LT3468/LT3468-1

7

34681i

APPLICATIO S I FOR ATIO

Choosing The Right Device (LT3468/LT3468-1)

The only difference between the two versions of the

LT3468 is the peak current level. For the fastest possible

charge time, use the LT3468. The LT3468-1 has a lower

peak current capability, and is designed for applications

that need a more limited drain on the batteries. Due to the

lower peak current, the LT3468-1 can use a physically

smaller transformer.

Transformer Design

The flyback transformer is a key element for any LT3468/

LT3468-1 design. It must be designed carefully and checked

that it does not cause excessive current or voltage on any

pin of the part. The main parameters that need to be

designed are shown in Table 1.

The first transformer parameter that needs to be set is the

turns ratio N. The LT3468/LT3468-1 accomplish output

voltage detection by monitoring the flyback waveform on

the SW pin. When the SW voltage reaches 31.5V higher

the choice of N sets the target output voltage as it changes

Table 1. Recommended Transformer Parameters

TYPICAL RANGE

TYPICAL RANGE

PARAMETER

NAME

LT3468

LT3468-1

UNITS

Primary Inductance

>5

>10

µH

Primary Leakage Inductance

100 to 300

200 to 500

nH

N

Secondary: Primary Turns Ratio

8 to 12

8 to 12

Secondary to Primary Isolation Voltage

>500

>500

V

Primary Saturation Current

>1.6

>0.8

A

Primary Winding Resistance

<300

<500

m

Ω

Secondary Winding Resistance

<40

<80

Ω

the amplitude of the reflected voltage from the output to

the SW pin. Choose N according to the following equation:

N

=

+ 2

31 5

.

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)

that the LT3468/LT3468-1 has adequate time to respond

to the flyback waveform.