LT1962 Series
W U U
U
APPLICATIO S I FOR ATIO
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.
TJMAX = 50°C + 35.3°C = 85.3°C
Protection Features
The LT1962 regulators incorporate several protection
featureswhichmakethemidealforuseinbattery-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.
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
Current limit protection and thermal overload protection
areintendedtoprotectthedeviceagainstcurrentoverload
conditions at the output of the device. For normal opera-
tion, the junction temperature should not exceed 125°C.
COPPER AREA
THERMAL RESISTANCE
TOPSIDE* BACKSIDE
BOARD AREA (JUNCTION-TO-AMBIENT)
2500mm2
1000mm2
225mm2
100mm2
50mm2
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
110°C/W
115°C/W
120°C/W
130°C/W
140°C/W
The input of the device will withstand reverse voltages of
20V.Currentflowintothedevicewillbelimitedtolessthan
1mA (typically less than 100µA) 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.
*Device is mounted on topside.
Calculating Junction Temperature
The output of the LT1962 can be pulled below ground
withoutdamagingthedevice.Iftheinputisleftopencircuit
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
flow to less than 40µA. For adjustable versions, the output
will act like an open circuit; no current will flow out of the
pin. If the input is powered by a voltage source, the output
will source the short-circuit current of the device and will
protect itself by thermal limiting. In this case, grounding
the SHDN pin will turn off the device and stop the output
from sourcing the short-circuit current.
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?
The power dissipated by the device will be equal to:
IOUT(MAX)(VIN(MAX) – VOUT) + IGND(VIN(MAX)
where,
)
IOUT(MAX) = 100mA
VIN(MAX) = 6V
IGND at (IOUT = 100mA, VIN = 6V) = 2mA
So,
The ADJ pin of the adjustable device can be pulled above
or below ground by as much as 7V without damaging the
device. Iftheinputisleftopencircuitorgrounded, theADJ
pin will act like an open circuit when pulled below ground
and like a large resistor (typically 100k) in series with a
diode when pulled above ground.
P = 100mA(6V – 3.3V) + 2mA(6V) = 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:
In situations where the ADJ pin is connected to a resistor
divider that would pull the ADJ pin above its 7V clamp
voltage if the output is pulled high, the ADJ pin input
current must be limited to less than 5mA. For example, a
resistor divider is used to provide a regulated 1.5V output
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:
14
Linear [ Linear ]
