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LT1121CS8-3.3 Datasheet(PDF) 9 Page - Linear Technology

Part # LT1121CS8-3.3
Description  Micropower Low Dropout Regulators with Shutdown
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LT1121CS8-3.3 Datasheet(HTML) 9 Page - Linear Technology

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9
LT1121/LT1121-3.3/LT1121-5
S
APPLICATI
I FOR ATIO
taken in still air, on 3/32" FR-4 board with 1oz copper. All
NC leads were connected to the ground plane.
Calculating Junction Temperature
Example: given an output voltage of 3.3V, an input voltage
range of 4.5V to 7V, an output current range of 0mA to
100mA, and a maximum ambient temperature of 50
°C,
what will the maximum junction temperature be?
Power dissipated by the device will be equal to:
IOUT MAX × (VIN MAX – VOUT) + (IGND × VIN)
where, IOUT MAX = 100mA
VIN MAX = 7V
IGND at (IOUT = 100mA, VIN = 7V) = 5mA
so,
P = 100mA
× (7V – 3.3V) + (5mA × 7V)
= 0.405W
If we use an SOT-223 package, then the thermal resistance
will be in the range of 50
°C/W to 65°C/W depending on
copper area. So the junction temperature rise above
ambient will be less than or equal to:
0.405W
× 60°C/W = 24°C
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
TJMAX = 50°C + 24°C = 74°C
Table 5. TO-92 Package
THERMAL
RESISTANCE
Package alone
220
°C/W
Package soldered into PC board with plated
175
°C/W
through holes only
Package soldered into PC board with 1/4 sq. inch of
145
°C/W
copper trace per lead
Package soldered into PC board with plated through holes
in board, no extra copper trace, and a clip-on type
heat sink:
Thermalloy type 2224B
160
°C/W
Aavid type 5754
135
°C/W
Output Capacitance and Transient Performance
The LTC1121 is designed to be stable with a wide range of
output capacitors. The minimum recommended value is
1
µF with an ESR of 3Ω or less. For applications where
space is very limited, capacitors as low as 0.33
µF can be
used if combined with a small series resistor. Assuming
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
Table 1. N8 Package*
COPPER AREA
TOPSIDE
BACKSIDE
BOARD AREA
2500 sq mm
2500 sq. mm
2500 sq. mm
80
°C/W
1000 sq mm
2500 sq. mm
2500 sq. mm
80
°C/W
225 sq mm
2500 sq. mm
2500 sq. mm
85
°C/W
1000 sq mm
1000 sq. mm
1000 sq. mm
91
°C/W
* Device is mounted on topside. Leads are through hole and are soldered
to both sides of board.
Table 2. S8 Package
COPPER AREA
TOPSIDE*
BACKSIDE
BOARD AREA
2500 sq. mm 2500 sq. mm
2500 sq. mm
120
°C/W
1000 sq. mm 2500 sq. mm
2500 sq. mm
120
°C/W
225 sq. mm
2500 sq. mm
2500 sq. mm
25
°C/W
100 sq. mm
1000 sq. mm
1000 sq. mm
131
°C/W
* Device is mounted on topside.
Table 4. SOT-223 Package
(Thermal Resistance Junction-to-Tab 20
°C/W)
COPPER AREA
TOPSIDE*
BACKSIDE
BOARD AREA
2500 sq. mm 2500 sq. mm
2500 sq. mm
50
°C/W
1000 sq. mm 2500 sq. mm
2500 sq. mm
50
°C/W
225 sq. mm
2500 sq. mm
2500 sq. mm
58
°C/W
100 sq. mm
2500 sq. mm
2500 sq. mm
64
°C/W
1000 sq. mm 1000 sq. mm
1000 sq. mm
57
°C/W
1000 sq. mm
0
1000 sq. mm
60
°C/W
* Tab of device attached to topside copper
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
Table 3. AS8 Package*
COPPER AREA
TOPSIDE**
BACKSIDE
BOARD AREA
2500 sq. mm 2500 sq. mm
2500 sq. mm
60
°C/W
1000 sq. mm 2500 sq. mm
2500 sq. mm
60
°C/W
225 sq. mm
2500 sq. mm
2500 sq. mm
68
°C/W
100 sq. mm
2500 sq. mm
2500 sq. mm
74
°C/W
*
Pins 3, 6, and 7 are ground.
** Device is mounted on topside.


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