The TPSM365R6 is a 3.5 × 4.5 mm2 0.6-A rated synchronous step-down power module that features a wide operating input range from 3.6 V to 65 V with an adjustable output voltage range from 1 V to 13 V. The TPSM365R6 can be configured into an inverting buck-boost (IBB) topology with the output voltage inverted with respect to input voltage. This application report shows how the TPSM365R6 EVM board can be configured for an inverting application. Additional level-shifter circuitry for the EN and PGOOD pins are also included in this application note.
All trademarks are the property of their respective owners.
In a standard buck configuration, the positive connection VOUT, is connected to the internal inductor, and the return connection is connected to the device ground.
In the IBB configuration, SYS_GND is connected to TPSM365R6's VOUT pin and the device return is now the negative output voltage, -VOUT. This shift in topology allows the output voltage to be inverted with respect to the input voltage. Pins connected to the TPSM365R6 IC GND in the standard buck configuration can still be referenced to the same pin in the inverting buck boost configuration. However, to use the EN and PGOOD pins, an appropriate level shifter must be added, which are detailed in the sections Section 5.1 and Section 5.2.
By changing the buck configuration into an IBB configuration, the average inductor current is affected. The output current capability in the IBB topology is less than the buck configuration. As a result, the maximum achievable output current is calculated using Equation 1.
The operating duty cycle for an inverting buck-boost converter can be found with Equation 2.
The efficiency term in Equation 2 adjusts the equations in this section for power conversion losses and yields a more accurate maximum output current result. This efficiency is found by averaging the efficiency of the TPSM365R6 buck in its standard configuration. Use Equation 1 and Equation 2 to calculate the recommended maximum output current. For example, with an input voltage of 24 V, an output voltage of –12 V, and an estimated efficiency of 0.9, the duty cycle will be calculated with Equation 2 as:
The result of Equation 4 is then used to calculate the maximum achievable output current with Equation 1:
Table 1-1 provides a general idea of the maximum output current allowed from the TPSM365R6.
VOUT (V) | VIN (V) | IL_max (A) | η | D | IOUT (A) |
---|---|---|---|---|---|
–1.8 | 24 | 0.65 | 0.6 | 0.111 | 0.58 |
–2.5 | 24 | 0.65 | 0.7 | 0.130 | 0.57 |
–3.3 | 24 | 0.65 | 0.7 | 0.164 | 0.54 |
–5 | 24 | 0.65 | 0.8 | 0.210 | 0.52 |
–12 | 24 | 0.65 | 0.9 | 0.357 | 0.42 |
In the standard configuration, the TPSM365R6 IC can tolerate a 65 V difference between its VIN and IC GND pins. In an IBB, however, TPSM365R6 IC GND is referenced to the negative output voltage pin, –VOUT, see Figure 1-1. The TPSM365R6 will experience a potential difference of VIN to –VOUT between its VIN and TPSM365R6 IC GND pins, which effectively limits the input voltage range of the TPSM365R6 module in its inverting buck-boost configuration. For example, for an output voltage of –12 V, the maximum input voltage will be 53 V, as this results in a 65 V potential difference between the TPSM365R6 IC's VIN and GND pins. The output voltage range in the TPSM365R6 IBB topology is –1.8 V to –13 V.