the rectified output of an inexpensive AC "wall wart" step-down transformer. This design may be preferred to Maxim's older MAX742 design, which requires a larger overall circuit.

The power supply consists of a MAX668 boost circuit or a MAX1846 inverting circuit. Each circuit operates at a 300kHz switching frequency, balancing cost, size, and performance. The circuits limit the maximum switch current, which ultimately limits output current for a given input voltage; however, maximum output current increases with increased input voltage.

The MAX668 Circuit

Figure 1 illustrates how to use the MAX668 boost circuit to implement a switch-mode power supply that provides a +15V at 0.5A output from a 4.5V to 12V input. This MAX668 circuit adds a few components to the minimum circuit implementation. C7 adds a pole to compensate for the ESR-zero of the output capacitor. R5 and C8 filter the current-sense voltage to prevent high-frequency switching noise from prematurely tripping the current limit. This functionality is in addition to the MAX668's internal 60ns current-sense blanking time.

The MAX668 output voltage can be changed to +12V by changing the value of the R2 resistor to 86.6kΩ. This change reduces the minimum input voltage to approximately 10V. Since the MAX668 provides internal compensation, no other changes are required for +12V output.

The output ripple voltage due to switching can be reduced an order of magnitude with a secondary output filter set to one-tenth of the switching frequency. A 1Ω, 0.5W resistor in series with a 10µF, 25V capacitor with less than 100mΩ ESR introduces a 0.5V decrease in output voltage at a 0.5A load. The feedback voltage must be sensed before it reaches the secondary filter for the MAX668 to maintain stability.

The MAX1846 Circuit

Figure 2 uses the MAX1846 inverting circuit to implement a switch-mode power supply that provides a -15V at 0.5A output from a 4.5V to 12V input. This MAX1846 circuit also adds a few components to the minimum circuit implementation. C20 adds a pole to compensate for the ESR-zero of the output capacitor, while R16 and C22 filter the current-sense voltage to prevent high-frequency switching noise from tripping the current limit. This functionality is in addition to the MAX1846's internal 100ns current-sense blanking time. The MAX1846 EXT pin implements controlled slew rate, which helps limit high-frequency switching noise.

The MAX1846 output voltage can be changed to -12V by changing the value of R13 to 97.6kΩ and R10 to 91kΩ. The maximum input voltage does not decrease, although duty cycle jitter increases somewhere between 10V and 12V input. Again, the output ripple voltage due to switching can be reduced an order of magnitude with the same secondary filter described in the MAX668 circuit above. As with the MAX668, the feedback voltage for the MAX1846 must be sensed before it reaches the secondary filter for the voltage inverter to work properly.