LT1761ES5-1.5 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1761ES5-1.5' PDF : 22 Pages View PDF
LT1761 Series
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,
similar to the way a piezoelectric accelerometer or micro-
phone works. For a ceramic capacitor the stress can be
induced by vibrations in the system or thermal transients.
The resulting voltages produced can cause appreciable
amounts of noise, especially when a ceramic capacitor is
used for noise bypassing. A ceramic capacitor produced
Figure 5’s trace in response to light tapping from a pencil.
Similar vibration induced behavior can masquerade as
increased output voltage noise.
VOUT
500μV/DIV
LT1761-5
COUT = 10μF
CBYP = 0.01μF
ILOAD = 100mA
100ms/DIV
1761 F05
Figure 5. Noise Resulting from Tapping on a Ceramic Capacitor
Thermal Considerations
The power handling capability of the device will be limited
by the maximum rated junction temperature (125°C). 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 ground pin current can be found by examining the
GND Pin Current curves in the Typical Performance Char-
acteristics section. Power dissipation will be equal to the
sum of the two components listed above.
The LT1761 series regulators have internal thermal limiting
designed to protect the device during overload conditions.
For continuous normal conditions, the maximum junction
temperature rating of 125°C must not be exceeded. It is
important to give careful consideration to all sources of
thermal resistance from junction to ambient. 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 table lists 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. Measured Thermal Resistance
COPPER AREA
TOPSIDE*
2500mm2
1000mm2
BACKSIDE
2500mm2
2500mm2
BOARD AREA
2500mm2
2500mm2
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
125°C/W
125°C/W
225mm2 2500mm2 2500mm2
130°C/W
100mm2
50mm2
2500mm2
2500mm2
2500mm2
2500mm2
135°C/W
150°C/W
*Device is mounted on topside.
Calculating Junction Temperature
Example: Given an output voltage of 3.3V, an input voltage
range of 4V to 6V, an output current range of 0mA to 50mA
1761sff
17
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