LTC3104 datasheet(18/26 Pages) LINER

18 / 26 page

LTC3630

3630fb

APPLICATIONS INFORMATION

RSW to RL and multiply the result by the square of the

average output current:

I2R Loss = IO2(RSW + RL)

Other losses, including CIN and COUT ESR dissipative

losses and inductor core losses, generally account for

less than 2% of the total power loss.

Thermal Considerations

In most applications, the LTC3630 does not dissipate much

heat due to its high efficiency. But, in applications where

the LTC3630 is running at high ambient temperature with

low supply voltage and high duty cycles, such as dropout,

the heat dissipated may exceed the maximum junction

temperature of the part.

To prevent the LTC3630 from exceeding the maximum

junction temperature, the user will need to do some thermal

analysis. The goal of the thermal analysis is to determine

whether the power dissipated exceeds the maximum junc-

tion temperature of the part. The temperature rise from

ambient to junction is given by:

TR = PD • θJA

where PD is the power dissipated by the regulator and θJA

is the thermal resistance from the junction of the die to

the ambient temperature.

The junction temperature is given by:

TJ = TA + TR

Generally, the worst-case power dissipation is in dropout

at low input voltage. In dropout, the LTC3630 can provide

a DC current as high as the full 1.2A peak current to the

output. At low input voltage, this current flows through a

higher resistance MOSFET, which dissipates more power.

As an example, consider the LTC3630 in dropout at an input

voltage of 5V, a load current of 500mA and an ambient

temperature of 85°C. From the Typical Performance graphs

of Switch On-Resistance, the RDS(ON) of the top switch

at VIN = 5V and 100°C is approximately 1.9Ω. Therefore,

the power dissipated by the part is:

PD = (ILOAD)2 • RDS(ON) = (500mA)2 • 1.9Ω = 0.475W

For the MSOP package the θJA is 45°C/W. Thus, the junc-

tion temperature of the regulator is:

TJ = 85°C+ 0.475W •

°C

= 106.4°C

which is below the maximum junction temperature of

150°C.

Note that the while the LTC3630 is in dropout, it can provide

output current that is equal to the peak current of the part.

This can increase the chip power dissipation dramatically

and may cause the internal overtemperature protection

circuitry to trigger at 180°C and shut down the LTC3630.

Design Example

As a design example, consider using the LTC3630 in an

application with the following specifications: VIN = 24V,

VIN(MAX) = 70V, VOUT = 3.3V, IOUT = 500mA, f = 200kHz.

Furthermore, assume for this example that switching

should start when VIN is greater than 12V.

First, calculate the inductor value that gives the required

switching frequency:

3.3V

200kHz • 1.2A

⎛

⎝

⎜

⎞

⎠

⎟ •1–

3.3V

24V

⎛

⎝

⎜

⎞

⎠

⎟ ≅ 10μH

Next, verify that this value meets the LMIN requirement.

For this input voltage and peak current, the minimum

inductor value is:

LMIN =

24V • 150ns

1.2A

≅ 3μH

Therefore, the minimum inductor requirement is satisfied

and the 10μH inductor value may be used.

Next, CIN and COUT are selected. For this design, CIN should

be sized for a current rating of at least:

IRMS = 500mA •

3.3V

24V

•

24V

3.3V

–1

≅ 175mARMS

LTC3104 датащи(PDF) 18 Page - Linear Technology

LTC3104 датащи(HTML) 18 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

датащи скачать

ссылки URL