Datasheet

RCLP =
16000:-A
I
limit
(A)
LM5070
SNVS308G –OCTOBER 2004–REVISED APRIL 2013 www.ti.com
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
below 1.5V with respect to V
EE
, the V
CC
regulator of the SMPS controller is enabled. The soft-start function is
enabled once the V
CC
regulator achieves minimum operating voltage. The RCLP programmed inrush current limit
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
The LM5070 contains an internal high voltage startup regulator that allows the input pin (V
IN
) to be connected
directly to line voltages as high as 75V. The regulator output is internally current limited to 15mA. The
recommended capacitance range for the V
CC
regulator output is 0.1uF to 10uF. When the voltage on the V
CC
pin
reaches the regulation point of 7.8V, the controller output is enabled. The controller will remain enabled until V
CC
falls below 6.25V.
In typical applications, a transformer auxiliary winding is diode connected to the V
CC
pin. This winding should
raise the V
CC
voltage above 8.1V to shut off the internal startup regulator. Though not required, powering V
CC
from an auxiliary winding improves conversion efficiency while reducing the power dissipated in the controller.
The external V
CC
capacitor must be selected such that the capacitor maintains the V
CC
voltage greater than the
V
CC
UVLO falling threshold (6.25V) during the initial start-up. During a fault condition when the converter auxiliary
winding is inactive, external current draw on the V
CC
line should be limited such that the power dissipated in the
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.
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