LT3748(RevA) Просмотр технического описания (PDF) - Linear Technology

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LT3748
(Rev.:RevA)

100V Isolated Flyback Controller

Linear Technology 

LT3748

APPLICATIONS INFORMATION

Selecting Actual RREF, RFB and RTC Resistor Values

The preceding equations define how the LT3748 would

regulate the output voltage if the system had no time de-

lays and no error sources. However, there are a number of

repeatable delays and parasitics in each application which

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

the RFB and RTC component values. The following approach

is the best method for selecting the correct values.

The expression for VOUT, developed in the Operation sec-

tion, can be rearranged to yield the following expression

for RFB:

R FB

=

RREF

• NPS ⎡⎣(VOUT

VBG

+

VF ) +

VTC ⎤⎦

where:

VOUT = Output voltage

VF = Output diode forward voltage

NPS = Effective primary-to-secondary turns ratio

VTC = 0.55V

The equation assumes the temperature coefficients of the

output diode and VTC are equal and substitutes RFB/NPS for

the value of RTC. This is a good first order approximation

but will be revisited later.

First, the value of RREF should be approximately 6.04k

since the LT3748 is trimmed and specified using this

value. If the impedance of RREF varies considerably from

6.04k, additional errors will result. However, a variation in

RREF of several percent is acceptable. This yields a bit of

freedom in selecting standard 1% resistor values to yield

nominal RFB/RREF ratios.

With starting values for RFB and RTC, an initial iteration

of the application should be built with final selections of

all external components (transformer, diode, MOSFET,

etc.). The resulting VOUT should be measured and used

to re-evaluate the value of RFB due to non-idealities in the

sampling system:

RFB(NEW )

=

VOUT(DESIRED)

VOUT(MEASURED)

• RFB(OLD)

10

With a new value of RFB selected, the temperature co-

efficient of the output diode in the application can be

tested to verify the nominal RTC value. The RTC resistor

should be removed from the circuit under test (this will

cause VOUT to increase for this step) and 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

LT3748—if freeze spray or a heat gun is used there can

be a significant mismatch in temperature between the

two devices that causes significant error. Attempting to

extrapolate the data from a diode datasheet or assuming

the nominal RTC value may yield a better result if there is

no method to apply uniform heat or cooling such as an

oven. With at least two data points (although more data

points from hot to cold are recommended), the change

in V/°C can be determined by:

ΔVOUT = VOUT1 – VOUT2

ΔTEMP TEMP1– TEMP2

Using the measured VOUT temperature coefficient, an exact

RTC value can be selected using the following equation:

RTC

=

RFB

NPS

•

1.85mV/°C

ΔVOUT

ΔTEMP

If the value of RTC has changed significantly, which can

happen with the use of some output diodes that have

a very low forward drop, the RFB value may need to be

changed to restore VOUT to the desired value. As in the

previous iteration, after measuring VOUT, a new RFB can

once again be selected using:

RFB(NEW )

=

VOUT(DESIRED)

VOUT(MEASURED)

• RFB(OLD)

Once the values of RFB and RTC are selected, the regulation

accuracy from board to board for a given application 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 of 1% or better). However, if the transformer,

the output diode or MOSFET switch are changed or the

layout is dramatically altered, there may be some change

in VOUT.

3748fa

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