LM5015 datasheet(19/28 Pages) TI

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Error Amplifier

( Internal or external)

V REF

RO1

RO2

VREF

VO - VREF

RO2

RO1

COMP

LM5015

CFB

VCC

LMV431A

Current

Mirror

PWM

1.26V

VDD

1.3V

VOUT

RO1

RO2

LM5015

www.ti.com

SNVS538C – NOVEMBER 2007 – REVISED APRIL 2013

Figure 22. High bandwidth Isolated Feedback Using CFB Pin

The two external pull-up and pull-down resistors are effectively connected in parallel for ac signal, yielding a

collector resistance on the opto-coupler of 0.5 k

Ω. This resistance is 10 times smaller than the internal pull up

resistor and the pole associated with the collector-base capacitance of the opto-coupler transistor is pushed out

to a decade higher frequency. Moving the opto-coupler pole to a higher frequency allows higher loop bandwidth

capability than conventional isolated feedback designs (see Figure 21).

OUTPUT VOLTAGE

Output voltage is normally set with a resistor divider shown in Figure 23 . To set the regulator’s output voltage to

VO, the two resistors must satisfy the following equation:

where

•

VREF is the reference voltage of the error amplifier, which is 1.26V for the LM5015’s internal error amplifier

(5)

Figure 23. Output Voltage Setting

In isolated designs, the reference and error amplifier are located on the secondary side. To obtain a 5V output in

an isolated feedback design using LMV431A (whose nominal VREF is1.24V), the feedback divider resistors ratio

specified by above equation is 0.330. Selecting 24.3 k

Ω for RO1, then RO2 should be 8.06 kΩ. In a similar non-

isolated converter design using the internal error amplifier, the divider’s resistors ratio is 0.337, therefore if 24.3

Ω is selected for RO1, then RO2 should be 8.20 kΩ.

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