The TPS2584x-Q1 provides an accurate current limit to protect the USB port from overload based upon the values of RSNS, RSET and RILIMIT. The design process is the same regardless of whether buck average current limiting or external NFET current limiting is chosen. The only difference is the current limit threshold voltage on the ILIMIT pin.
- RSNS is the current sense resistor. The recommended voltage across RSNS under current limit must be approximately 50 mV as a compromise between accuracy and power dissipation. For example, if current limiting is desired for IOUT(MAX) ≥ 3.3 A, then RSNS = 0.05 V / 3.3 A = 0.01515 Ω. Choose a standard value of 15 mΩ.
- RSET determines the input current to the transconductance amplifier
and current mirror. The amplifier balances the voltage to be equal to that
across RSNS. Choose a RSET value to produce an
ISET current between 75 –180 µA at the desired
IOUT(MAX). Considering 50 mV across RSET, a value of
300 Ω provides approximately 166 µA of ISET current to the amplifier
and mirror circuit. Care must be taken to limit the ISET current
below 200 µA to avoid saturating the internal amplifier circuit.
- Buck average current limiting occurs when VILIMIT = 1 V.
RILIMIT is calculated as 1 V × 300 Ω / [ 0.5 × (3.3 A × 15 mΩ +
0.7 mV) ] = 11.95 kΩ. A standard 11.8 kΩ value is chosen.