6.

Theory of Operation

The PD70101/PD70201 device theory of operation is described in the following sections.

The PD70101/PD70201 IC integrates IEEE 802.3af/at compliant PD front-end functions including Detection, Physical

Layer Classification, Two-events Classification (PD70201 only), Power Good, Soft Start Current Limiting, Over-

Current Protection, and Bulk Capacitor Discharge with a PWM controller. The integrated PWM controller function

provides a PWM controller solution with a minimum requirement of external components.

6.1

Detection

IEEE 802.3af/at-compliant detection is provided by means of a 24.9 KΩ resistor connected between VPP and RDET

pin. RDET pin is connected to VPN_IN via an open drain MOSFET with a maximum specified RDSON of 50 Ω.

Internal logic monitors VPP to VPN_IN and connects the RDET pin to VPN_IN when the rising VPP to VPN_IN

voltage is between 1.1 V and 10.1 V. When rising VPP to VPN_IN voltages exceed 10.1 V, the MOSFET is switched

off. Once above 10.1 V, falling VPP to VPN_IN voltage between 2.45 V and 4.85 V will reconnect RDET pin to

VPN_IN.

6.2

Physical Layer Classification

Physical Layer (hardware) Classification per IEEE 802.3af/at is generated via a regulated reference voltage of 1.2 V,

switched onto the RCLASS pin. Internal logic monitors the VPP to VPN_IN voltage and connects the 1.2 V reference

to RCLASS pin at a rising VPP to VPN_IN voltage threshold between 11.4 V and 13.7 V. Once VPP to VPN_IN has

exceeded the rising threshold, there is a 1 V typical hysteresis between the VPP rising (turn-on) threshold and the

VPP falling (turn-off) threshold.

The1.2 V reference stays connected to the RCLASS pin until the VPP to VPN_IN rising voltage exceeds the upper

turn-off threshold of 20.9 V to 23.9 V. The 1.2 V reference voltage is disconnected from the RCLASS pin at VPP to

VPN_IN voltages above the upper threshold.

Classification current signature is provided via a resistor connected between RCLASS pin and VPN_IN. The

classification current is therefore the current drawn by the PD70101/PD70201 IC during the classification phase, and

is simply the 1.2 V reference voltage divided by the RCLASS resistor value. The maximum current available at the

RCLASS pin is current limited to 68 mA (typical).

6.3

Two-Events Detection and AT Flag

The PD70201 IC provides IEEE 802.3at Type 2 compliant detection of the two-events classification signature, and

generation of the AT flag. This feature is available on the PD70201 IC only.

The two-events classification signature is a means by which an IEEE 802.3at Type 2 Power Source can inform a

compliant PD that it is AT Type 2 compliant, and as such is capable of providing AT Type 2 power levels.

The Power Source communicates the Type 2-compliant signature by toggling the VPP to VPN_IN voltage twice (2

events) during the Physical Layer Classification phase. The VPP to VPN_IN voltage is toggled from the Physical

Layer Classification’s voltage level (13.5 V to 20.9 V) down to a voltage Mark level. Voltage Mark level is specified as

a VPP to VPN_IN voltage of 4.9 V to 10.1 V.

PD70201 IC recognizes a VPP to VPN_IN falling edge from Classification level to Mark level as being one event of

the Two-Events Signature. If two such falling edges are detected, PD70201 will assert with 80 ms delay AT flag by

means of an open drain MOSFET connected between AT_FLAG pin and VPN_OUT.

AT_FLAG pin is active low; a low impedance state between AT_FLAG and VPN_OUT indicates a valid Two- Events

Classification Signature was received, and the Power Source is AT Type 2 compliant.

AT_FLAG MOSFET is capable of 5 mA of current and can be pulled up to VPP.

PD70101 and PD70201

Theory of Operation

Datasheet

DS00003738A-page 21