LT3579IUFD-1-PBF データシートの表示（PDF） - Linear Technology

部品番号

コンポーネント説明

メーカー

LT3579IUFD-1-PBF

6A Boost/Inverting DC/DC Converter with Fault Protection

Linear Technology 

LT3579/LT3579-1

APPLICATIONS INFORMATION

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

packaging/Linear_Technology_Thermal_Resistance_

Table.pdf) θJA value is 38°C/W for the TSSOP Exposed Pad

package and 34°C/W for the 4mm × 5mm QFN 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 ~22°C/W was consistently achieved for both TSSOP

and QFN packages of the LT3579 (at VIN = 5V, VOUT = 12V,

IOUT = 1.7A, fOSC = 1MHz) when board layout was optimized

as per the suggestions in the Layout Guidelines section.

Junction Temperature Measurement

The duty cycle of the CLKOUT signal on the LT3579 is linearly

proportional to die junction temperature, TJ (the CLKOUT

duty cycle on the LT3579-1 is fixed at ~50%). To get an

accurate 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 absolute

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. A

3% increase in CLKOUT duty cycle corresponds to ~10°C

increase 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 – FAULT 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 operating

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 switching losses due to

NPN base charge (see Thermal Considerations), Schottky

diode charge, and other capacitive loss terms increase

proportionally with frequency.

Oscillator Timing Resistor (RT)

The operating frequency of the LT3579 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 kΩ. Conversely, RT (in

kΩ) can be calculated from the desired frequency (in

MHz) using:

RT

=

87.6

fOSC

–1

35791f

20

Share Link: