PD70101 datasheet(7/24 Pages) MICROCHIP

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

another way, the VPNOUT to VPNIN differential voltage. Two modes are defined below:

Table 1-1. Isolation Switch Modes

VPNOUT to VPNIN

Mode

Description

>0.7 V

Soft Start Mode

Limits VPNOUT current to 240 mA (typical)

≤0.7 V

Normal Operating Mode

Limits VPNOUT current to 1.8 A (typical)

By controlling the MOSFET current based on VPNOUT to VPNIN 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 the following figure.

Figure 1-3. Startup Comparison

Once bulk capacitance has charged up to a point where VPNOUT to VPNIN 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.

PD70100 and PD70200 ICs are different in their respective isolation MOSFET’s continuous current handling

capability:

• PD70100: 450 mA (max.)

• PD70200: 1123 mA (max.)

An adequate heat sink for the PD70100/PD70200 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 is

recommended. Electrically the exposed pad ground plane should be connected to VPNIN.

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

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

1.1.5

Over-Current Protection

Over-current protection is provided on the PD70100/PD70200 IC using the Isolation MOSFET Switch, which limits

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

PD70100/PD70200

Functional Descriptions

Datasheet

DS00003639A-page 7

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