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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

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/LT8302-3 uses a unique sampling scheme

to regulate the isolated output voltage. Due to the sam-

pling 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 recom-

mended 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/LT8302-3 is trimmed and specified

using this value. If the RREF resistor value varies consid-

erably 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 com-

ponents connected, and measure the regulated output

voltage, 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/LT8302-3. 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 coeffi-

cient 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 reg-

ulation accuracy from board to board for a given appli-

cation will be very consistent, typically under ±5% when

including device variation of all the components in the

system (assuming resistor tolerances and transformer

windings matching within ±1%). However, if the trans-

former or the output diode is changed, or the layout is

dramatically altered, there may be some change in VOUT.

Rev. G

12

For more information www.analog.com

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