RT =

1

F x 330 x 10

-12

RT =

1

F x 165 x 10

-12

LM5071

www.ti.com

SNVS409D – NOVEMBER 2005 – REVISED APRIL 2006

Error Amplifier

An internal high gain error amplifier is provided within the LM5071. 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 LM5071 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.

The LM5071 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 transformer 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 connected together

local to the controller and a single connection 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 LM5071 oscillator frequency.

Internal to the LM5071–50 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 internally to create a precise 50% duty cycle

limit function. Because of this divide by two, the internal oscillator actually operates at twice the frequency of the

output (OUT). For the LM5071–80 device the oscillator frequency and the operational output frequency are the

same. To set a desired output operational frequency (F), the RT resistor can be calculated from:

LM5071-80:

(1)

LM5071-50:

(2)

The LM5071 can also be synchronized to an external clock. The external clock must have a higher frequency

than the free running oscillator frequency set by the RT resistor. The clock signal should be capacitively coupled

into the RT pin with a 100pF capacitor. A peak voltage level greater than 3.7 volts at the RT pin is required for

detection of the sync pulse. The sync pulse width should be set between 15 to 150ns by the external

components. The RT resistor is always required, whether the oscillator is free running or externally synchronized.

The voltage at the RT pin is internally regulated to a 2 volts. The RT resistor should be located very close to the

device and connected directly to the pins of the controller (RT and ARTN).

Copyright © 2005–2006, Texas Instruments Incorporated

Submit Documentation Feedback

13

Product Folder Links: LM5071