DG-ACC-POE Digi International, DG-ACC-POE Datasheet - Page 7

• Manufacturer Part Number: DG-ACC-POE
• Description: KIT POE APPLICATION
• Manufacturer: Digi International

Specifications of DG-ACC-POE

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

+12V Power-Over-Ethernet Application Kit - Hardware Reference
2.1.3 Current Limit / Current Sense
2.1.4 Working Frequency
2.1.5 Error Amplifier
2.2 PoE module usage
The LM5070 provides a cycle-by-cycle over current protection function. Current limit is
accomplished by an internal current sense comparator. If the voltage at the current sense
comparator input CS exceeds 0.5V with respect to RTN/ARTN, the output pulse will be
immediately terminated. A small RC filter, located near the CS pin of the controller, is
recommended to filter noise from the current sense signal because the LM5070 current sense and
PWM comparators are very fast, and may respond to short duration noise pulses.
The PoE application kit has a 0.33R sense resistor that limits the output current to 1.5A for the 12V
signal.
A single external resistor connected between the RT and ARTN pins sets the LM5070 oscillator
frequency. In the LM5070-50 the internal oscillator operates at twice the frequency of the output
(OUT). To set a desired output operational frequency, the RT resistor can be calculated from:
In the PoE application kit the RT resistor is 15KHz, so the output operational frequency is set to
20KHz.
An internal high gain error amplifier is provided within the LM5070. The amplifier’s non-inverting
reference is set to a fixed reference voltage of 1.25V. The inverting input is connected to the FB pin
For the PoE application kit the error amplifier function is implemented on the secondary side of the
converter and the internal error amplifier is not used. The internal error amplifier is configured as
an open drain output and can be disabled by connecting the FB pin to ARTN. An internal 5K pull-
up resistor between a 5V reference and COMP can be used as the pull-up for an optocoupler in
isolated applications.
The POE_12V_JS application kit provides a +12V supply voltage. This +12V_POE supply output
is connected directly to the power switch of the JumpStart kit board through a diode and so we
reuse the Jump Start power controller, which provides +5V (and also +/-12V) from the input power
supply 9-30V (in our case the input power supply is +12V_POE).
The POE module can be used directly once it’s plugged on the Jump Start boards. In this case, the
module replaces the input power supply plug and so provides all internal power supplies.
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