CS

ARTN

RTN

OUT

R SENSE

1 nF

100:

LM5072

+

CLK

LEB

current sense

65 ns blanking

To PWM

Comparator

To Drain of Hot

Swap MOSFET

12

9

16

8

Isolation

5k

COMP

15

14

FB

SS

to PWM

16

ARTN

From

Secondary

Error-

Amplifier

5 V

.

1 25 V

LM 5072

Boundary

5k

COMP

15

14

FB

SS

to PWM

R2

16

ARTN

R1

Vo

Zcomp

5V

.

1 25V

LM 5072

LM5072

www.ti.com

SNVS437D – MARCH 2006 – REVISED APRIL 2013

Figure 26. Internal Error Amplifier – Used for Non-isolated Output Applications

For isolated applications, the error amplifier function is located on the isolated secondary side. The LM5072’s

error amplifier can be disabled by connecting the FB pin to the ARTN pin. As shown in Figure 27, an opto-

coupler is normally used to send the feedback signal across the isolation boundary to the COMP pin. The

internal pull-up resistor on the COMP pin now serves as the pull-up bias for the opto-coupler transistor.

Figure 27. The Internal Error Amplifier – Bypassed in Isolated Output Applications

Current Sense and Limit

The LM5072 CS pin senses the transformer primary current signal for current mode control and current limiting of

the supply. As shown in Figure 28, the current sense function can be fulfilled by a simple sense resistor RSENSE

inserted between the RTN and the source of the primary MOSFET switch.

The RSENSE resistor should be non-inductive, and a low pass filter should be used to reject the switching noise on

the sensed signal. A simple RC filter using 100

Ω and 1 nF is typically sufficient. The filter capacitor must be

located close to the CS and ARTN pins. In order to prevent noise propagation and to improve the noise immunity

of the current sense, it is very important to minimize the return path of the current sense signal. This is

accomplished with direct connection to the ARTN pin and a single point connection to the RTN pin on the PC

Board layout.

Figure 28. Current Sense Schemes

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