LM5070 datasheet(12/17 Pages) NSC

12 / 17 page

Power Supply Operation / Current

Limit Programming (Continued)

capacitance provides input filtering for the power converter

section and must be at least 5uF per the IEEE 802.3af

specification. To accomplish the charging in a controlled

manner, the power MOSFET is current limited to 375mA.

The IEEE 802.3af specification requires that the load capaci-

tance be charged within 75ms.

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 program-

mable 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:

The SMPS controller will not initiate operation until the load

capacitor is completely charged. The power sequencing be-

tween 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

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 regu-

lator 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 aux-

iliary winding improves conversion efficiency while reducing

the power dissipated in the controller. The external V

capacitor must be selected such that the capacitor maintains

the V

CC voltage greater than the VCC 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 con-

nected 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 con-

nected 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 config-

ured as an open drain output and can be disabled by con-

necting 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 protec-

tion 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.

The LM5070 current sense and PWM comparators are very

fast, and may respond to short duration noise pulses. Layout

considerations are critical for the current sense filter and

sense resistor. The capacitor associated with the CS filter

must be located very close to the device and connected

directly to the pins of the controller (CS and ARTN). If a

current sense transformer is used, both leads of the trans-

former secondary should be routed to the sense resistor and

the current sense filter network. A sense resistor located in

the source of the primary power MOSFET may be used for

current sensing, but a low inductance resistor is required.

When designing with a current sense resistor, all of the noise

sensitive low power ground connections should be con-

nected together local to the controller and a single connec-

tion should be made to the high current power return (sense

resistor ground point).

Oscillator, Shutdown and Sync

Capability

A single external resistor connected between the RT and

ARTN pins sets the LM5070 oscillator frequency. Internal to

the LM5070–50 device (50% duty cycle limited option) is an

oscillator divide by two circuit. This divide by two circuit

creates an exact 50% duty cycle clock which is used inter-

nally to create a precise 50% duty cycle limit function. Be-

cause of this divide by two, the internal oscillator actually

operates at twice the frequency of the output (OUT). For the

LM5070–80 device the oscillator frequency and the opera-

tional output frequency are the same. To set a desired output

operational frequency (F), the RT resistor can be calculated

from:

LM5070-80:

LM5070 Datasheet(PDF) 12 Page - National Semiconductor (TI)

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