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

must be taken into account for the dynamic blocking

voltage capabilities of the LT3420/LT3420-1. The dynamic

blocking voltage capability of both parts is 38V.

Table 1 summarizes the various breakdown voltages of the

SW pin for both parts.

Table 1. SW Pin Voltage Ratings

PART

SW PIN DC RATING

LT3420

38V

LT3420-1

50V

SW PIN DYNAMIC RATING

38V

38V

Figure 7 shows what to examine in a new transformer

design to determine if the specifications for the SW pin are

met.

The first leakage inductance spike labeled “A” must not

exceed the dynamic rating of the SW pin. If it does exceed

the rating, then the transformer leakage inductance must

be lowered. The flyback waveform after the initial spike

labeled “B” must not exceed the DC rating of the SW pin.

If it does exceed the rating, then the turns ratio of the

transformer must be lowered. In measuring the voltage on

the SW pin, care must be taken in minimizing the ground

loop of the voltage probe. Careless probing will result in

inaccurate readings.

Note also the magnitude of the initial current spike in the

primary of the transformer labeled “C” when the power

switch turns on. If the leakage inductance is lowered to a

very low level, the internal capacitances of the transformer

will be high. This will result in the initial spike of current in

the primary becoming excessively high. The level of “C”

should be kept to 4A or less in a typical design for both the

LT3420 and LT3420-1. Please note that by inserting a loop

of wire in the primary to measure the primary current, the

leakage inductance of the primary will be made artificially

high. This may result in erroneous voltage measurements

on the SW pin.

The measurements shown in Figure 7 should be made

with both VOUT and VBAT at the maximum levels for the

given application. This results in the highest voltage and

current stress on the SW pin.

Transformer Secondary Capacitance

The total capacitance of the secondary should be mini-

mized for both efficient and proper operation of the LT3420/

LT3420-1. Since the secondary of the transformer under-

goes large voltage swings (approaching 600VP-P), any

capacitance on the secondary can severely affect the

MUST BE LESS THAN 4A

FOR BOTH THE LT3420

“C” AND LT3420-1

IPRI

0A

MUST BE LESS THAN 38V

FOR THE LT3420

“B”

MUST BE LESS THAN 50V

FOR THE LT3420-1

“A”

VSW

MUST BE LESS THAN 38V

FOR BOTH THE LT3420

AND LT3420-1

0V

3420 F07

Figure 7. New Transformer Design Check (Not to Scale)

3420fa

11

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