LT1761ES5-2.8 LINER [Linear Technology], LT1761ES5-2.8 Datasheet - Page 15

• Manufacturer Part Number: LT1761ES5-2.8
• Description: 100mA, Low Noise, LDO Micropower Regulators in SOT-23
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
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:
where,
So,
The thermal resistance will be in the range of 125 C/W to
150 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:
TOPSIDE*
2500mm
1000mm
225mm
100mm
50mm
I
I
V
I
P = 50mA(6V – 3.3V) + 1mA(6V) = 0.14W
0.14W(150 C/W) = 21.2 C
T
OUT(MAX)
OUT(MAX)
GND
JMAX
IN(MAX)
COPPER AREA
2
at (I
2
2
2
2
= 50 C + 21.2 C = 71.2 C
= 6V
OUT
(V
BACKSIDE
= 50mA
2500mm
2500mm
2500mm
2500mm
2500mm
IN(MAX)
= 50mA, V
U
2
2
2
2
2
– V
BOARD AREA (JUNCTION-TO-AMBIENT)
2500mm
2500mm
2500mm
2500mm
2500mm
INFORMATION
OUT
U
IN
) + I
= 6V) = 1mA
2
2
2
2
2
GND
W
THERMAL RESISTANCE
(V
IN(MAX)
125 C/W
125 C/W
130 C/W
135 C/W
150 C/W
)
U
Protection Features
The LT1761 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 LT1761-X 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
flow to typically less than 100 A. 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.
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.
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
from the 1.22V reference when the output is forced to 20V.
LT1761 Series
15