LT1763 Series
APPLICATIONS INFORMATION
Table 2. SO-8 Package, 8-Lead SO
Protection Features
COPPER AREA
The LT1763 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.
THERMAL RESISTANCE
BOARD AREA (JUNCTION-TO-AMBIENT)
TOPSIDE* BACKSIDE
2500mm2
1000mm2
225mm2
100mm2
50mm2
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
2500mm2
60°C/W
60°C/W
68°C/W
74°C/W
86°C/W
Current limit protection and thermal overload protection
are intended to protect the device against current overload
conditionsattheoutputofthedevice.Fornormaloperation,
the junction temperature should not exceed 125°C.
* Device is mounted on topside
Calculating Junction Temperature
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 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.
Example: Given an output voltage of 3.3V, an input voltage
rangeof4Vto6V,anoutputcurrentrangeof0mAto250mA
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
(V
– V ) + I (V
)
OUT(MAX) IN(MAX)
OUT
GND IN(MAX)
The output of the LT1763-X can be pulled below ground
withoutdamagingthedevice.Iftheinputisleftopen-circuit
or grounded, the output can be pulled below ground by
20V. For fixed voltage versions, the output will act like a
largeresistor,typically500korhigher,limitingcurrentflow
to less than 100μA. 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.
where,
I
= 250mA
= 6V
OUT IN
OUT(MAX)
V
IN(MAX)
I
at (I
= 250mA, V = 6V) = 5mA
GND
So,
P = 250mA(6V – 3.3V) + 5mA(6V) = 0.71W
The thermal resistance will be in the range of 60°C/W to
86°C/W, depending on the copper area. So, the junction
temperature rise above ambient will be approximately
equal to:
The ADJ pin of the adjustable device can be pulled above
or below ground by as much as 7V without damaging the
device. If the input is left open-circuit or grounded, the
ADJ 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.
0.71W(75°C/W) = 53.3°C
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature, or:
T
= 50°C + 53.3°C = 103.3°C
JMAX
In situations where the ADJ pin is connected to a resistor
divider that would pull the ADJ pin above its 7V clamp
voltageiftheoutputispulledhigh,theADJpininputcurrent
must be limited to less than 5mA. For example, a resistor
divider is used to provide a regulated 1.5V output from the
1.22V reference when the output is forced to 20V.
1763fe
17
Linear [ Linear ]
