LT1121-3.3 Linear Technology, LT1121-3.3 Datasheet - Page 9

• Manufacturer Part Number: LT1121-3.3
• Description: Micropower Low Dropout Regulators with Shutdown
• Manufacturer: Linear Technology

A
taken in still air, on 3/32" FR-4 board with 1oz copper. All
NC leads were connected to the ground plane.
Table 3. AS8 Package*
TOPSIDE**
2500 sq. mm 2500 sq. mm 2500 sq. mm
1000 sq. mm 2500 sq. mm 2500 sq. mm
225 sq. mm
100 sq. mm
*
** Device is mounted on topside.
Table 4. SOT-223 Package
(Thermal Resistance Junction-to-Tab 20 C/W)
TOPSIDE*
2500 sq. mm 2500 sq. mm
1000 sq. mm 2500 sq. mm
225 sq. mm
100 sq. mm
1000 sq. mm 1000 sq. mm
1000 sq. mm
* Tab of device attached to topside copper
Table 1. N8 Package*
TOPSIDE
2500 sq mm
1000 sq mm
225 sq mm
1000 sq mm
* Device is mounted on topside. Leads are through hole and are soldered
Table 2. S8 Package
TOPSIDE*
2500 sq. mm 2500 sq. mm 2500 sq. mm
1000 sq. mm 2500 sq. mm 2500 sq. mm
225 sq. mm
100 sq. mm
* Device is mounted on topside.
PPLICATI
to both sides of board.
Pins 3, 6, and 7 are ground.
COPPER AREA
COPPER AREA
COPPER AREA
COPPER AREA
2500 sq. mm 2500 sq. mm
2500 sq. mm 2500 sq. mm
2500 sq. mm
2500 sq. mm
2500 sq. mm 2500 sq. mm
2500 sq. mm 2500 sq. mm
2500 sq. mm 2500 sq. mm
1000 sq. mm 1000 sq. mm
2500 sq. mm 2500 sq. mm
1000 sq. mm 1000 sq. mm
BACKSIDE
BACKSIDE
BACKSIDE
BACKSIDE
0
O
U
S
BOARD AREA
BOARD AREA
BOARD AREA
BOARD AREA
2500 sq. mm
2500 sq. mm
2500 sq. mm
2500 sq. mm
1000 sq. mm
1000 sq. mm
I FOR ATIO
U
(JUNCTION-TO-AMBIENT)
W
(JUNCTION-TO-AMBIENT)
(JUNCTION-TO-AMBIENT)
THERMAL RESISTANCE
THERMAL RESISTANCE
THERMAL RESISTANCE
120 C/W
120 C/W
131 C/W
60 C/W
60 C/W
68 C/W
74 C/W
50 C/W
50 C/W
58 C/W
64 C/W
57 C/W
60 C/W
80 C/W
80 C/W
85 C/W
91 C/W
25 C/W
U
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:
where, I
so,
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:
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
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 F with an ESR of 3 or less. For applications where
space is very limited, capacitors as low as 0.33 F can be
used if combined with a small series resistor. Assuming
Table 5. TO-92 Package
Package alone
Package soldered into PC board with plated
Package soldered into PC board with 1/4 sq. inch of
Package soldered into PC board with plated through holes
through holes only
copper trace per lead
in board, no extra copper trace, and a clip-on type
heat sink:
LT1121/LT1121-3.3/LT1121-5
I
V
I
P = 100mA (7V – 3.3V) + (5mA 7V)
0.405W 60 C/W = 24 C
T
OUT MAX
OUT MAX
GND
JMAX
IN MAX
= 0.405W
at (I
= 50 C + 24 C = 74 C
Thermalloy type 2224B
Aavid type 5754
= 7V
OUT
= 100mA
(V
IN MAX
= 100mA, V
– V
OUT
IN
) + (I
= 7V) = 5mA
GND
V
RESISTANCE
THERMAL
IN
220 C/W
175 C/W
145 C/W
160 C/W
135 C/W
)
9