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LT3748H
(Rev.:RevA)

100V Isolated Flyback Controller

Linear Technology 

LT3748

APPLICATIONS INFORMATION

Pseudo-DC Theory of Operation

The RREF and RFB resistors as depicted in the Block Diagram

are external resistors used to program the output voltage.

The LT3748 operates much the same way as traditional

current mode switchers with the exception of the unique

error amplifier which derives its feedback information

from the flyback pulse.

Operation is as follows: when the NMOS output switch

turns off, its drain voltage rises above VIN. The amplitude

of this flyback pulse (i.e., the difference between it and

VIN) is given as:

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

VF = DOUT forward voltage

ISEC = Transformer secondary current

ESR = Total impedance of secondary circuit

NPS = Transformer effective primary-to-secondary

turns ratio

The flyback voltage is converted to a current by RFB and

Q2. Nearly all of this current flows through resistor RREF to

form a ground-referred voltage. This voltage is fed into the

flyback error amplifier. The flyback error amplifier samples

this output voltage information when the secondary-side

winding current reaches zero. The error amplifier uses a

bandgap voltage, 1.223V, as the reference voltage.

The relatively high gain in the overall loop will then cause

the voltage at the RREF resistor to be nearly equal to the

bandgap reference voltage, VBG. The relationship between

VFLBK and VBG may then be expressed as:

⎛

⎝⎜⎜

VFLBK

RFB

⎞

⎠⎟⎟

=

VBG

RREF

or

VFLBK

=

VBG ⎛⎝⎜⎜RRRFEBF

⎞

⎠⎟⎟

VBG = Internal bandgap reference

Combining with the previous VFLBK expression yields an

expression for VOUT, in terms of the internal reference,

programming resistors, transformer turns ratio and diode

forward voltage drop:

VOUT

=

VBG

⎛

⎝⎜⎜

RFB

RREF

⎞⎛

⎠⎟⎟⎝⎜⎜

1

NPS

⎞

⎠⎟⎟ −

VF

− ISEC

(ESR)

Additionally, it includes the effect of nonzero secondary

output impedance (ESR). This term can be assumed to

be zero in boundary control mode.

Temperature Compensation

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

perature dependence, but the diode forward drop, VF, has a

significant negative temperature coefficient. To compensate

for this, a positive temperature coefficient current source

is internally connected to the RREF pin. The current is set

by resistor RTC to ground connected between the TC pin

and ground. To cancel the temperature coefficient, the

following equation is used:

δVF = − RFB • 1 • δVTC or,

δT RTC NPS δT

RTC

= −RFB

NPS

•

1

δVF / δT

•

δVTC

δT

≈ RFB

NPS

(δVF/δT) = Diode’s forward voltage temperature coef-

ficient

(δVTC/δT) = 1.85mV/°C

VTC = 0.55V

The resistor value given by this equation should also

be verified experimentally and adjusted, if necessary, to

achieve optimal regulation over temperature.

The revised output voltage is as follows:

VOUT

=

VBG

⎛

⎝⎜⎜

RFB

RREF

⎞⎛

⎠⎟⎟⎝⎜⎜

1

NPS

⎞

⎠⎟⎟ −

VF

−

⎛

⎝⎜⎜

VTC

RTC

⎞

⎠⎟⎟

•

RFB

NPS

– ISEC

(ESR)

3748fa

9

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