Datasheet

12Maxim Integrated
MAX5988A/MAX5988B
IEEE 802.3af-Compliant, High-Efficiency, Class 1/Class 2,
Powered Devices with Integrated DC-DC Converter
Detailed Description
PD Interface
The MAX5988A/MAX5988B include complete interface
functions for a PD to comply with the IEEE 802.3af stan-
dard as a Class 1/Class 2 PD. The devices provide the
detection and classification signatures using a single
external signature resistor. An integrated MOSFET pro-
vides isolation from the buck converter when the PSE has
not applied power. The devices guarantee a leakage cur-
rent 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, classifi-
cation, and power-on/-off modes.
Operating Modes
The devices operate in three different modes depending
on V
DD
. The three modes are detection mode, classifica-
tion mode, and power mode. The device is in detection
mode when V
DD
is between 1.4V and 10.1V, classifica-
tion 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 minimum 1V
increment. The PSE computes the differential resistance
to ensure the presence of the 24.9kω signature resis-
tor. 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 signifi-
cantly affect the signature resistance measurement.
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 classifi-
cation current at 10.5mA (Class 1, CLASS1 = GND) or
18mA (Class 2, CLASS2 = VDRV). The PSE uses this to
determine the maximum power to deliver. The classifica-
tion current includes current drawn by the supply current
of the device so the total current drawn by the PD is with-
in 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 devices have the
buck regulator enabled and the LDO enabled. The
devices can be in either wake mode, sleep mode, or
ultra-low-power mode. The buck regulator and LDO are
only enabled 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-chan-
nel 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 isola-
tion MOSFET is fully turned on, the device changes the
current limit to 321mA (typ). 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 38.8V (typ), and
a turn-off UVLO threshold (V
OFF
) at 31.5V (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
The devices 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 modu-
lated 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.