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LT3574

3574f

applications inForMation

ERROR AMPLIFIER—DYNAMIC THEORY

Due to the sampling nature of the feedback loop, there

are several timing signals and other constraints that are

required for proper LT3574 operation.

Minimum Current Limit

The LT3574 obtains output voltage information from the

SW pin when the secondary winding conducts current.

The sampling circuitry needs a minimum amount of time

to sample the output voltage. To guarantee enough time,

a minimum inductance value must be maintained. The

primary side magnetizing inductance must be chosen

above the following value:

µH

PRI

OUT

MIN

OUT

≥









•

tMIN = minimum off-time, 350ns

IMIN = minimum current limit, 175mA

The minimum current limit is higher than that on the Elec-

trical Characteristics table due to the overshoot caused by

the comparator delay.

Leakage Inductance Blanking

When the output switch first turns off, the flyback pulse

appears.However,ittakesafinitetimeuntilthetransformer

primary-side voltage waveform approximately represents

the output voltage. This is partly due to the rise time on

the SW node, but more importantly due to the trans-

former leakage inductance. The latter causes a very fast

voltage spike on the primary side of the transformer that

is not directly related to output voltage (some time is also

required for internal settling of the feedback amplifier

circuitry). The leakage inductance spike is largest when

the power switch current is highest.

In order to maintain immunity to these phenomena, a fixed

delay is introduced between the switch turn-off command

andthebeginningofthesampling.Theblankingisinternally

set to 150ns. In certain cases, the leakage inductance may

not be settled by the end of the blanking period, but will

not significantly affect output regulation.

Selecting RFB and RREF Resistor Values

The expression for VOUT, developed in the Operation sec-

tion, can be rearranged to yield the following expression

for RFB:

REF

OUT

(

) +





•

where,

VOUT = Output voltage

VF = Switching diode forward voltage

a = Ratio of Q1, IC to IE, typically 0.986

NPS = Effective primary-to-secondary turns ratio

VTC = 0.55V

The equation assumes the temperature coefficients of

the diode and VTC are equal, which is a good first-order

approximation.

Strictly speaking, the above equation defines RFBnotasan

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

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

is that RREF should be approximately 6.04k. The LT3574

is trimmed and specified using this value of RREF. If the

impedance of RREF varies considerably from 6.04k, ad-

ditional 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. The RFB resistor given by this equation

should also be verified experimentally, and adjusted if

necessary for best output accuracy.

Tables 1-4 are useful for selecting the resistor values for

RREF and RFB with no equations. The tables provide RFB,

RREF and RTC values for common output voltages and

common winding ratios.

Table 1. Common Resistor Values for 1:1 Transformers

VOUT (V)

NPS

RFB (kΩ)

RREF (kΩ)

RTC (kΩ)

3.3

1.00

18.7

6.04

19.1

1.00

27.4

6.04

1.00

64.9

6.04

66.5

1.00

80.6

6.04

80.6

1.00

107

6.04

105

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