6.6

Power Good

During Soft Start mode, the PD70101/PD70201 IC monitors VPN_OUT to VPN_IN differential voltage. When this

voltage is less than 0.7 V (maximum), the IC enters normal operation mode and the isolation switch current limit is

increased to 1.8 A (typical). At this same 0.7 V (maximum) threshold the Power Good signal is asserted by means of

an open drain MOSFET between PGOOD and VPN_OUT.

PGOOD pin is active low; a low impedance state between PGOOD and VPN_OUT indicates the Soft Start mode has

finished and the isolation switch has transitioned into normal operating mode.

PGOOD MOSFET can handle current of 5 mA and can be pulled up to VPP.

The application load should begin after no less than 80 ms after PGOOD is activated according to IEEE 802.3.

6.7

Start-Up Supply

PD70101/PD70201 IC provides a 10.5 V (typical) regulated output used as a start-up supply for the integrated

DC/DC controller when VCC is provided via a bootstrap winding. This regulated supply is available at VAUX pin, and

is referenced to VPN_OUT pin. The VAUX start-up supply is current-limited at 10 mA (minimum).

For stability, the start-up regulator requires a minimum of 4.7 µF ceramic capacitor connected directly between VAUX

and PGND pins (most applications will connect PGND to VPN_OUT).

For applications where power to the DC-DC controller is provided by POE only, the VAUX pin is connected directly to

VCC. For applications which have alternate power sources (such as a wall adaptor), the VAUX pin output is

connected to the VCC pin through a series diode. This diode is typically a low current diode with a 30 V rating.

6.8

PD Interface Thermal Protection

Both PD70101 and PD70201 IC contain temperature sensors which individually monitor both the isolation MOSFET

and the classification current source for over temperature conditions. In case of an over temperature condition, the

sensor will activate protection circuitry which will disconnect its respective monitored function.

6.9

Bulk Capacitor Discharge

The bulk capacitor discharge circuitry eliminates the need to place a diode in series with the VPP line to prevent an

application’s bulk capacitance from discharging through the detection resistor and the isolation switch MOSFET’s

body diode. Discharge current through the detection resistor can cause failure of the detection signature in cases

where a PD is connected and the bulk capacitance is not fully discharged.

During normal operation, PD70101/PD70201 IC continuously monitors voltage at VPP to VPN_IN. Should VPP to

VPN_IN voltage fall below isolation switch turn-off threshold (31 V to 34 V), isolation switch MOSFET is immediately

placed in a high-impedance state. At this point the internal logic monitors the voltage at VPP to VPN_OUT.

If VPP to VPN_OUT voltage is between 1.5 V to 32 V, minimum 23 mA constant current source is connected across

the VPP and VPN_OUT pins. This constant current source provides bulk capacitor discharge.

A 220 µF bulk capacitance can be discharged from 32 V to 1.5 V in a maximum period of 292 ms.

6.10

DC-DC Start-Up

The DC-DC controller starts up when it receives the PGOOD high signal from the front-end, or ENABLE goes high

provided that VCC UVLO have passed. When the PGOOD signal or ENABLE goes high, the start- up sequence

begins with ramping up the SS pin from GND to 1.2 V. For isolated applications the output voltage may reach the

maximum level before the SS reaches 1.2 V, depending on the output loading condition. In applications with lighter

loads, the output reaches regulation level sooner than in heavy loads, as in this mode the SS voltage directly controls

the peak inductor current; hence the energy is delivered to the load. The external secondary error amplifier regulates

the output voltage and controls the peak inductor current via the opto-coupler across the isolation barrier. For non-

PD70101 and PD70201

Theory of Operation

Datasheet

DS00003738A-page 23