LT1762 LINER [Linear Technology], LT1762 Datasheet - Page 13

• Manufacturer Part Number: LT1762
• Description: 150mA, Low Noise, LDO Micropower Regulators
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
across temperature, while the X5R is less expensive and
is available in higher values.
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 6’s trace in response to light tapping from a pencil.
Similar vibration induced behavior can masquerade as
increased output voltage noise.
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
2. GND pin current multiplied by the input voltage:
Figure 6. Noise Resulting from Tapping on a Ceramic Capacitor
differential: (I
(I
GND
500 V/DIV
)(V
V
OUT
IN
).
LT1762-5
C
C
I
LOAD
OUT
BYP
OUT
= 10 F
= 0.01 f
= 100mA
U
)(V
IN
INFORMATION
– V
U
100ms/DIV
OUT
), and
W
1762 F05
U
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 LT1762 series regulators have internal thermal limit-
ing designed to protect the device during overload condi-
tions. 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
*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
and a maximum ambient temperature of 50 C, what will
the maximum junction temperature be?
The power dissipated by the device will be equal to:
TOPSIDE*
2500mm
1000mm
225mm
100mm
I
50mm
OUT(MAX)
COPPER AREA
2
2
2
2
2
BACKSIDE
(V
2500mm
2500mm
2500mm
2500mm
2500mm
IN(MAX)
2
2
2
2
2
– V
BOARD AREA (JUNCTION-TO-AMBIENT)
2500mm
2500mm
2500mm
2500mm
2500mm
OUT
) + I
2
2
2
2
2
GND
LT1762 Series
THERMAL RESISTANCE
(V
IN(MAX)
110 C/W
115 C/W
120 C/W
130 C/W
140 C/W
)
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