LT3585EDDB-2 datasheet(11/20 Pages) LINER

Part #	LT3585EDDB-2

Description	Photofl ash Chargers with Adjustable Input Current and IGBT Drivers

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Manufacturer	LINER [Linear Technology]

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LT3585-0/LT3585-1

LT3585-2/LT3585-3

11

3585f

APPLICATIO S I FOR ATIO

Choosing the Right Device

(LT3585-0/LT3585-1/LT3585-2/LT3585-3)

The only difference between the four versions of the

LT3585 series is the peak current level. For the fastest

possible charge time, use the LT3585-3. The LT3585-1

has the lowest peak current capability, and is designed

for applications that need a more limited drain on the

batteries. Due to the lower peak current, the LT3585-1

can use a physically smaller transformer. The LT3585-0

and LT3585-2 have a current limit in between that of the

LT3585-1 and the LT3585-3.

Transformer Design

The ﬂyback transformer is a key element for any LT3585-0/

LT3585-1/LT3585-2/LT3585-3 design. It must be designed

carefully and checked that it does not cause excessive cur-

rent 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 LT3585-0/LT3585-1/LT3585-2/LT3585-3

accomplish output voltage detection by monitoring the

ﬂyback waveform on the SW pin. When the SW voltage

reaches 31.5V higher than the VBAT voltage, the part halts

power delivery. Thus, the choice of N sets the target output

voltage and changes the amplitude gain of the reﬂected

voltage from the output to the SW pin. Choose N according

to the following equation:

N

VOUT

=

+ 2

31 5

.

where VOUT is the desired output voltage. The number 2

in the numerator is used to include the forward 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:

L

V

NI

PRI

OUT

PK

≥

••

•

–

200 10

9

where VOUT is the desired output voltage. N is the trans-

former turns ratio. IPK is 1.4 (LT3585-0), 0.7 (LT3585-1),

1 (LT3585-2) and 2 (LT3585-3). LPRI needs to be equal

or larger than this value to ensure that the LT3585 series

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 important

parameter is the leakage inductance, LLEAK. When the

power switch of the LT3585 series turns off, the leakage

inductance on the primary of the transformer causes a

voltage spike to occur on the SW pin. The height of this

spike must not exceed 50V, even though the absolute

maximum rating of the SW pin is 60V. The 60V absolute

maximum rating is a DC blocking voltage speciﬁcation,

which assumes that the current in the power NPN is zero.

Figure 4 shows the SW voltage waveform for the circuit

of Figure 8 (LT3585-0). Note that the absolute maximum

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 volt-

age. Figure 5 shows the various limits on the SW voltage

during switch turn off.

Table 1. Recommended Transformer Parameters

PARAMETER

NAME

TYPICAL RANGE

LT3585-0

TYPICAL RANGE

LT3585-1

TYPICAL RANGE

LT3585-2

TYPICAL RANGE

LT3585-3

UNITS

LPRI

Primary Inductance

>5

>10

>7

>3.5

µH

LLEAK

Primary Leakage Inductance

100 to 300

200 to 500

100 to 300

nH

N

Secondary/Primary Turns Ratio

8 to 12

VISO

Secondary to Primary Isolation

Voltage

>500

V

ISAT

Primary Saturation Current

>1.6

>0.8

>1.0

>2

A

RPRI

Primary Winding Resistance

<300

<500

<400

<200

m

Ω

RSEC

Secondary Winding Resistance

<40

<80

<60

<30

Ω

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LT3585EDDB-2 Datasheet(PDF) 11 Page - Linear Technology

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