The most common use for a synchronous buck controller is high-efficiency conversion of a positive voltage to a lower positive voltage. But it can also produce a negative voltage from a positive voltage. In negative output applications, a buck controller can be configured as an inverting buck/boost device, where the negative output voltage has an absolute value either higher or lower than its positive input.
To transform a buck converter to a buck/boost, simply reference the circuit to the negative rail instead of ground, tie the (+) end of Cout to ground instead of Vout, and connect the input voltage from the drain of the top MOSFET to the new ground (Fig. 1). The hookup is otherwise the same as a standard positive buck configuration, and the top MOSFET is still the control MOSFET.
The design requirements for a buck/boost converter are, however, more demanding. For instance, the off-state voltage stress of the MOSFETs for a buck/boost configuration is higher for a given input voltage. That's because it's now equal to the difference between Vin and Vout, which is below ground. Also, the dc inductor current is higher for a given load since it's now equal to the sum of the load current and input current. As a result, the inductor must have a higher saturation-current rating and lower DCR than required for a standard buck design.
Due to the higher voltage stress and dc inductor current, the transition and conduction losses of the MOSFETs are higher. Because the output capacitor for a buck/boost converter is only recharged when the bottom MOSFET is on, the output capacitor has pulsed current flowing through it, where the peak-to-peak magnitude is equal to the dc inductor current (assuming a very large inductor is used).
On the other hand, the output-capacitor ripple current for a buck converter is only equal to the inductor ripple current. As a result, the output capacitors used for a buck/boost design need to have much lower ESR and ESL to maintain low output-voltage ripple.
Figure 2 shows an example of an inverting buck/boost circuit that meets the above challenges. This circuit is a +12-V to 5.2-V/15-A dual-phase dual-output converter controlled by Linear Technology's LTC3728. Both phases of the converter are tied together and separated by 180°, which provides ripple-current cancellation for both the output and input capacitors. The combination of dual-phase operation, 400-kHz switching frequency, and low-ESR and low-ESL ceramics in parallel with POSCAPs at the output yields an output-voltage ripple of only 39 mV p-p. The low output-voltage ripple suits it for biasing negative ECL circuits.
Despite the higher losses of a buck/boost configuration, this circuit does have high efficiency. At full load, the efficiency is 91.4% and the peak efficiency is 92.9%. The high efficiency is the result of using low RDS(ON) and low QG MOSFETs, using two phases instead of one, and the LTC3728's strong gate drivers. The switch node pin and VIN pin of the LTC3728 are both rated at 36 V, and the MOSFETs are rated at 30 V, which allows them to easily handle the stress created by converting +12 V to 5 V.
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