8.2.2.9.1 Output Voltage Limitations
Due to the internal design of the TPS54218, there are limitations to the minimum and maximum achievable output voltages. The output voltage can never be lower than the internal voltage reference of 0.8 V. Above 0.8 V, the output voltage can be limited by the minimum controllable on-time. The minimum output voltage in this case is given by Equation 35. There is also a maximum achievable output voltage which is limited by the minimum off-time. The maximum output voltage is given by Equation 36. These equations represent the results when the power MOSFETs are matched. Refer to the Understanding Output Voltage Limitations of DC/DC Buck Converters Technical Brief for more information.
Equation 35.
where
- VOUT(min) is the minimum achievable output voltage
- tON(min) is the minimum controllable on-time (110 nsec typical)
- fSW(max) is the maximum switching frequency including tolerance
- VIN(max) is the maximum input voltage
- IOUT(min) is the minimum load current
- RLS(min) is the minimum low-side MOSFET on-resistance (30 mΩ typical)
- RDCR is the series resistance of output inductor
Equation 36.
where
- VOUT(max) is the maximum achievable output voltage
- tOFF(max) is the maximum, minimum controllable off-time (60 ns typical)
- fSW(max) is the maximum switching frequency including tolerance
- VIN(min) is the minimum input voltage
- IOUT(max) is the maximum load current
- RHS(max) is the maximum high-side MOSFET on-resistance (70 mΩ max)
- RDCR is the series resistance of output inductor