LTC3441 datasheet(16/20 Pages) AD

16 / 20 page

LTC3246

3246fa

For more information www.linear.com/LTC3246

APPLICATIONS INFORMATION

siderably. Poor board layout and failure to connect the die

pad (Pin 17) to a large ground plane can result in thermal

junction to ambient impedance well in excess of 40°C/W.

It is also possible to get thermal rates less than 40°C/W

with good airflow over the part and PC board.

Because of the wide input operating range, it is possible

to exceed the specified operating junction temperature

and even reach thermal shutdown (175°C typ). Figure 6

and Figure 7 show the available output current vs ambi-

ent temperature to ensure the 150°C operating junction

temperature is not exceeded.

Thefiguresassumeworst-caseoperatingconditionsanda

thermal impedance of 40°C/W. It is always safe to operate

under the line shown on the graph. Operation above the

line is conditional and is the responsibility of the user to

calculate worst-case operating conditions (temperature

and power) to make sure the part does not exceed the

150°C operating junction temperature for extended pe-

riods of time.

The 2:1 Step-Down Charge Pump Operation, 1:1 Step-

Down Charge Pump Operation, and 1:2 Step-Up Charge

PumpOperationsectionsprovideequationsforcalculating

power dissipation (PD) in each mode.

For example, if it is determined that the maximum power

dissipation (PD) is 1.2W under normal operation, then the

junction to ambient temperature rise will be:

TJA = 1.2W • 40°C/W = 48°C

Thus, the ambient temperature under this condition can-

not exceed 102°C if the junction temperature is to remain

below 150°C, and, if the ambient temperature exceeds

about 127°C, the device will cycle in and out of the thermal

shutdown.

Every application will have a slightly different thermal rise

than the specified 40°C/W, especially applications with

good airflow. Calculating the actual thermal rate for a

specific application circuit is too complex to be presented

here, but the thermal rate can be measured in application.

This is done by first taking the final application circuit and

enabling the LTC3246 under a known power dissipation

(PD) and raising the ambient temperature slowly until

the LTC3246 shuts down. Note this temperature as T1.

Now, remove the load from the part and raise the ambi-

ent temperature slowly until the LTC3246 shuts down

again. Note this temperature as T2. The thermal rate can

be calculated as:

JA = PD/(T2 – T1)

Another method for determining maximum safe operating

temperature in application is to configure the LTC3246 to

operate under the worst case operating power dissipa-

tion. Then slowly raise the ambient temperature until the

LTC3246 shuts down. At this point the LTC3246 junction

temperature will be about 175°C, so simply subtract

25°C from the shutdown temperature and this is the safe

operating temperature for the application.

Figure6.

AMBIENT TEMPERATURE (°C)

0.5

0.4

0.1

0.3

0.2

0.0

125

3246 F06

150

100

2.7V < VIN < 22V

θJA = 40°C/W

CONDITIONAL

OPERATION

SAFE OPERATION

5V Output Operation vs Ambient Temperature

Figure7.

AMBIENT TEMPERATURE (°C)

0.5

0.4

0.1

0.3

0.2

0.0

125

3246 F07

150

100

2.7V < VIN < 15V

θJA = 40°C/W

CONDITIONAL

OPERATION

SAFE OPERATION

3.3V Output Operation vs Ambient Temperature

LTC3441 Datasheet(PDF) 16 Page - Analog Devices

LTC3441 Datasheet(HTML) 16 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL