参数资料
| 型号： | LTC4258CGW#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 9/28页 |
| 文件大小： | 0K |
| 描述： | IC POE 802.3AF QUAD 36-SSOP |
| 标准包装： | 1,000 |
| 控制器类型： | 以太网供电控制器（POE） |
| 接口： | IEEE 802.3af |
| 电源电压： | 3 V ~ 4 V |
| 电流 - 电源： | 2.5mA |
| 工作温度： | 0°C ~ 70°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 36-BSOP（0.295"，7.50mm 宽） |
| 供应商设备封装： | 36-SSOP |
| 包装： | 带卷 (TR) |
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LTC4258
17
4258fb
Dual-Level Current Limit
A PD is permitted to draw up to 15.4W continuously and
up to 400mA for 50ms. The LTC4258 has two correspond-
ing current limit thresholds, ICUT (375mA typ) and ILIM
(425mA typ). These are given by the equations:
ICUT = VCUT/RS, ILIM = VLIM/RS
RS is the sense resistor and should be 0.5 for IEEE
802.3af compliance. While the LTC4258 allows the port
current to exceed ICUT for a limited time period (see tICUT
timing below), it does not allow the current to exceed ILIM.
The current limit circuit monitors the port current by
monitoring the voltage across the sense resistor and re-
duces the MOSFET gate voltage as needed to keep the
current at or below ILIM. When the current drops below
ILIM, the gate voltage is restored to the full value to keep
the MOSFET resistance to a minimum.
tICUT Timing
Whenever more than ICUT = VCUT/RS flows through a port,
the port’s sense voltage is above VCUT and the tICUT timer
counts up. If the sense voltage is still above VCUT when the
tICUT timer expires, the LTC4258 will turn off the power to
the port immediately and set the appropriate tICUT Fault bit
in register 06h/07h. The tICUT timer duration can be pro-
grammed via register 16h, bits 3 and 2 (Table 1).
The tICUT timer is an up/down counter that is designed to
protect the external MOSFET from thermal stress caused
by repeatedly operating in current limit. The counter
counts up whenever the current is above ICUT and counts
down at 1/16th the rate when it is not. The counter will
bottom out at zero to prevent underflow. Full count indi-
cates that the tICUT timer has expired and the port will be
turned off.
This count up/count down behavior implements duty cycle
protection, preventing intermittent current limit faults from
causing cumulative thermal stress in the MOSFET. If the port
enters current limit but then exits before the timer expires,
the count will decrease slowly, giving the ICUT timer the
ability to turn off sooner in the case of a repetitive fault. If
the overcurrent duty cycle is less than 6.3% the tICUT timer
will be fully reset.
If the tICUT timer expires and causes the port to shut off, the
timer will continue to run, counting down at the slow
1/16th rate and preventing the port from being repowered
until the count returns to zero. This protects the MOSFET
from damage due to a faulty PD that may still have a valid
signature, or from errant software that repeatedly writes to
the Power On bit.
The port will not repower until after the tICUT counter
returns to zero. In manual and semiauto modes the power
enable command must be received after the tICUT counter
reaches zero. In auto mode the LTC4258 must complete a
valid detection cycle after the tICUT counter reaches zero.
tSTART Timing
To distinguish between normal turn-on current limit be-
havior and current limit faults which occur after power-up
is complete, the LTC4258 starts a timer (the tSTART timer)
whenever a power-up sequence begins.
The tSTART timer serves three functions. First and fore-
most, it allows the user to specify a different current limit
timeout (tSTART instead of tICUT) during turn-on (current
limit duty cycle protection remains functional). Second,
the DC disconnect timer is disabled during this period and
can only begin counting up after the tSTART timer has
expired. Together, these two features let the PD draw the
maximum current IINRUSH to charge its input capacitance,
boot up and begin drawing power without triggering a
tSTART fault. Finally, if the device is in current limit for the
entire tSTART period, a tSTART fault will be generated
instead of a tICUT fault. This can be useful for tracking down
the cause of a current fault.
As long as the PD draws less than ICUT at the end of tSTART
and begins drawing the minimum current within tDIS after
tSTART expires (if DC disconnect is enabled), no faults will
be indicated.
The tSTART timer also implements the duty cycle protec-
tion described under tICUT timing and its duration can be
programmed via register 16h, bits 5 and 4 (Table 1).
APPLICATIO S I FOR ATIO
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