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LTC3246 데이터시트(HTML) 16 Page - Analog Devices

부품명 LTC3246
상세내용  Wide VIN Range Buck-Boost Charge Pump with Watchdog Timer
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LTC3246 데이터시트(HTML) 16 Page - Analog Devices

 
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LTC3246
16
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
0.5
0.4
0.1
0.3
0.2
0.0
125
75
3246 F06
150
100
50
25
2.7V < VIN < 22V
θJA = 40°C/W
CONDITIONAL
OPERATION
SAFE OPERATION
5V Output Operation vs Ambient Temperature
Figure7.
AMBIENT TEMPERATURE (°C)
0
0.5
0.4
0.1
0.3
0.2
0.0
125
75
3246 F07
150
100
50
25
2.7V < VIN < 15V
θJA = 40°C/W
CONDITIONAL
OPERATION
SAFE OPERATION
3.3V Output Operation vs Ambient Temperature


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