Refer to Figure 10-14. The TPS2584x-Q1 can establish current limit by two methods.

• Using external a single or back-to-back N-Channel MOFETs between CSN/OUT and BUS: a voltage of 0.49 V on the ILIMIT pin initiates current limiting using the external MOSFET by decreasing the LS_GD voltage causing the FET to operate in the saturation region. To protect the MOSFETs from damage a hiccup timer limits the duty cycle to prevent thermal runaway. Refer to the Specifications for MOSFET hiccup timing.
• Buck average current limit: no MOSFET, CSN/OUT connected to BUS. The LS_GD must be pulled up through a 2.2-kΩ resistor. In this configuration, a voltage of 1 V across R_ILIMIT on the ILIMIT pin initiates average current limiting of the buck regulator.

The 2-level current limit is described below:

• With external MOSFET Figure 10-15:

  • Isolating a fault on the USB port from other loads connected to the CSP output of the TPS2584x-Q1. In some applications, it can be useful to power additional circuitry (for example: USB HUB) from the output of the TPS2584x-Q1 and maintain operation of these circuits in the event of a short circuit downstream of the BUS pin. To prevent triggering the MOSFET current limit below the programmed ILIMIT threshold, external circuits must be supplied after the inductor and before the current sense resistor, R_SNS.

  • After R_SNS and R_SET are determined and the full load I_SET current is known, the resistor value, R_ILIMIT, can be determined by:

    Equation 6.

  • In most cases, the recommended voltage across R_SNS under current limit must be approximately 50 mV as a compromise between accuracy and power dissipation. While in some application, R_ILIMIT is the only resistor that can be changed to achieve different current limit. Typical R_ILIMIT resistors value are listed in Table 10-2 given the condition R_SNS = 15 mΩ and R_SET = 300 Ω

    Table 10-2 Setting the Current Limit with R_ILIMIT

    Current-Limit Threshold (mA)	RILIMIT (kΩ)
    	With External MOSFET	Without External MOSFET
    700	26.1	53.6
    1500	12.7	26.1
    1700	11.3	22.6
    2700	7.15	14.7
    3000	6.49	13
    3400	5.62	11.5
    3800	5.11	10.5

• Buck Average Current Limit Figure 10-16:

  1. CSN/OUT connected directly to BUS, LS_GD must be pulled up through 2.2-kΩ resistor. The TPS2584x-Q1 can operate as a stand-alone USB charging port. In this configuration, the internal buck regulator operates with average current limiting as programmed by the ILIMIT pin, potentially producing less heat compared to N-channel MOSFET current limiting

  2. After R_SNS and R_SET are determined and the full load I_SET current is known, the resistor value R_ILIMIT can be determined by:

     Equation 7.

  3. Table 10-2 lists Typical R_ILIMIT resistors values given the condition R_SNS = 15 mΩ and R_SET = 300 Ω.

Figure 10-15 Current Limit With External MOSFET

Figure 10-16 Buck Average Current Limit