3439EFE データシートの表示（PDF） - Linear Technology

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3439EFE

Slew Rate Controlled Ultralow Noise1A Isolated DC/DC Transformer Driver

Linear Technology 

LT3439

APPLICATIO S I FOR ATIO

mean greater switching losses in the internal output

switches. However, efficiency is only modestly reduced

for a large improvement in EMI.

Care should be taken to ensure that the worst-case input

voltage and load current conditions do not cause an

excessive die temperature. The total power dissipation of

the IC is dominated by three loss terms, regulator losses,

saturation losses and switching losses. The following

formulas may be used to approximate these losses:

1. Regulator Dissipation:

PVIN

=

VIN

12mA

+

I

60



where I is the average switch current.

2. Switch Saturation Dissipation:

PVSAT = (VSAT)(I)

3. Switch Switching Dissipation:

( ) 

PSW = 10–6 • VIN • I • fOSC

I

–2.3 • 10–4 • RSL + 10.8

+

( ) V

–1.7 • 10–3 • RSL

+ 65.8





Total IC power dissipation (PD) is the sum of these three

terms. Die junction temperature can be computed as

follows:

TJ = TAMB + (PD)(θJA)

where TAMB is the ambient temperature, TJ is the junction

temperature and θJA is the thermal resistance from junc-

tion to ambient.

The LT3439 comes in the 16-pin TSSOP with exposed

backside package that has a very low junction-to-ambient

thermal resistance (θJA) of approximately 40°C/W.

Transformer Design

Table 1 lists recommended center tapped transformers for

a variety of input voltage, output voltage and power

combinations.

8

Table 1

NOMINAL

INPUT

VOLTAGE

5V

5V

5V

5V

5V

5V

12V

NOMINAL

OUTPUT

VOLTAGE

12V

12V

±15V

±15V

12V

12V

– 12V

OUTPUT

POWER

1.5W

3.0W

1.5W

3.0W

1.5W

10W

6W

COILTRONICS

PART NUMBER

CTX02-13716-X1

CTX02-13665-X1

CTX02-13713-X1

CTX02-13664-X1

CTX02-13834-X3

CTX02-13949-X1

CTX02-16076

These transformers will yield slightly high output voltages

so that they can accommodate an LDO regulator on the

output.

If your application is not listed, the LTC Applications group

is available to assist in the choice and/or the design of the

transformer.

In the design/selection of the transformer the following

characteristics are critical and should be considered.

Turns Ratio

The turns ratio of the transformer determines the output

voltage. The following equation can be used as a first pass

to calculate the turns ratio:

NS = VOUT + VF

NP VIN – VSW

where VF is the forward voltage of the output diode and

VSW is the voltage drop across the internal switches (see

Typical Performance curves).

Sufficient margin should be added to the turns ratio to

account for voltage drops due to transformer winding

resistances. Also, if using an LDO for regulating the output

voltage, don’t forget to take into account the voltage drop

that should be added to VOUT.

Magnetizing Current

The primary inductance of the transformer causes a ripple

current that is independent of load current. The ripple

current manifests itself in the output voltage through the

parasitic resistances of the supply. Increasing the trans-

former magnetizing inductance can reduce the ripple

sn3439 3439fs

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