Datasheet

MAX5986A–MAX5986C/MAX5987A
IEEE 802.3af-Compliant, High-Efficiency, Class 1/
Class 2, PDs with Integrated DC-DC Converter
13Maxim Integrated
Detailed Description
PD Interface
The devices include complete interface functions for
a PD to comply with the IEEE 802.3af standard as a
Class 1/Class 2 PD. The devices provide the detection
and classification signatures using a single external
signature resistor. An integrated MOSFET provides
isolation from the buck converter when the PSE has
not applied power. The devices guarantee a leakage
current offset of less than 10µA during the detection
phase. The devices feature power-mode undervoltage-
lockout (UVLO) with wide hysteresis and long deglitch
time to compensate for twisted-pair-cable resistive drop
and to ensure glitch-free transitions between detection,
classification, and power-on/-off modes.
Operating Modes
The devices operate in three different modes depend-
ing on V
DD
. The three modes are detection mode,
classification mode, and power mode. The device is in
detection mode when V
DD
is between 1.4V and 10.1V,
classification mode when V
DD
is between 12.6V and 20V,
and power mode when the input voltage exceeds V
ON
.
Detection Mode (1.4V < V
DD
< 10.1V)
In detection mode, the devices provide a signature
differential resistance to V
DD
. During detection, the
power-sourcing equipment (PSE) applies two voltages
to V
DD
, both between 1.4V and 10.1V with a mini-
mum 1V increment. The PSE computes the differential
resistance to ensure the presence of the 24.9kω signature
resistor. Connect the 24.9kω signature resistor (R
SIG
)
from RREF to GND for proper signature detection. The
device applies V
DD
to RREF when in detection mode,
and the V
DD
offset current due to the device is less than
10µA. The DC offset due to protection diodes does not
significantly affect the signature resistance measure-
ment.
Classification Mode (12.6V < V
DD
< 20V)
In classification mode, the devices sink Class 1/Class 2
classification currents. The PSE applies a classification
voltage between 12.6V and 20V, and measures the clas-
sification currents. The devices use the external 24.9kω
resistor (R
SIG
) and the CLASS2 pin to set the classifica-
tion current at 10.5mA (Class 1) or 18.5mA (Class 2).
The devices provide either Class 1 (CLASS2 = GND) or
Class 2 (CLASS2 = VDRV). The PSE uses this to deter-
mine the maximum power to deliver. The classification
current includes current drawn by the supply current of
the device so the total current drawn by the PD is within
the IEEE 802.3af standard. The classification current is
turned off when the device leaves classification mode.
Power Mode (V
DD
> V
ON
)
In power mode, the devices have the isolation MOSFET
between V
DD
and V
CC
fully on. The MAX5987A has
the buck regulator enabled and the LDO enabled. The
MAX5986A–MAX5986C can be in either wake mode,
sleep mode, or ultra-low power mode. The buck regulator
is enabled when the MAX5986A–MAX5986C is in wake
mode.
The devices enter power mode when V
DD
rises above
the undervoltage lockout threshold (V
ON
). When V
DD
rises above V
ON
, the device turns on the internal
p-channel isolation MOSFET to connect V
CC
to V
DD
with inrush current limit internally set to 49mA (typ). The
isolation MOSFET is fully turned on when V
CC
is near
V
DD
and the inrush current is below the inrush limit.
Once the isolation MOSFET is fully turned on, the device
changes the current limit to 323mA. The buck converter
turns on 123ms after the isolation MOSFET turns on fully.
Undervoltage Lockout
The devices operate with up to a 60V supply voltage with
a turn-on UVLO threshold (V
ON
) at 35.4V/38.7V (typ),
and a turn-off UVLO threshold (V
OFF
) at 31.4V (typ).
When the input voltage is above V
ON
, the device enters
power mode and the internal isolation MOSFET is turned
on. When the input voltage is below V
OFF
for more than
t
OFF_DLY
, the MOSFET and the buck converter are off.
LED Driver (MAX5986A–MAX5986C)
The MAX5986A–MAX5986C drive an LED, or multiple
LEDs in series, with a maximum LED voltage of 6.5V.
In sleep mode and ultra-low power mode, the LED
current is pulse width modulated with a duty cycle of 25%
and the amplitude is set by R
SL
. The LED driver current
amplitude is programmable from 10mA to 20mA using R
SL
according to the formula:
I
LED
= 646/R
SL
(mA)
where R
SL
is in kω.