RCLP =

16000:-A

LM5070

SNVS308F – OCTOBER 2004 – REVISED APRIL 2006

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Some legacy PSEs may not be able to supply the IEEE maximum power of 15W to the PD, and this can be a

problem during startup. Low power PDs that are used in these legacy systems will require a lower startup current

limit. The LM5070 can be programmed for a reduced inrush current limit level with a resistor at RCLP pin. The

programmable inrush current limit range is 75mA to 390mA. If the RCLP pin is left open, the LM5070 will default

to 390mA, near the maximum allowed per the IEEE 802.3 specification. To set a desired inrush current limit

(limit), the RCLP resistor can be calculated from:

(1)

The SMPS controller will not initiate operation until the load capacitor is completely charged. The power

sequencing between the interface circuitry and the SMPS controller occurs automatically within the LM5070.

Detection circuitry monitors the RTN pin to detect interface startup completion. When the RTN pin potential drops

only applies to the initial charging phase. The interface power MOSFET current limit will revert to the fixed default

protection current limit of 390mA once the SMPS is powered up and the soft-start pin sequence begins.

High Voltage Start-Up Regulator

IN) to be connected directly to line voltages as high as 75V. The regulator output is internally current limited to

from an auxiliary winding improves conversion efficiency while reducing the power dissipated in the controller.

start-up regulator does not exceed the maximum power dissipation capability of the LM5070 package.

Error Amplifier

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. In non-isolated applications,

the power converter output is connected to the FB pin via voltage scaling resistors. Loop compensation

components are connected between the COMP and FB pins. For most isolated applications 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.

Current Limit / Current Sense

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. The CS input has an internal

MOSFET which discharges the CS pin capacitance at the conclusion of every cycle. The discharge device

remains on an additional 50ns after the beginning of the new cycle to attenuate the leading edge spike on the

current sense signal.

12

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