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LT3511

3511fa

applicaTions inForMaTion

PSUEDO DC THEORY

In the Block Diagram, RREF (R4) and RFB (R3) are external

resistors used to program the output voltage. The LT3511

operates similar to traditional current mode switchers,

except in the use of a unique error amplifier, which derives

its feedback information from the flyback pulse.

Operation is as follows: when the output switch, Q1, turns

off, its collector voltage rises above the VIN rail. The am-

plitude of this flyback pulse, i.e., the difference between

it and VIN, is given as:

VFLBK = (VOUT + VF + ISEC • ESR) • NPS

VF = D1 forward voltage

ISEC = Transformer secondary current

ESR = Total impedance of secondary circuit

NPS = Transformer effective primary-to-secondary turns

ratio

RFBandQ2converttheflybackvoltageintoacurrent.Nearly

all of this current flows through RREF to form a ground-

referred voltage. The resulting voltage forms the input

to the flyback error amplifier. The flyback error amplifier

samples the voltage information when the secondary side

winding current is zero. The bandgap voltage, 1.20V, acts

as the reference for the flyback error amplifier.

The relatively high gain in the overall loop will then cause

the voltage at RREF to be nearly equal to the bandgap

reference voltage VBG. The resulting relationship between

VFLBK and VBG approximately equals:

VFLBK

RFB

⎛

⎝

⎜

⎞

⎠

⎟ =

VBG

RREF

or VFLBK = VBG

RFB

RREF

⎛

⎝

⎜

⎞

⎠

⎟

VBG = Internal bandgap reference

Combination of the preceding expression with earlier

derivation of VFLBK results in the following equation:

VOUT = VBG

RFB

RREF

⎛

⎝

⎜

⎞

⎠

⎟

NPS

⎛

⎝

⎜

⎞

⎠

⎟ – VF – ISEC ESR

(

)

The expression defines VOUT in terms of the internal ref-

erence, programming resistors, transformer turns ratio

and diode forward voltage drop. Additionally, it includes

the effect of nonzero secondary output impedance (ESR).

Boundary control mode minimizes the effect of this im-

pedance term.

Temperature Compensation

The first term in the VOUT equation does not have tem-

perature dependence, but the diode forward drop has a

significant negative temperature coefficient. A positive

temperature coefficient current source connects to the

RREF pin to compensate. A resistor to ground from the

TC pin sets the compensation current.

The following equation explains the cancellation of the

temperature coefficient:

δVF

δT

= –

RFB

RTC

• 1

NPS

• δ

VTC

δT

or,

RTC =

–RFB

NPS

•

δVF / δT

• δ

VTC

δT

≈

RFB

NPS

(δVF/δT)=Diode’sforwardvoltagetemperaturecoefficient

(δVTC/δT) = 2mV

VTC = 0.55V

ExperimentallyverifytheresultingvalueofRTCandadjustas

necessary to achieve optimal regulation over temperature.

The addition of a temperature coefficient current modifies

the expression of output voltage as follows:

VOUT = VBG

RFB

RREF

⎛

⎝

⎜

⎞

⎠

⎟

NPS

⎛

⎝

⎜

⎞

⎠

⎟ – VF

–

VTC

RTC

⎛

⎝

⎜

⎞

⎠

⎟ •

RFB

NPS

– ISEC ESR

(

)

Output Power

A flyback converter has a complicated relationship be-

tween the input and output current compared to a buck

or a boost. A boost has a relatively constant maximum

input current regardless of input voltage and a buck has a

relatively constant maximum output current regardless of

input voltage. This is due to the continuous nonswitching

behavior of the two currents. A flyback converter has both

discontinuous input and output currents which makes it

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