Design Note 1021: How to Produce Negative Output Voltages from Positive Inputs Using a uModule Step-Down Regulator
Introduction
Linear Technology’s DC/DC step-down μModule® regulators are complete switchmode power supplies in a surface-mount package. They include the DC/DC controller, inductor, power switches and supporting circuitry. These highly integrated regulators also provide an easy solution for applications that require negative output voltages. In other words, these products can operate as inverting buck-boost regulators. As a result, the lowest potential in the circuit is not the standard 0V, but –VOUT, which must be tied to the μModule regulator’s GND. All signals are now referred to –VOUT.
For this discussion, the LTM8025 (36V, 3A) is used to demonstrate how a buck μModule regulator can be altered to produce a negative output voltage with level-shifting circuitry for synchronization. This approach can be applied to other μModule regulators, such as the LTM8022 (36V, 1A), LTM8023 (36V, 2A) and LTM8027 (60V, 4A).
Design Guide
A conventional buck (step-down) μModule regulator can be easily configured to generate negative output voltages by configuring it as an inverting buck-boost converter, as illustrated in Figure 1. The negative terminal of the input supply is connected to the VOUT pin of the μModule regulator and the GND pin is tied to the –VOUT rail. The actual input voltage (VIN’) seen by the μModule regulator is the difference between the input supply (VIN) and the output voltage (–VOUT). This voltage must be within the allowable input range of the part. Additionally, the absolute value of the output voltage must not exceed the maximum output voltage rating of the μModule regulator. Since the part is now operating as an inverting buck-boost, the switch current is larger than in its buck counterpart. Hence, parameters such as output current, switching frequency, thermal performance, etc. must be considered to stay within the part’s limits. Refer to Appendix for detailed discussions and calculations. Refer to Table 1 for a selection guide of example buck μModule regulators configured as inverters.
–12V Output Application
The LTM8025 is a 36VIN, 3A step-down μModule converter that can support output voltages up to 24V. With minimal design effort, it can be easily configured to generate negative output voltages. Figure 2 shows an LTM8025 schematic generating –12V at 2A from an input range of 20V to 24V. The actual input voltage seen by the LTM8025 is VIN’ = VIN – (–VOUT). For instance, if VIN = 20V, VIN’ = 20V – (–12V) = 32V. Because the maximum input rating of the LTM8025 is 36V, the input supply in this specific application is limited to 24V. Additionally, the internal oscillator of the LTM8025 can be synchronized by applying an external 250kHz to 2MHz clock signal to the SYNC pin. For negative output voltages, the clock must be level-shifted to account for the lower potential. This example has a 0V to 5V, 750kHz input clock signal. By adding a few passive components, the input clock is level-shifted to produce a –12V to –7V signal, which is then applied to the SYNC pin of the LTM8025. Figure 3 shows the start-up waveforms for the –12V output application.
Run/Shutdown
The LTM8025 has a RUN/SS pin that provides shutdown along with soft-start functions. In order to shut down the part, the RUN/SS pin must be pulled below 0.2V. For negative output applications, the LTM8025 GND is tied to –VOUT. So, the RUN/SS voltage must be below 0.2V above –VOUT to turn off the part, whereas it must be tied to 2.5V above –VOUT for normal operation.
Conclusion
Step-down μModule regulators, such as the LTM8025, can be easily configured for negative output voltages. For negative outputs, the LTM8025 operates as an inverting buck-boost, so the maximum allowable output current is lower than typical buck topologies. If synchronization is desired, proper level-shifting circuitry is required. For a complete description of the LTM8025, including operation and applications information, refer to the data sheet.
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