Datasheet

9
LT1121/LT1121-3.3/LT1121-5
U
S
A
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PP
L
IC
AT
I
WU
U
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:
I
OUT MAX
× (V
IN MAX
– V
OUT
) + (I
GND
× V
IN
)
where, I
OUT MAX
= 100mA
V
IN MAX
= 7V
I
GND
at (I
OUT
= 100mA, V
IN
= 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:
T
JMAX
= 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
1µF with an ESR of 3Ω or less. For applications where
space is very limited, capacitors as low as 0.33µF 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.