PDA-300 Reference Manual
January 21, 2014 Sifos Technologies Page
Type-2 PSE’s have the option to utilize Link Layer Discovery Protocol (LLDP) as an alternative to providing two-
event classification for the purpose of granting power levels beyond 13 watts to the Type-2 PD. Many Type-2 end-
span PSE’s will in fact choose to use LLDP instead of two-event classification because LLDP offers much better power
management granularity. LLDP is described below in section 1.5.
Type-2 PSE’s also have the freedom to utilize two-event classification with all PD’s, not just Type-2 PD’s. This is
because older 802.3af (Type-1) PD’s should be insensitive to the occurrence of a repeat class measurement preceding a
power-up.
1.4. Power Application & Overload Conditions
PSE’s will apply DC power to a PD only if PD Detection completes successfully and if the PSE has sufficient power
available as determined through PD Classification and/or knowledge of available power resources. Power is provided
with a DC voltage ranging from 44V to 57V at the PSE end of the connection. Because of resistive cable loss,
voltages present at the PD input can range from 37V to 57V. The PD must receive this DC power from either the
ALT-A or ALT-B pairs (see section 1.2) and accept
either positive or negative polarity.
During initial power-up, a PD may draw a surge current
to charge power supply capacitances. Surge loads must
be tolerated by the PSE for up to at least 50 msec
(Tinrush) so long as they do not exceed 400mA
(Iinrush_pd). PD’s are responsible for regulating
surge current to 400mA or less if they present very large
load capacitances. Capacitive charging “energy” can be
expressed as the product of power and time so a 48V
PSE supplying 400mA for 50msec furnishes 0.96 watt-
sec of charging energy to the PD.
Following the Inrush interval, PD’s are required to restrict average (or continuous) power draw to a level below
Pclass_pd(max), or the maximum power allowed to the specified PD Class. The PD may briefly exceed this power
level with load transients up to the value of Ppeak_pd(max), so long as those load transients do not exceed a duration
of Tcut_min, or 50msec. These requirements are depicted graphically in Figure 1.6. Values for Pclass_pd(max) and
Ppeak_pd(max) are shown in the table below.
A peak power violation occurs
when a PD instantaneously draws
more power than specified by
Ppeak_pd(max) for that PD
Class. A class power violation
occurs when a PD exceeds
Pclass_pd(max) for more than 50
msec (contiguous) or for greater
than 5% of time on average (see
Figure 1.6). Any violation may
lead to power removal by a PSE.
PSE’s are allowed very wide
discretion in monitoring and
reacting to class and peak power violations. Many Type-1 PSE’s will treat all Type-1 PD’s as Class 0 meaning Class 1
or 2 power violations are completely ignored. Many other PSE’s use more conservative thresholds than
Pclass_pd(max) and Ppeak_pd(max) meaning they will tolerate PD’s that over-draw power relative to their specified
classification. LLDP capable PSE’s may use granted power values for Pclass_pd(max).For these reasons, PSE’s
generally are not useful as test vehicles for assessing PD power compliance.
IEEE 802.3at places a special requirement on Type-2 PD’s that restricts initial
start-up power draw to Type-1 levels (13 watts or less) until either the PSE is
assured that the initial Inrush period is completed or until an LLDP
negotiation is completed. If the PSE provides two-event classification as a
means to grant Type-2 power to a Type-2 PD, then the PD must wait for at
least 80 msec (Tdelay) before stepping up to a power draw in excess of 13
watts (see Figure 1.7). If a Type-2 PSE does not provide two-event
classification, then the PD may need to wait up to tens seconds to start a link
layer LLDP negotiation with the PSE and subsequently may need to wait
indefinitely for a power grant from the PSE authorizing power draw in excess
Type-1 Pclass
Type-2 Pclass
Two-Event
Class
> 80msec
Inrush
Power-
Up 2-Event Grant
Response
Type-1 Pclass
Type-2 Pclass
Two-Event
Class
> 80msec
Inrush
Power-
Up 2-Event Grant
Response
Figure 1.7 Type-2 PD Start-Up
Figure 1.6 Powered PD Loading Requirements
Tcut_min
(50msec)
400mA
Inrush
Interval
(50msec)
Valid
Transient
Power-
Up
Ppeak_pd(max)
Pclass_pd(max)
Tcut_min
(50msec)
Ppeak
Violation
Pclass
Violation
Tcut_min
(50msec)
400mA
Inrush
Interval
(50msec)
Valid
Transient
Power-
Up
Ppeak_pd(max)
Pclass_pd(max)
Tcut_min
(50msec)
Ppeak
Violation
Pclass
Violation
