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參數(shù)資料
| 型號(hào)： | 2377-1 |
| 廠商： | Texas Instruments, Inc. |
| 英文描述： | IEEE 802.3af PoE POWERED DEVICE CONTROLLERS WITH AUTO-RETRY |
| 中文描述： | IEEE 802.3af標(biāo)準(zhǔn)的PoE用電設(shè)備控制器，自動(dòng)重試 |
| 文件頁數(shù)： | 12/23頁 |
| 文件大?。?/td> | 649K |
| 代理商： | 2377-1 |
www.ti.com
UNDERVOLTAGE LOCKOUT (UVLO)
The TPS2375-1 incorporates an undervoltage lockout (UVLO) circuit that monitors line voltage to determine
when to apply power to the downstream load and allow the PD to power up. The IEEE 802.3af specification
dictates a maximum PD turnon voltage of 42 V and a minimum turnoff voltage of 30 V ( Figure 17). The IEEE
802.3af standard assumes an 8-V drop in the cabling based on a 20-
feed resistance and a 400-mA maximum
inrush limit. Because the minimum PSE output voltage is 44 V, the PD must continue to operate properly with
input voltages as low as 36 V. The TPS2375-1 UVLO limits are designed to meet the turnon, turnoff, and
hysteresis requirements.
PROGRAMMABLE INRUSH CURRENT LIMIT AND FIXED OPERATIONAL CURRENT LIMIT
Inrush limiting has several benefits. First, it provides a mechanism to keep the inrush current below the 400 mA,
50 ms, maximum inrush allowed by the standard. Second, by keeping the current below the PSE operational
limit, which can be as low as the classification power divided by the PSE voltage, it allows an arbitrarily
large-bulk capacitor to be charged. Third, some legacy PSEs may not tolerate large inrush currents while
powering their outputs up.
PROTECTION OPERATION
TPS2375-1
TPS2377-1
SLVS570–MARCH 2005
APPLICATION INFORMATION (continued)
Various legacy PSE systems in the field do not meet the minimum output voltage of 44 V. The TPS2377-1 UVLO
limits are designed to support these systems with a lower turnon voltage and smaller hysteresis. Although the
TPS2377-1 works with compliant PSEs, it could potentially exhibit startup instabilities if the PSE output voltage
rises slowly. The TPS2375-1 is recommended for applications with compliant PSEs.
The TPS2375-1 uses the UVLO function to control the load through an integrated MOSFET switch. Figure 17
graphically shows the relationships of the UVLO thresholds defined in the Electrical Characteristics section to the
TPS2375-1 operational states.
The TPS2375-1 operational current limit protects the internal power switch from sudden output faults and current
surges. The minimum operational current limit level of 405 mA lies above the maximum PSE output current limit
of 400 mA. This current limit enables the PD to draw the maximum available power and also allows the PSE to
detect fault conditions. The IEEE 802.3af standard allows PDs to draw momentary currents up to 400 mA, which
can be prevented if the current limit is set lower.
The TPS2375-1 incorporates a state machine that controls the inrush and operational current limit states. When
V
is below the lower UVLO threshold, the current limit state machine is reset. In this condition, the RTN pin
is high impedance, and floats to V
(VDD)
once the output capacitor is discharged. When V
(VDD)
rises above the
UVLO turnon threshold, the TPS2375-1 enables the internal power MOSFET with the current limit set to the
value programmed by R
. The load capacitor charges and the RTN pin voltage falls from V
to nearly
V
(VSS)
. Once the inrush current falls about 10% below the programmed limit for 150-μs, the current limit switches
to the 450-mA operational level and PG goes open-drain. This switchover is seen in the operation of PG in
Figure 1. The internal power MOSFET is disabled if the input voltage drops below the lower UVLO threshold and
the state machine is reset
The inrush limit, bulk capacitor size, and downstream dc/dc converter startup method must be chosen so that the
converter input current does not exceed the inrush current limit while it is active. This is achieved by using the
PG output to enable the downstream converter after inrush finishes, by delaying the converter startup until inrush
finishes, or by increasing the value of the inrush current limit.
Practical values of R
lie between 62.5 k
and 500 k
; however, selecting lower inrush current values
reduces peak stresses under output-short circuit conditions. An inrush level of 140 mA, set by an R
(ILIM)
of 178
k
, is used with TPS2377 applications for compatibility with legacy systems. This same inrush current level is
recommended for most TPS2375 applications, and it should be kept below 250 mA if the application allows.
The TPS2375-1 protects itself, the load, and the PSE using a number of mechanisms while the input voltage is
above the lower UVLO. This operation is illustrated by considering the cases of soft overload, hard overload, and
input-voltage steps.
A soft overload is one that forces the internal MOSFET into current limit with V
less than 10 V. Provided
that the PSE does not respond to this event, the bulk capacitor recharges after a momentary overload, and
operation returns to normal. If the overload persists, the TPS2375-1 may overheat and go into thermal shutdown
12
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