The TPS62120 is a synchronous buck dc-to-dc converter designed for low-power applications. It features a wide operating input voltage range from 2 V to 15 V, up to 96% efficiency, and 75-mA output current. This device family is well-suited for many applications such as ultra low-power microprocessors, energy harvesting, and low-power RF applications. Even though this application note describes the TPS62120 in an inverting buck-boost topology for use in low current negative rails for operational amplifier biasing and other low-power applications. This document also applies to the TPS62122.
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The TPS62120 for inverting buck-boost application is very risky. We strongly recommend using our new generation buck converter TPS629203 or -Q1 family (including TPS629206 or -Q1 and TPS629210 or -Q1) instead of the TPS62120 for inverting buck-boost applications. The TPS629203 family not only has a higher current limit threshold, but most importantly, it does not require the inductor current to fall to zero before starting a new switching cycle. For example TPS629203 family has 0.9A typ low-side current limit threshold, thus, the device will continue to switch as long as the DC bias current of downstream circuitry is below this 0.9A typ threshold. More detailed information for TPS629203 inverting buck-boost application can be found in Using the TPS629210-Q1 in an Inverting Buck-Boost Topology
The TPS62120 integrates a high-side MOSFET current limit ILIMF to protect the device against over current or short circuit fault. The current in high-side MOSFET is monitored by current limit comparator and once the current reaches the limit of ILIMF, the high-side MOSFET is turned off and low-side MOSFET is turned on to ramp down the inductor current. The high-side MOSFET is turned on again once zero current comparator trips and the inductor current has become zero.
The inverting buck-boost application is most commonly used to drive differential (+V/-V) rails. At some scenarios, the downstream devices become active as soon as the input voltage is present, their Iq current feeds into the negative (-V) rail even if the negative (-V) rail is not being enabled. There is a positive DC bias voltage is likely existed on the negative (-V) rail, then TPS62120 is more prone to get stuck as it may never see a zero-crossing current while getting into an over current fault due to this positive DC bias voltage.
A possible workaround solution for TPS62120 in inverting buck-boost application: Adjust the system power up sequence to prevent the downstream devices from loading the negative (-V) rail prior to the negative rail being enabled. That means to enable the negative rail (-V) first and then enable other downstream devices. Otherwise TPS62120 likely gets stuck during power up.
The inverting buck-boost topology is very similar to the buck topology. In the buck configuration shown in Figure 1-1, the positive connection (VOUT) is connected to the inductor and the return connection is connected to the integrated circuit (IC) ground. However, in the inverting buck boost configuration shown in Figure 1-2, the IC ground is used as the negative output voltage pin (labeled as –VOUT ). What used to be the positive output in the buck configuration is used as the ground (GND). This inverting topology allows the output voltage to be inverted and is always lower than the ground.
The circuit operation is different in the inverting buck boost topology than in the buck topology. Figure 1-3 (a) illustrates that the output voltage terminals are reversed, though the components are wired the same as a buck converter. During the on time of the control MOSFET, shown in Figure 1-3 (b), the inductor is charged with current while the output capacitor supplies the load current. The inductor does not provide current to the load during that time. During the off time of the control MOSFET and the on time of the synchronous MOSFET, shown in Figure 1-3 (c), the inductor provides current to the load and the output capacitor. These changes affect many parameters described in the upcoming sections.