Other Parts Discussed in Thread: TPS629206, , TPS629210, TPS629203, TPS62122
The TPS62120 or TPS62122 configured as 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/TPS62122 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. Mode detail 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 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 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.