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LT1425

Isolated Flyback Switching Regulator

Linear Technology 

LT1425

APPLICATIONS INFORMATION

SELECTING RFB AND RREF RESISTOR VALUES

The expression for VOUT developed in the Operation

section can be rearranged to yield the following expres-

sion for RFB:

) ) RFB = RREF

VOUT + VF + ISEC(ESR)

VBG

α

NSP

The unknown parameter α, which represents the fraction

of RFB current flowing into the RREF node, can be repre-

sented instead by specified data sheet values as follows:

(IREF)(α)(3k) = VBG

) α =

VBG

(IREF)(3k)

Allowing the expression for RFB to be rewritten as:

) RFB = RREF

VOUT + VF + ISEC(ESR)

IREF(3k)NSP

where,

VOUT = Desired output voltage

VF = Switching diode forward voltage

(ISEC)(ESR) = Secondary resistive losses

IREF = Data sheet reference current value

NSP = Effective secondary-to-primary turns ratio

Strictly speaking, the above equation defines RFB not as an

absolute value, but as a ratio of RREF. So the next question

is, “What is the proper value for RREF?” The answer is that

RREF should be approximately 3k. This is because the

LT1425 is trimmed and specified using this value of RREF.

If the impedance of RREF varies considerably from 3k,

additional errors will result. However, a variation in RREF

of several percent or so is perfectly acceptable. This yields

a bit of freedom in selecting standard 1% resistor values

to yield nominal RFB/RREF ratios.

SELECTING ROCOMP RESISTOR VALUE

The Operation section previously derived the following

expressions for ROUT, i.e., effective output impedance and

ROCOMP, the external resistor value required for its nomi-

nal compensation:

) ROUT = ESR

1

1 – DC

) ) ROCOMP

=

K1

∆VRCCOMP

∆ISW

RFB

ROUT

While the value for ROCOMP may therefore be theoretically

determined, it is usually better in practice to employ

empirical methods. This is because several of the required

input variables are difficult to estimate precisely. For

instance, the ESR term above includes that of the trans-

former secondary, but its effective ESR value depends on

high frequency behavior, not simply DC winding resis-

tance. Similarly, K1 appears to be a simple ratio of VIN to

VOUT times (differential) efficiency, but theoretically esti-

mating efficiency is not a simple calculation. The sug-

gested empirical method is as follows:

Build a prototype of the desired supply using the

eventual secondary components. Temporarily ground

the RCCOMP pin to disable the load compensation func-

tion. Operate the supply over the expected range of

output current loading while measuring the output

voltage deviation. Approximate this variation as a single

value of ROUT (straight line approximation). Calculate a

value for the K1 constant based on VIN, VOUT and the

measured (differential) efficiency. They are then com-

bined with the data sheet typical value for (∆VRCCOMP/

∆ISW ) to yield a value for ROCOMP.

Verify this result by connecting a resistor of roughly this

value from the ROCOMP pin to ground. (Disconnect the

ground short to RCCOMP and connect the requisite

0.1µF filter capacitor to ground.) Measure the output

impedance with the new compensation in place. Modify

the original ROCOMP value if necessary to increase or

decrease the effective compensation.

Once the proper load compensation resistor has been

chosen, it may be necessary to adjust the value of the

RFB resistor. This is because the load compensation

system exhibits some nonlinearity. In particular, the

circuit can shift the reference current by a noticeable

11

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