LT3468 데이터 시트보기 (PDF) - Linear Technology

부품명

상세내역

일치하는 목록

LT3468

Photoflash Capacitor Chargers in ThinSOT™

Linear Technology 

LT3468/LT3468-1/LT3468-2

APPLICATIO S I FOR ATIO

Where: VOUT is the desired output voltage. The number

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

inductance, LPRI. Choose LPRI according to the following

formula:

LPRI

≥

VOUT

• 200 • 10−9

N • IPK

Where: VOUT is the desired output voltage. N is

the transformer turns ratio. IPK is 1.4 (LT3468), 0.7

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

LPRI needs to be equal or larger than this value to ensure

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-

tant parameter is the leakage inductance, LLEAK. When the

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

Table 1. Recommended Transformer Parameters

PARAMETER

LPRI

LLEAK

N

VISO

ISAT

RPRI

RSEC

NAME

Primary Inductance

Primary Leakage Inductance

Secondary: Primary Turns Ratio

Secondary to Primary Isolation Voltage

Primary Saturation Current

Primary Winding Resistance

Secondary Winding Resistance

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

check the SW voltage waveform with VOUT near the target

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.

TYPICAL RANGE

LT3468

>5

100 to 300

8 to 12

>500

>1.6

<300

<40

TYPICAL RANGE

LT3468-1

>10

200 to 500

8 to 12

>500

>0.8

<500

<80

TYPICAL RANGE

LT3468-2

>7

200 to 500

8 to 12

>500

>1.0

<400

<60

UNITS

µH

nH

V

A

mΩ

Ω

VIN = 5V

VOUT = 320V

VSW

10V/DIV

100ns/DIV

3468 G18

Figure 3. LT3468 SW Voltage Waveform

8

“B”

“A”

VSW

MUST BE

LESS THAN 50V

MUST BE

LESS THAN 40V

0V

3420 F07

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

346812f

Share Link: