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LT3511

3511fa

Selecting RFB and RREF Resistor Values

The following section provides an equation for setting

RFB and RREF values. The equation should only serve

as a guide. Follow the procedure outlined in the Design

Procedure to set accurate values for RFB, RREF and RTC

using the iterative design procedure.

Rearrangement of the expression for VOUTintheTempera-

ture Compensation section, developed in the Operations

section, yields the following expression for RFB:

RFB =

RREF • NPS VOUT + VF

(

)+ VTC

⎡⎣

⎤⎦

VBG

where:

VOUT = Output voltage

VF = Switching diode forward voltage

NPS = Effective primary-to-secondary turns ratio

VTC = 0.55V

This equation assumes:

RTC =

RFB

NPS

The equation assumes the temperature coefficients of

the diode and VTC are equal, which is a good first order

approximation.

Strictly speaking, the above equation defines RFBnotasan

absolute value, but as a ratio of RREF. So the next question

is, what is the proper value for RREF? The answer is that

RREFshouldbeapproximately10k.TheLT3511istrimmed

and specified using this value of RREF. If the impedance of

RREF varies considerably from 10k, additional errors will

result. However, a variation in RREF of several percent is

acceptable. This yields a bit of freedom in selecting stan-

dard 1% resistor values to yield nominal RFB/RREF ratios.

Undervoltage Lockout (UVLO)

A resistive divider from VIN to the EN/UVLO pin imple-

ments undervoltage lockout (UVLO). Figure 7 shows this

configuration. The EN/UVLO pin threshold is set at 1.21V.

In addition, the EN/UVLO pin draws 2.6μA when the volt-

age at the pin is below 1.21V. This current provides user

programmable hysteresis based on the value of R1. The

effective UVLO thresholds are:

VIN(UVLO,RISING) =

1.2V • R1+R2

(

)

+ 2.6µA • R1

VIN(UVLO,FALLING) =

1.2V • R1+R2

(

)

Figure 7 also shows the implementation of external shut-

down control while still using the UVLO function. The

NMOS grounds the EN/UVLO pin when turned on, and

puts the LT3511 in shutdown with quiescent current draw

of less than 1μA.

Minimum Load Requirement

The LT3511 recovers output voltage information using the

flyback pulse. The flyback pulse occurs once the switch

turns off and the secondary winding conducts current. In

order to regulate the output voltage, the LT3511 needs to

sample the flyback pulse. The LT3511 delivers a minimum

amount of energy even during light load conditions to

ensureaccurateoutputvoltageinformation.Theminimum

delivery of energy creates a minimum load requirement

of 10mA to 15mA depending on the specific application.

Verify minimum load requirements for each application.

A Zener diode with a Zener breakdown of 20% higher

than the output voltage can serve as a minimum load if

pre-loading is not acceptable. For a 5V output, use a 6V

Zener with cathode connected to the output.

LT3511

EN/UVLO

GND

VIN

3511 F07

RUN/STOP

CONTROL

(OPTIONAL)

Figure 7. Undervoltage Lockout (UVLO)

applicaTions inForMaTion

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