LT1962 Series
applicaTions inForMaTion
ꢀoltage 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
microphone works. For a ceramic capacitor the stress can
beinducedbyvibrationsinthesystemorthermaltransients.
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 6’s trace in response to light tapping from a pencil.
Similar vibration induced behavior can masquerade as
increased output voltage noise.
The following table lists thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on 1/16" FR-4 board with one ounce
copper.
Table 1. Measured Thermal Resistance
COPPER AREA
THERMAL RESISTANCE
TOPSIDE*
BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
2
2
2
2
2
2
2
2
2
2
2
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
2500mm
110°C/W
115°C/W
120°C/W
130°C/W
140°C/W
2
1000mm
2
225mm
2
100mm
2
50mm
*Device is mounted on topside.
Thermal Considerations
Calculating Junction Temperature
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:
Example: Given an output voltage of 3.3ꢀ, an input volt-
age range of 4ꢀ to 6ꢀ, an output current range of 0mA
to 100mA and a maximum ambient temperature of 50°C,
what will the maximum junction temperature be?
1. Output current multiplied by the input/output voltage
The power dissipated by the device will be equal to:
differential: (I )(ꢀ – ꢀ ), and
OUT
IN
OUT
I
(ꢀ
– ꢀ ) + I (ꢀ
)
OUT(MAX) IN(MAX)
OUT
GND IN(MAX)
2. GND pin current multiplied by the input voltage:
(I )(ꢀ ).
where,
GND
IN
I
= 100mA
= 6ꢀ
OUT IN
OUT(MAX)
The GND pin current can be found by examining the GND
Pin Current curves in the Typical Performance Character-
istics section. Power dissipation will be equal to the sum
of the two components listed above.
ꢀ
IN(MAX)
I
at (I
= 100mA, ꢀ = 6ꢀ) = 2mA
GND
So,
The LT1962 series regulators have internal thermal
limiting designed to protect the device during overload
conditions. For continuous normal conditions, the maxi-
mum 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 ambi-
ent. Additional heat sources mounted nearby must also
be considered.
P = 100mA(6ꢀ – 3.3ꢀ) + 2mA(6ꢀ) = 0.28W
The thermal resistance will be in the range of 110°C/W to
140°C/W depending on the copper area. So the junction
temperature rise above ambient will be approximately
equal to:
0.28W(125°C/W) = 35.3°C
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
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.
T
JMAX
= 50°C + 35.3°C = 85.3°C
1962fba
14
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