LTC3441 データシートの表示（PDF） - Linear Technology

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LTC3441

Photoflash Capacitor Chargers with Automatic Refresh

Linear Technology 

LT3420/LT3420-1

APPLICATIO S I FOR ATIO

COMPONENT SELECTION

Choosing the Right Transformer

The flyback transformer plays a key role in any LT3420/

LT3420-1 application. A poorly designed transformer can

result in inefficient operation. Linear Technology Corpora-

tion has worked with a number of transformer manufactur-

ers to develop specific transformers for use with the

LT3420/LT3420-1. These predesigned transformers are

sufficient for a large majority of the applications that may

be encountered. In some cases, the reader may choose to

design his own transformer or may simply be curious about

the issues involved in designing the transformer. The fol-

lowing is a brief discussion of the issues relating to trans-

former design.

Transformer Turns Ratio

The turns ratio for the transformer, N, should be high

enough so that the absolute maximum voltage rating for

the NPN power switch is not exceeded. When the power

switch turns off, the voltage on the collector of the switch

(SW Pin) will “fly” up to the output voltage divided by N

plus the battery voltage (neglecting the voltage drop

across the rectifying diodes). This voltage should not

exceed the 38V (LT3420) or 50V (LT3420-1) breakdown

rating of the power switch.

Choose the minimum N by the following formula.

NMIN

≥

VOUT

38 – VBAT

(LT3420)

NMIN

≥

VOUT

50 – VBAT

(LT3420 − 1)

For an LT3420 design, a 5V battery voltage and a 330V

output results in a NMIN of 10 so a turns ratio of 10 or greater

should be used.

Transformer Primary Inductance

A flyback transformer needs to store substantial amounts

of energy in the core during each switching cycle. The

transformer, therefore, will generally require an air gap.

The use of an air gap in the core makes the energy storage

ability, or inductance, much more stable with temperature

and variations in the core material. Most core manufactur-

ers will supply standard sizes of air gaps with a given type

of core, resulting in different AL values. AL is the induc-

tance of a particular core per square turns of winding. To

get a certain inductance, simply divide the desired induc-

tance by the AL value and take the square root of the result

to find the number of turns needed on the primary of the

transformer.

The LT3420/LT3420-1 detect the output voltage via the

flyback pulse on the SW pin. Since this can only occur

while the power switch is off, an important criteria is that

the value of the primary inductance of the transformer be

larger than a certain minimum value. The switch off time

should be 500ns or larger for the LT3420 and 350ns or

larger for the LT3420-1. The minimum inductance can be

calculated with the following formula:

LPRI

≥

500 • 10–9 • VOUT

N • (1.4 – 0.04N)

LPRI

≥

350 • 10–9 • VOUT

N • (1.0 – 0.015N)

(LT3420)

(LT3420 − 1)

VOUT: Target Output Voltage

N: Transformer Turns Ratio

Transformer Leakage Inductance

The leakage inductance of the transformer must be care-

fully minimized for both proper and efficient operation of

the part. The DC voltage rating of the SW pin on the LT3420

is 38V while on the LT3420-1 it is 50V. These ratings are

for DC blocking voltages only and additional precautions

3420fa

10

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