The TPS7H4011 can be configured as an
inverting buck-boost in order to create a negative output voltage as shown in Figure 9-5.
Additional considerations for
designing an inverting buck-boost are described in the application note, Working With
Inverting Buck-Boost Converters. While many details and equations are
provided within the application note, a few considerations for the TPS7H4011 are as
follows:
- Ensure the recommended
maximum input voltage of 14V is followed. This means VIN + |VOUT| ≤ 14V. For
example, an inverting buck-boost configured from 5V to –5V is acceptable
(10V differential) but 12V to –12V would not be acceptable (24V
differential).
- Be sure to select the correct
current limit for ILIM. The average inductor current for an inverting
buck-boost is greater than the load current. This may result in higher peak
currents than expected when compared to a buck converter. Additionally, this
means that the average inductor current must be kept lower than the
TPS7H4011 recommended maximum of 12A; therefore, the maximum output current
available to the load must always be under 12A.
- CIO in the Figure 9-5 is the standard input capacitor that would be utilized in a buck
converter. CIN is an input capacitor with respect to system
ground which provides a low impedance path at the regulator input.
- Be sure that device logic
input pins such as EN and FAULT never exceed the recommended maximum rating
of 7V. For example, if EN or FAULT was driven to 5V from an external source
and the inverting buck-boost is configured for a –5V output, this would
apply 10V to EN (with respect to the device GND pin) which would exceed the
rating. Take care with selecting the input voltage signals to avoid this
condition. Alternatively, logic shift the signals so they are referenced
with respect to -VOUT (which is the device GND pin).