LT8302HS8E-3-WPBF Просмотр технического описания (PDF) - Analog Devices

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LT8302HS8E-3-WPBF

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

Analog Devices 

LT8302/LT8302-3

APPLICATIONS INFORMATION

Output Voltage

The RFB and RREF resistors as depicted in the Block

Diagram are external resistors used to program the out-

put voltage. The LT8302/LT8302-3 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 sam-

ple-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 amplifier.

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

⎞

⎟

⎠

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 tem-

perature dependence, but the output diode forward

voltage, VF, has a significant negative temperature coef-

ficient (–1mV/°C to –2mV/°C). Such a negative tem-

perature coefficient produces 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

effect 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/LT8302-3 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-tem-

perature (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

VREF = Internal reference voltage 1.00V

For more information www.analog.com

Rev. G

11

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