LT3581IMSE-TRPBF Просмотр технического описания (PDF) - Linear Technology

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LT3581IMSE-TRPBF

3.3A Boost/Inverting DC/DC Converter with Fault Protection

Linear Technology 

LT3581

APPLICATIONS INFORMATION

where TJ = Die Junction Temperature, TA = Ambient Tem-

perature, PTOTAL is the final result from the calculations

shown in Table 4, and θJA is the thermal resistance from

the silicon junction to the ambient air.

The published (http://www.linear.com/designtools/pack-

aging/Linear_Technology_Thermal_Resistance_Table.

pdf) θJA value is 43°C/W for the 4mm × 3mm 14-pin DFN

package and 45°C/W for the 16-lead MSOP package. In

practice, lower θJA values are realizable if board layout is

performed with appropriate grounding (accounting for heat

sinking properties of the board) and other considerations

listed in the Layout Guidelines section. For instance, a

θJA value of ~24°C/W was consistently achieved for both

MSE and DFN packages of the LT3581 (at VIN = 5V, VOUT =

12V, IOUT = 0.83A, fOSC = 2MHz) when board layout was

optimized as per the suggestions in the Board Layout

Guidelines section.

Junction Temperature Measurement

The duty cycle of the CLKOUT signal is linearly propor-

tional to die junction temperature, TJ. To get a temperature

reading, measure the duty cycle of the CLKOUT signal and

use the following equation to approximate the junction

temperature:

TJ

=

DCCLKOUT –

0.3%

35%

where DCCLKOUT is the CLKOUT duty cycle in % and TJ

is the die junction temperature in °C. Although the actual

die temperature can deviate from the above equation by

±15°C, the relationship between change in CLKOUT duty

cycle and change in die temperature is well defined. Basi-

cally a 1% change in CLKOUT duty cycle corresponds to a

3.33°C change in die temperature. Note that the CLKOUT

pin is only meant to drive capacitive loads up to 50pF.

Thermal Lockout

A fault condition occurs when the die temperature exceeds

165°C (see Operation Section), and the part goes into

thermal lockout. The fault condition ceases when the die

temperature drops by ~5°C (nominal).

SWITCHING FREQUENCY

There are several considerations in selecting the operat-

ing frequency of the converter. The first is staying clear

of sensitive frequency bands, which cannot tolerate any

spectral noise. For example, in products incorporating RF

communications, the 455kHz IF frequency is sensitive to

any noise, therefore switching above 600kHz is desired.

Some communications have sensitivity to 1.1MHz and in

that case a 1.5MHz switching converter frequency may be

employed. The second consideration is the physical size

of the converter. As the operating frequency goes up, the

inductor and filter capacitors go down in value and size.

The tradeoff is efficiency, since the losses due to switch-

ing dynamics (see Thermal Considerations), Schottky

diode charge, and other capacitive loss terms increase

proportionally with frequency.

Oscillator Timing Resistor (RT)

The operating frequency of the LT3581 can be set by the

internal free-running oscillator. When the SYNC pin is driven

low (< 0.4V), the frequency of operation is set by a resistor

from the RT pin to ground. An internally trimmed timing

capacitor resides inside the IC. The oscillator frequency

is calculated using the following formula:

fOSC

=

87.6

RT + 1

where fOSC is in MHz and RT is in k. Conversely, RT (in k)

can be calculated from the desired frequency (in MHz)

using:

RT

=

87.6

fOSC

–1

18

For more information www.linear.com/LT3581

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