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Microsemi

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Rev.2.0, 17-Jan-2017

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PD70101 / PD70201

Datasheet

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 36V to 42V. 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 below:

Isolation Switch Modes

VPN_OUT to VPN_IN

Mode

Description

≥ 0.7V

Soft Start Mode

Limits VPN_OUT current to 240mA

(typical)

≤ 0.7V

Normal Operating Mode

Limits VPN_OUT current to 1.8A

(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 350mA, 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.

Figure 5 · 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 0.7V,

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

provide overcurrent protection.

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

capability:

PD70101: 450mA (max.)

PD70201: 1123mA (max.)

An adequate heatsink 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 31V to 34V.

10

20

30

40

50

60

VPD

[V]

T [ms]

Soft startup

Hard startup

Voltage sag!

DC

DC

Inrush

C

I

Slope

/

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