![](http://datasheet.mmic.net.cn/ON-Semiconductor/NCP1092DBRG_datasheet_99124/NCP1092DBRG_7.png)
NCP1090, NCP1091, NCP1092
http://onsemi.com
7
Description of Operation
Powered Device Interface
The integrated PD interface supports the IEEE 802.3af
defined operating modes: detection signature, current
source classification, undervoltage lockout, inrush and
operating current limits. The following sections give an
overview of these previous processes.
Detection
During the detection phase, the incremental equivalent
resistance seen by the PSE through the cable must be in the
IEEE 802.3af standard specification range (23.70 k
W to
26.30 k
W) for a PSE voltage from 2.7 V to 10.1 V. In order
to compensate for the nonlinear effect of the diode bridge
and satisfy the specification at low PSE voltage, the
NCP1090/91/92 present a suitable impedance in parallel
with the 24.9 k
W Rdet external resistor. For some types of
diodes (especially Schottky diodes), it may be necessary to
adjust this external resistor.
The Rdet resistor has to be inserted between VPORTP and
DET pins. During the detection phase, the DET pin is pulled
to ground and goes in high impedance mode (opendrain)
once the device exit this mode, reducing thus the current
consumption on the cable.
Classification
Once the PSE device has detected the PD device, the
classification process begins. In classification, the PD
regulates a constant current source that is set by the external
resistor RCLASS value on the CLASS pin. Figure
4 shows
the schematic overview of the classification block. The
current source is defined as:
Iclass +
9.8 V
Rclass
Figure 4. Classification Block Diagram
CLASS
VPORTP
1.2 V
EN
Class_enable
VPORTP
VPORTN
9.8 V
Power Mode
When the classification handshake is completed, the
PSE and PD devices move into the operating mode.
Under Voltage Lock Out (UVLO)
The NCP1090/91/92 incorporate a fixed under voltage
lock out (ULVO) circuit which monitors the input voltage
and determines when to turn on the pass switch and charge
the dcdc converter input capacitor before the power up of
the application.
The NCP1091 offers a fixed or adjustable Vuvlo_on
threshold depending if the UVLO pin is used or not. In
Figure
5, the UVLO pin is strapped to ground and the
Vuvlo_on threshold is defined by the internal level.
Figure 5. Default Internal UVLO Configuration
(NCP1091 only)
UVLO
VPORTP
VPORTN1,2
VPORT
To define the UVLO threshold externally, the ULVO pin
must be connected to the center of an external resistor
divider between VPORTP and VPORTN as shown in
In order to guarantee the detection signature, the
equivalent input resistor made of the Ruvlo1, Ruvlo2 and
Rdet should be equal to 24.9 k
W.
UVLO
VPORTN1,2
VPORT
Ruvlo2
Ruvlo1
VPORTP
NCP1091
DET
Rdet
Figure 6. Default Internal UVLO Configuration
(NCP1091 only)
For a Vuvlo_on desired turnon voltage threshold,
Ruvlo1 and Ruvlo2 can be calculated using the following
equations:
Ruvlo +
24.9 k @ Rdet
Rdet * 24.9 k
Ruvlo1 ) Ruvlo2 + Ruvlo
with
Ruvlo2 +
1.2
Vuvlo_on @
Ruvlo
and
With:
Vuvlo_on: Desired TurnOn voltage threshold