LT1962 Linear Technology, LT1962 Datasheet - Page 7

• Manufacturer Part Number: LT1962
• Description: 300mA/ Low Noise/ Micropower LDO Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
1. Output current multiplied by the input/output voltage
2. GND pin current multiplied by the input voltage:
The GND pin current can be estimated using specification
in the Electrical Characteristics table. Power dissipation
will be equal to the sum of the two components listed
above.
The LT1962 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 1/16" 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
100mA and a maximum ambient temperature of 50 C,
what will the maximum junction temperature be?
TOPSIDE*
2500mm
1000mm
225mm
100mm
differential: (I
(I
50mm
GND
COPPER AREA
2
)(V
2
2
2
2
IN
BACKSIDE
2500mm
2500mm
2500mm
2500mm
2500mm
).
OUT
2
2
2
2
2
U
)(V
IN
BOARD AREA (JUNCTION-TO-AMBIENT)
2500mm
2500mm
2500mm
2500mm
2500mm
– V
U
OUT
2
2
2
2
2
), and
W
THERMAL RESISTANCE
110 C/W
115 C/W
120 C/W
130 C/W
140 C/W
U
The power dissipated by the device will be equal to:
where,
So,
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:
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
Protection Features
The LT1962 regulators incorporate several protection
features which make them ideal for use in battery-powered
circuits. In addition to the normal protection features
associated with monolithic regulators, such as current
limiting and thermal limiting, the devices are protected
against reverse input voltages, reverse output voltages
and reverse voltages from output to input.
Current limit protection and thermal overload protection
are intended to protect the device against current overload
conditions at the output of the device. For normal opera-
tion, the junction temperature should not exceed 125 C.
The input of the device will withstand reverse voltages of
20V. Current flow into the device will be limited to less than
1mA (typically less than 100 A) and no negative voltage
will appear at the output. The device will protect both itself
and the load. This provides protection against batteries
which can be plugged in backward.
The output of the LT1962 can be pulled below ground
without damaging the device. If the input is left open circuit
or grounded, the output can be pulled below ground by
20V. For fixed voltage versions, the output will act like a
large resistor, typically 500k or higher, limiting current
I
I
V
I
P = 100mA(6V – 3.3V) + 2mA(6V) = 0.28W
0.28W(125 C/W) = 35.3 C
T
OUT(MAX)
OUT(MAX)
GND
JMAX
IN(MAX)
at (I
= 50 C + 35.3 C = 85.3 C
= 6V
OUT
(V
= 100mA
IN(MAX)
= 100mA, V
– V
OUT
IN
) + I
= 6V) = 2mA
LT1962 Series
GND
(V
IN(MAX)
)
7