6.4

Soft Start and Inrush Current Protection

PD70101/PD70201 IC contains an internal isolation switch that provides ground isolation between power source and

PD application during Detection and Classification phases. The isolation switch is a N- channel MOSFET, wired in a

common source configuration where the MOSFET’s Source is connected to Power Source ground at VPN_IN, and

the MOSFET’s Drain is connected to application’s primary ground at VPN_OUT.

Internal logic monitors VPP to VPN_IN voltage and keeps the MOSFET in a high impedance state until VPP to

VPN_IN voltage reaches turn-on threshold of 36 V to 42 V. Once VPP to VPN_IN voltage exceeds this threshold, the

MOSFET is switched into one of two modes.

Mode into which the MOSFET is switched is determined by the voltage developed across the MOSFET, or put

another way, the VPN_OUT to VPN_IN differential voltage. Two modes are defined as in the following table.

Table 6-1. Isolation Switch Modes

VPN_OUT to VPN_IN

Mode

Description

≥0.7 V

Soft start

Limits VPN_OUT current to 240 mA (typical)

≤0.7 V

Normal operating

Limits VPN_OUT current to 1.8 A (typical)

By controlling the MOSFET current based on VPN_OUT to VPN_IN voltage, inrush currents generated by fully

discharged bulk capacitors can be limited. This method limits current to a maximum of 350 mA, compliant with IEEE

802.3af/at specification.

Soft Start current limiting is required to reduce occurrences of voltage sag at the PD input during device power-up. A

comparison is shown in figure 3 (see page 7).

Figure 6-1. Comparison of Input Voltages without Soft-start (Hard Startup) and Soft-start (Soft Startup)

Once bulk capacitance has charged up to a point where VPN_OUT to VPN_IN differential voltage is less than 0.7 V,

the isolation MOSFET is switched into normal operating mode with MOSFET current limit set at 1.8 A (typical), to

provide over-current protection.

PD70101 and PD70201 ICs are different in their respective isolation MOSFET’s continuous current handling

capability. The maximum isolation MOSFET's continuous current handling capability is 450 mA for PD70101 and

1123 mA for PD70201.

An adequate heat sink for the PD70101/PD70201 IC’s exposed pad must be provided to achieve these current levels

without damaging the IC. A large, heavy copper fill area and/or a heavy ground plane with Thermal vias are

recommended.

Internal logic monitoring VPP to VPN_IN will place the isolation switch MOSFET in a high impedance state if voltage

between VPP and VPN_IN drops below 31 V to 34 V.

6.5

Over-Current Protection

An over-current protection is provided on the PD70101/PD70201 IC using the Isolation Switch MOSFET, which limits

the VPN_OUT current to 1.8 A during normal operation. See previous description of Soft Start.

PD70101 and PD70201

Theory of Operation

Datasheet

DS00003738A-page 22