LT3468/LT3468-1/LT3468-2

8

sn346812 346812fs

2 in the numerator is used to include the effect of the

voltage drop across the output diode(s).

Thus for a 320V output, N should be 322/31.5 or 10.2.

For a 300V output, choose N equal to 302/31.5 or 9.6.

The next parameter that needs to be set is the primary

formula:

L

V

NI

PRI

OUT

PK

≥

−

••

•

(LT3468-1), and 1.0 (LT3468-2).

that the LT3468/LT3468-1/LT3468-2 has adequate time

to respond to the flyback waveform.

All other parameters need to meet or exceed the recom-

mended limits as shown in Table 1. A particularly impor-

power switch of the LT3468/LT3468-1/LT3468-2 turns

off, the leakage inductance on the primary of the trans-

former causes a voltage spike to occur on the SW pin. The

height of this spike must not exceed 40V, even though the

absolute maximum rating of the SW Pin is 50V. The 50V

absolute maximum rating is a DC blocking voltage speci-

fication, which assumes that the current in the power NPN

is zero. Figure 3 shows the SW voltage waveform for the

circuit of Figure 6(LT3468). Note that the absolute maxi-

mum rating of the SW pin is not exceeded. Make sure to

output voltage, as this is the worst case condition for SW

voltage. Figure 4 shows the various limits on the SW

voltage during switch turn off.

It is important not to minimize the leakage inductance to

a very low level. Although this would result in a very low

leakage spike on the SW pin, the parasitic capacitance of

the transformer would become large. This will adversely

effect the charge time of the photoflash circuit.

Linear Technology has worked with several leading mag-

netic component manufacturers to produce pre-designed

flyback transformers for use with the LT3468/LT3468-1/

LT3468-2. Table 2 shows the details of several of these

transformers.

APPLICATIO S I FOR ATIO

Figure 4. New Transformer Design Check (Not to Scale).

Figure 3. LT3468 SW Voltage Waveform

3468 G18

100ns/DIV

10V/DIV

Table 1. Recommended Transformer Parameters

TYPICAL RANGE

TYPICAL RANGE

TYPICAL RANGE

PARAMETER

NAME

LT3468

LT3468-1

LT3468-2

UNITS

Primary Inductance

>5

>10

>7

µH

Primary Leakage Inductance

100 to 300

200 to 500

200 to 500

nH

N

Secondary: Primary Turns Ratio

8 to 12

8 to 12

8 to 12

Secondary to Primary Isolation Voltage

>500

>500

>500

V

Primary Saturation Current

>1.6

>0.8

>1.0

A

Primary Winding Resistance

<300

<500

<400

mΩ

Secondary Winding Resistance

<40

<80

<60

Ω

3420 F07

0V

“A”

“B”

MUST BE

LESS THAN 40V

MUST BE

LESS THAN 50V