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LT8302MPS8E

42VIN Micropower No-Opto Isolated Flyback Converter with 65V/3.6A Switch

Linear Technology 

LT8302

Applications Information

Output Voltage

The RFB and RREF resistors as depicted in the Block Diagram

are external resistors used to program the output voltage.

The LT8302 operates similar to traditional current mode

switchers, except in the use of a unique flyback pulse

sense circuit and a sample-and-hold error amplifier, which

sample and therefore regulate the isolated output voltage

from the flyback pulse.

Operation is as follows: when the power switch M1 turns

off, the SW pin voltage rises above the VIN supply. The

amplitude of the flyback pulse, i.e., the difference between

the SW pin voltage and VIN supply, is given as:

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

VF = Output diode forward voltage

ISEC = Transformer secondary current

ESR = Total impedance of secondary circuit

NPS = Transformer effective primary-to-secondary

turns ratio

The flyback voltage is then converted to a current, IRFB,

by the RFB resistor and the flyback pulse sense circuit

(M2 and M3). This current, IRFB, also flows through the

RREF resistor to generate a ground-referred voltage. The

resulting voltage feeds to the inverting input of the sample-

and-hold error amplifier. Since the sample-and-hold error

amplifier samples the voltage when the secondary current

is zero, the (ISEC • ESR) term in the VFLBK equation can be

assumed to be zero.

The internal reference voltage, VREF, 1.00V, feeds to the

noninverting input of the sample-and-hold error ampli-

fier. The relatively high gain in the overall loop causes the

voltage at the RREF pin to be nearly equal to the internal

reference voltage VREF. The resulting relationship between

VFLBK and VREF can be expressed as:

⎛

⎝⎜

VFLBK

RFB

⎞

⎠⎟

• RREF

=

VREF

or

VFLBK

=

VREF

•

⎛

⎝⎜

RFB

RREF

⎞

⎠⎟

VREF = Internal reference voltage 1.00V

Combination with the previous VFLBK equation yields an

equation for VOUT, in terms of the RFB and RREF resistors,

transformer turns ratio, and diode forward voltage:

VOUT

=

VREF

•

⎛

⎝⎜

RFB

RREF

⎞

⎠⎟

•

⎛

⎝⎜

1

NPS

⎞

⎠⎟

–

VF

Output Temperature Compensation

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

ture dependence, but the output diode forward voltage, VF,

has a significant negative temperature coefficient (–1mV/°C

to –2mV/°C). Such a negative temperature coefficient pro-

duces approximately 200mV to 300mV voltage variation

on the output voltage across temperature.

For higher voltage outputs, such as 12V and 24V, the

output diode temperature coefficient has a negligible ef-

fect on the output voltage regulation. For lower voltage

outputs, such as 3.3V and 5V, however, the output diode

temperature coefficient does count for an extra 2% to 5%

output voltage regulation.

The LT8302 junction temperature usually tracks the output

diode junction temperature to the first order. To compensate

the negative temperature coefficient of the output diode,

a resistor, RTC, connected between the TC and RREF pins

generates a proportional-to-absolute-temperature (PTAT)

current. The PTAT current is zero at 25°C, flows into the

RREF pin at hot temperature, and flows out of the RREF pin

at cold temperature. With the RTC resistor in place, the

output voltage equation is revised as follows:

( ) ( ) VOUT = VREF •

RFB

RREF

•1

NPS

– VF TO –

VTC / T •

(T –TO)•

RFB

RTC

•

1

NPS

– ( VF /

T ) • ( T – TO)

TO=Room temperature 25°C

(

VF /

T) = Output diode forward voltage

temperature coefficient

( VTC / T) = 3.35mV/°C

For more information www.linear.com/LT8302

8302fa

11

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