LTC4276 datasheet(13/26 Pages) LINER

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LT4276

4276fa

For more information www.linear.com/LT4276

The block diagram shows VSENSE is compared against

the voltage across the current sense resistor, V(ISEN+)-

V(ISEN–) modified by the internal slope compensation

voltage discussed subsequently.

LOAD COMPENSATION

As can be seen in Figure 13, the voltage on the FB31 pin

droops slightly during the flyback period. This is mostly

caused by resistances of components of the secondary

side such as: the secondary winding, RDS(ON) of the syn-

chronous MOSFET, ESR of the output capacitor, etc. These

resistances cause a feedback error that is proportional to

the current in the secondary loop at the time of feedback

sample window. To compensate for this error, the LT4276

places a voltage proportional to the peak current in the

primary winding on the RLDCMP pin.

Determining Feedback and Load Compensation

Resistors

Because the resistances of components on the secondary

side are generally not well known, an empirical method

must be used to determine the feedback and load com-

pensation resistor values.

INITIALLY SET RFB2 = 2kΩ

RFB1≈RFB2

VOUT

VFB

NTHIRD

NSECONDARY

–RFB2

ConnecttheresistorRLDCMPbetweentheRLDCMPpinand

GND. RLDCMP must be at least 10kΩ. Adjust RLDCMP for

minimumchangeofVOUToverthefullinputandoutputload

range. A potentiometer in series with 10kΩ may be initially

used for RLDCMP and adjusted. The potentiometer+10kΩ

may then be removed, measured, and replaced with the

equivalent fixed resistor. The resulting VOUT differs from

the desired VOUT due to offset injected by load compensa-

tion. The change to RFB2 to correct this is predicted by:

ΔRFB2 =

ΔVOUT

VFB

NTHIRD

NSECONDARY

RFB22

RFB1

applicaTions inForMaTion

Where: ΔVOUT is the desired change to VOUT

ΔRFB2 is the required change to RFB2

NTHIRD/NSECONDARY is the transformer third

winding to secondary winding

OPTO-ISOLATOR FEEDBACK

For forward mode operation, the flyback voltage cannot be

sensed across the transformer. Thus, opto-isolator feed-

back must be used. When using opto-isolator feedback,

connect the FB31 pin to GND and leave the RLDCMP pin

open. In this condition, the feedback amplifier sinks an

average current of ISINK into the ITHB pin. An example for

feedback connections is shown in Figure 14. Note that

since ISINK is time-averaged over the switching period,

the sink current varies as a function of the user-selected

switching frequency.

Figure 14: Opto-isolator Feedback

Connections in the Forward Mode

LT4276

ITHB

4276 F13

VCC

VOUT

FB31

GND

SOFT-START

In PoE applications, a proper soft-start design is required

to prevent the PD from drawing more current than the

PSE can provide.

The soft-start time, tSFST, is approximately the time in

which the power supply output voltage, VOUT, is charg-

ing its output capacitance, COUT. This results in an inrush

current at the port of the PD, Iport_inrush. Care must be

taken in selecting tSFST to prevent the PD from drawing

more current than the PSE can provide.

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