LT3012HFEPBF Просмотр технического описания (PDF) - Linear Technology
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LT3012HFEPBF Datasheet PDF : 16 Pages
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LT3012
APPLICATIONS INFORMATION
Voltage and temperature coefficients are not the only
sources of problems. Some ceramic capacitors have a
piezoelectric response. A piezoelectric device generates
voltage across its terminals due to mechanical stress, simi-
lar to the way a piezoelectric accelerometer or microphone
works. For a ceramic capacitor the stress can be induced
by vibrations in the system or thermal transients.
Current Limit and Safe Operating Area Protection
Like many IC power regulators, the LT3012 has safe oper-
ating area protection. The safe operating area protection
decreases the current limit as the input voltage increases
and keeps the power transistor in a safe operating region.
The protection is designed to provide some output current
at all values of input voltage up to the device breakdown
(see curve of Current Limit vs Input Voltage in the Typical
Performance Characteristics).
The LT3012 is limited for operating conditions by maximum
junction temperature. While operating at maximum input
voltage, the output current range must be limited; when
operating at maximum output current, the input voltage
range must be limited. Device specifications will not apply
for all possible combinations of input voltage and output
current. Operating the LT3012 beyond the maximum junc-
tion temperature rating may impair the life of the device.
Thermal Considerations
The power handling capability of the device will be limited
by the maximum rated junction temperature of (125°C for
LT3012E, or 140°C for LT3012HFE). The power dissipated
by the device will be made up of two components:
1. Output current multiplied by the input/output voltage
differential: IOUT • (VIN – VOUT) and,
2. GND pin current multiplied by the input voltage:
IGND • VIN.
The GND pin current can be found by examining the GND
Pin Current curves in the Typical Performance Character-
istics. Power dissipation will be equal to the sum of the
two components listed above.
The LT3012 has internal thermal limiting designed to pro-
tect the device during overload conditions. For continuous
normal conditions the maximum junction temperature
rating of 125°C (E-Grade) or 140°C (H-Grade)must not
be exceeded. It is important to give careful consideration
to all sources of thermal resistance from junction to ambi-
ent. Additional heat sources mounted nearby must also
be considered.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Copper board stiffeners and plated
through-holes can also be used to spread the heat gener-
ated by power devices.
The following tables list thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on 3/32" FR-4 board with one ounce
copper.
Table 1. DFN Measured Thermal Resistance
COPPER AREA
TOPSIDE
BOARD AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
2500 sq mm
2500 sq mm
40°C/W
1000 sq mm
2500 sq mm
45°C/W
225 sq mm
2500 sq mm
50°C/W
100 sq mm
2500 sq mm
62°C/W
Table 2. TSSOP Measured Thermal Resistance
COPPER AREA
TOPSIDE
BOARD AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
2500 sq mm
2500 sq mm
40°C/W
1000 sq mm
2500 sq mm
45°C/W
225 sq mm
2500 sq mm
50°C/W
100 sq mm
2500 sq mm
62°C/W
The thermal resistance junction-to-case (θJC), measured
at the exposed pad on the back of the die, is 16°C/W.
Continuous operation at large input/output voltage dif-
ferentials and maximum load current is not practical
due to thermal limitations. Transient operation at high
input/output differentials is possible. The approximate
thermal time constant for a 2500sq mm 3/32" FR-4 board
with maximum topside and backside area for one ounce
copper is 3 seconds. This time constant will increase as
more thermal mass is added (i.e., vias, larger board, and
other components).
3012fd
10
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