Refer to Figure 10-15. The TPS2583x-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 Switching Characteristics for MOSFET hiccup timing.
• Buck average current limit: No MOSFET, CSN/OUT connected to BUS. For TPS25831-Q1, the LS_GD pin can be left floating. For TPS25830-Q1, the LS_GD pin 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 two level current limit is described below:

• With external MOSFET Figure 10-16, Figure 10-17:

  • Isolating a fault on the USB port from other loads connected to the CSP output of the TPS2583x-Q1. In some applications, it may be useful to power additional circuitry (example USB HUB) from the output of the TPS2583x-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 case, the recommended voltage across R_SNS under current limit should 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	Buck Average
    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 10-18, Figure 10-19:

  1. CSN/OUT connected directly to BUS. For TPS25831-Q1, LS_GD pin can be floating. For TPS25830-Q1, LS_GD pin must be pulled up through a 2.2-kΩ resistor. The TPS2583x-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. Typical R_ILIMIT resistors value are listed in Table 10-2 given the condition R_SNS= 15 mΩ and R_SET= 300 Ω

Figure 10-16 TPS25830-Q1 Current Limit with External MOSFET

Figure 10-18 TPS25830-Q1 Buck Average Current Limit

Figure 10-17 TPS25831-Q1 Current Limit with External MOSFET

Figure 10-19 TPS25831-Q1 Buck Average Current Limit