LT1579CS8-3.3 Linear Technology, LT1579CS8-3.3 Datasheet - Page 17

• Manufacturer Part Number: LT1579CS8-3.3
• Description: IC LDO REG SMART DUAL 3.3V 8SOIC
• Manufacturer: Linear Technology

Specifications of LT1579CS8-3.3

• Function: Back-Up Management
• Battery Type: Smart Batteries
• Operating Temperature: 0°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIONS
resistance from the junction to a ground lead is 40 C/W for
16-lead SSOP, 32 C/W for 16-lead SO and 35 C/W for
8-lead S0.
Table 2. 8-Lead SO Package (S8)
*Device is mounted on topside.
Table 3. 16-Lead SO Package (S)
*Device is mounted on topside.
Table 4. 16-Lead SSOP Package (GN)
*Device is mounted on topside.
Calculating Junction Temperature
Example: Given an output voltage of 5V, an input voltage
range of 5V to 7V for V
output current range of 10mA to 150mA and a maximum
ambient temperature of 50 C, what will the maximum
junction temperature be?
When run from the primary input, current drawn from the
secondary input is negligible and worst-case power dissi-
pation will be:
Where:
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TOPSIDE*
TOPSIDE*
TOPSIDE*
(I
I
V
I
OUT(MAX)
GND
IN1(MAX)
OUT(MAX)
COPPER AREA
COPPER AREA
COPPER AREA
at (I
OUT
BACKSIDE
BACKSIDE
BACKSIDE
= 150mA
= 7V
)(V
= 150mA, V
IN1(MAX)
U
IN1
BOARD AREA (JUNCTION-TO-AMBIENT)
BOARD AREA (JUNCTION-TO-AMBIENT)
BOARD AREA (JUNCTION-TO-AMBIENT)
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INFORMATION
– V
U
and 8V to 10V for V
IN1
OUT
= 7V) = 2mA
) + (I
W
THERMAL RESISTANCE
THERMAL RESISTANCE
THERMAL RESISTANCE
GND
)(V
73 C/W
75 C/W
80 C/W
90 C/W
55 C/W
58 C/W
60 C/W
68 C/W
70 C/W
75 C/W
80 C/W
95 C/W
IN1(MAX)
IN2
U
, with an
)
Therefore,
When switched to the secondary input, current from the
primary input is negligible and worst-case power dissipa-
tion will be:
Where:
Therefore,
Using a 16-lead SO package, the thermal resistance will be
in the range of 55 C/W to 68 C/W dependent upon 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 temperature rise above ambient plus the
maximum ambient temperature or:
Protection Features
The LT1579 incorporates several protection features that
make it ideal for use in battery-powered circuits. In addi-
tion to the normal protection features associated with
monolithic regulators, such as current limiting and ther-
mal limiting, the device is 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.
Current limit protection is designed to protect the device
if the output is shorted to ground. With the output shorted
to ground, current will be drawn from the primary input
until it is discharged. The current drawn from V
increase until the primary input is discharged. This pre-
vents a short-circuit on the output from discharging both
inputs simultaneously.
P = (150mA)(7V – 5V) + (2mA)(7V) = 0.31W
(I
I
V
I
P = (150mA)(10V – 5V) + (2mA)(10V) = 0.77W
(0.77W)(65 C/W) = 50.1 C
T
OUT(MAX)
GND
JMAX
OUT(MAX)
IN2(MAX)
at (I
= 50.1 C + 50 C = 100.1 C
OUT
= 150mA
= 10V
)(V
= 150mA, V
IN2(MAX)
– V
IN2
OUT
= 10V) = 2mA
) + (I
GND
)(V
IN2(MAX)
LT1579
IN2
17
will not
)