7.1.1 Detailed Pin Description

The following descriptions refer to the pinout and the functional block diagram.

DRAIN1-DRAIN4: Port 1-4 output voltage monitor and detect sense. Used to measure the port output voltage, for port voltage monitoring, port-power-good detection and foldback action. Detection probe currents also flow into this pin. The TPS23861 uses an innovative 4-point technique in order to provide a reliable PD detection. Detection is performed by sinking two different current levels via the DRAINn pin, while the PD voltage is measured from VPWR to DRAINn. The 4-point measurement provides the capability to distinguish between an IEEE-standard-compliant PD and a capacitive or legacy load. If the Port n is not used, DRAINn can be left floating or tied to AGND.

GATE1-GATE4: Port 1-4 gate drive output used for external N-channel MOSFET gate control. At port turn on, it is driven positive by a low-current source to turn the MOSFET on. GATEn is pulled low whenever any of the input supplies are low or if an over-current timeout has occurred. GATEn will also be pulled low if its port is turned off during fast shutdown. Leave floating if unused. For a robust design, a current-foldback function limits the power dissipation of the MOSFET during low resistance load or a short-circuit event. The foldback mechanism measures the port voltage across AGND and DRAINn to reduce the current-limit threshold as shown in Figure 14, Figure 57, and Figure 58. The fast overload protection is for major faults like a direct short. This forces down the current within the current limit in less than a microsecond. When ICUT threshold is exceeded while a port is on, a timer starts. During that time, linear current limiting makes sure the current will not exceed ILIM combined with current-foldback action. When the timer reaches its t_OVLD (or t_START if at port turn on) limit, the port shuts off. When the port current goes below I_CUT , the counter counts down at a rate 1/16^th of the increment rate, and it must reach a count of zero before the port can be turned on again.

KSENSA, KSENSB: Kelvin point connection used to perform a differential voltage measurement across the associated current sense resistors. KSENSA is shared between SEN1 and SEN2, while KSENSB is shared between SEN3 and SEN4. In order to optimize the accuracy of the measurement, the PCB layout (see Figure 61) must be done carefully to minimize impact of PCB trace resistance.

SHTDWN: Shutdown, active low. This pin is internally pulled up to VDD, with internal 1-µs to 5-µs deglitch filter. The Port Power Priority register is used to determine which port(s) is (are) shut down in response to an external assertion of the SHTDWN pin. The turn-off procedure is similar to a port reset or a reset command (Reset register).

RESET: Reset input, active low. When asserted, the TPS23861 resets, turning off all ports and forcing the registers to their power-up state. This pin is internally pulled up to VDD, with internal 1-µs to 5-µs deglitch filter. External RC network can be used to delay the turn-on. There is also an internal power-on-reset which is independent of the RESET input.

SEN1- SEN4: Port 1-2 current sense input relative to KSENSA, and port 3-4 current sense relative to KSENSB. A differential measurement is performed using KSENSA and KSENSB Kelvin point connection. It monitors the external MOSFET current by use of either a 255-mΩ (two 510 mΩ in parallel) or a 250-mΩ (four 1 Ω in parallel) current-sense resistors connected to AGND. Used by current foldback engine and also during classification. Can be used to perform load current monitoring via A/D conversion.

For the current limit with foldback function, there is an internal 2-µs analog filter on the SEN1-4 pins to provide glitch filtering. For measurements through an A/D converter, an anti-aliasing filter is present on the SEN1-4 pins. This includes the port-powered current monitoring and disconnect. If the port is not used, tie SENn to AGND.

VDD: 3.3-V logic power supply input.

VPWR: High-voltage power supply input. Nominally 48 V.