LT8302MPS8E 查看數據表（PDF） - Linear Technology

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LT8302MPS8E

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

Linear Technology 

LT8302

Applications Information

To cancel the output diode temperature coefficient, the

following two equations should be satisfied:

( ) VOUT = VREF •

RFB

RREF

•1

NPS

– VF TO

( VTC/ T) •

RFB

RTC

•

1

NPS

= –( VF/ T)

Selecting Actual RREF, RFB, RTC Resistor Values

The LT8302 uses a unique sampling scheme to regulate

the isolated output voltage. Due to the sampling nature,

the scheme contains repeatable delays and error sources,

which will affect the output voltage and force a re-evaluation

of the RFB and RTC resistor values. Therefore, a simple

2-step sequential process is recommended for selecting

resistor values.

Rearrangement of the expression for VOUT in the previous

sections yields the starting value for RFB:

( ( )) RFB

=

RREF

• NPS

• VOUT

VREF

+

VF

TO

VOUT = Output voltage

VF (TO) = Output diode forward voltage at 25°C = ~0.3V

NPS = Transformer effective primary-to-secondary

turns ratio

The equation shows that the RFB resistor value is indepen-

dent of the RTC resistor value. Any RTC resistor connected

between the TC and RREF pins has no effect on the output

voltage setting at 25°C because the TC pin voltage is equal

to the RREF regulation voltage at 25°C.

The RREF resistor value should be approximately 10k

because the LT8302 is trimmed and specified using this

value. If the RREF resistor value varies considerably from

10k, additional errors will result. However, a variation in

RREF up to 10% is acceptable. This yields a bit of freedom

in selecting standard 1% resistor values to yield nominal

RFB/RREF ratios.

First, build and power up the application with the starting

RREF, RFB values (no RTC resistor yet) and other compo-

nents connected, and measure the regulated output volt-

age, VOUT(MEAS). The new RFB value can be adjusted to:

RFB(NEW)

=

VOUT

VOUT(MEAS)

• RFB

Second, with a new RFB resistor value selected, the output

diode temperature coefficient in the application can be

tested to determine the RTC value. Still without the RTC

resistor, the VOUT should be measured over temperature

at a desired target output load. It is very important for

this evaluation that uniform temperature be applied to

both the output diode and the LT8302. If freeze spray or

a heat gun is used, there can be a significant mismatch

in temperature between the two devices that causes sig-

nificant error. Attempting to extrapolate the data from a

diode data sheet is another option if there is no method

to apply uniform heating or cooling such as an oven. With

at least two data points spreading across the operating

temperature range, the output diode temperature coef-

ficient can be determined by:

–

(δVF

/δT)

=

VOUT

( T1) –

T1–

VOUT

T2

(T2)

Using the measured output diode temperature coefficient,

an exact RTC value can be selected with the following

equation:

RTC

=

(δVTC/δT)

– (δVF /δT )

•

⎛

⎝⎜

RFB

NPS

⎞

⎠⎟

Once the RREF, RFB, and RTC values are selected, the regula-

tion accuracy from board to board for a given application

will be very consistent, typically under ±5% when includ-

ing device variation of all the components in the system

(assuming resistor tolerances and transformer windings

matching within ±1%). However, if the transformer or

the output diode is changed, or the layout is dramatically

altered, there may be some change in VOUT.

8302fa

12

For more information www.linear.com/LT8302

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