Part Number:TPS54331
Tool/software:TINA-TI or Spice Models
PROBLEM
Buck converter design was made to operate with minimum input voltage of 5V. However, the lower limit is showing bad results on the startup of the buck converter TPS54331 (datasheet).
OBSERVATIONS
Figure 1 shows part of the schematic with the problematic circuit.
- Buck converter is not starting when super capacitor is charged. It was observed a relation between super capacitors voltage Supercap and input voltage, when:
- Supercap voltage 3.50V, buck startup Vin 5.2V.
- Supercap voltage 2.90V, buck startup Vin 5.05V.
- Supercap voltage 2.50V, buck startup Vin 4.70V.
- Issue is more likely to happen at 85°C.
- The board starts at 5.6V when the super capacitors are with 3.22V.
The circuit was simulated with different parameters using TINA Software. However, none of them worked fine, and, the IC is not working with 5V input voltage, it starts the operation at 5.84V on the software.
Using the online tool to select power supply in TI website (WEBENCH) they recommend the use of TPS54331, for this very same application.
START UP ONLY
When applied 7V to the input and this voltage is slowly decreased, buck converter keeps operating until the external undervoltage diables the EN pin, at 4.45V.
ENABLE PIN
There is a recommendation to use external VIN UVLO to add hysteresis when VIN is lower than Vout + 2V. However, as we are applying 2.8V directly through an inverter port to this pin, the IC is activated.
VOUT LIMITATIONS
Since the converter has a limit on minimum and maximum duty cycle. These limit values are calculated and they are within the duty cycle range.
RL=35mΩ
VD=0.7V
IOmax=Iomin=3ARds(on)max = 200mΩ
VIN(MIN)=5V
VO(MAX)= 3.85V
The Maximum Output Voltage (VO(MAX)) of 3.85V is the higher output voltage that the IC can deliver with a 5V input voltage. So, 3.52V is within the output voltage range and it is ok.
Temperature might cause some variations on these resistance and diode drop, however, the circuit was not tested with this maximum output current (IOmax), but 300mA, which will result in a VO(MAX) of 4.44V.