Datasheet

LT1762 Series
14
Rev B
For more information www.analog.com
APPLICATIONS INFORMATION
significant enough to drop capacitor values below appro-
priate levels. Capacitor DC bias characteristics tend to
improve as component case size increases, but expected
capacitance at operating voltage should be verified.
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.
The LT1762 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.
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.
Table1 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
BOARD AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)TOPSIDE* BACKSIDE
2500mm
2
2500mm
2
2500mm
2
110°C/W
1000mm
2
2500mm
2
2500mm
2
115°C/W
225mm
2
2500mm
2
2500mm
2
120°C/W
100mm
2
2500mm
2
2500mm
2
130°C/W
50mm
2
2500mm
2
2500mm
2
140°C/W
*Device is mounted on topside.
Calculating Junction Temperature
Example: Given an output voltage of 3.3V, an input volt-
age 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:
I
OUT(MAX)
(V
IN(MAX)
– V
OUT
) + I
GND
(V
IN(MAX)
)
where,
I
OUT(MAX)
= 150mA
V
IN(MAX)
= 6V
I
GND
at (I
OUT
= 150mA, V
IN
= 6V) = 5mA
So,
P = 150mA(6V – 3.3V) + 5mA(6V) = 0.44W
Figure6. Noise Resulting from Tapping on a Ceramic Capacitor
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
differential: (I
OUT
)(V
IN
– V
OUT
), and
2. GND pin current multiplied by the input voltage:
(I
GND
)(V
IN
).
The GND pin current can be found by examining the
GND Pin Current curves in the Typical Performance
Characteristics. Power dissipation will be equal to the
sum of the two components listed above.
V
OUT
500μV/DIV
1762 F06
100ms/DIV
LT1762-5
C
OUT
= 10μF
C
BYP
= 0.01μF
I
L
= 100mA
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.