Datasheet

LT1121/LT1121-3.3/LT1121-5
10
1121fg
APPLICATIONS INFORMATION
Table 1. N8 Package*
COPPER AREA
THERMAL RESISTANCE
JUNCTION TO AMBIENTTOPSIDE BACKSIDE BOARD AREA
2500 sq mm 2500 sq mm 2500 sq mm 80ºC/W
1000 sq mm 2500 sq mm 2500 sq mm 80ºC/W
225 sq mm 2500 sq mm 2500 sq mm 85ºC/W
1000 sq mm 1000 sq mm 1000 sq mm 91ºC/W
* Device is mounted on topside. Leads are through hole and are
soldered to both sides of board.
NC leads were connected to the ground plane.
Table 2. S8 Package
COPPER AREA
THERMAL RESISTANCE
JUNCTION TO AMBIENT
TOPSIDE* BACKSIDE BOARD AREA
2500 sq mm 2500 sq mm 2500 sq mm 120ºC/W
1000 sq mm 2500 sq mm 2500 sq mm 120ºC/W
225 sq mm 2500 sq mm 2500 sq mm 125ºC/W
100 sq mm 1000 sq mm 1000 sq mm 131ºC/W
* Device is mounted on topside.
Table 3. AS8 Package*
COPPER AREA
THERMAL RESISTANCE
JUNCTION TO AMBIENT
TOPSIDE** BACKSIDE BOARD AREA
2500 sq mm 2500 sq mm 2500 sq mm 60ºC/W
1000 sq mm 2500 sq mm 2500 sq mm 60ºC/W
225 sq mm 2500 sq mm 2500 sq mm 68ºC/W
100 sq mm 2500 sq mm 2500 sq mm 74ºC/W
* Pins 3, 6 and 7 are ground.
** Device is mounted on topside.
Table 4. SOT-223 Package
(Thermal Resistance Junction-to-Tab 20ºC/W)
COPPER AREA
THERMAL RESISTANCE
JUNCTION TO AMBIENT
TOPSIDE* BACKSIDE BOARD AREA
2500 sq mm 2500 sq mm 2500 sq mm 50ºC/W
1000 sq mm 2500 sq mm 2500 sq mm 50ºC/W
225 sq mm 2500 sq mm 2500 sq mm 58ºC/W
100 sq mm 2500 sq mm 2500 sq mm 64ºC/W
1000 sq mm 2500 sq mm 1000 sq mm 57ºC/W
1000 sq mm 0 1000 sq mm 60ºC/W
* Tab of device attached to topside copper.
Table 5. TO-92 Package THERMAL RESISTANCE
Package alone 220ºC/W
Package soldered into PC board with plated through
holes only
175ºC/W
Package soldered into PC board with 1/4 sq. inch of
copper trace per lead
145ºC/W
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
160ºC/W
Aavid type 5754 135ºC/W
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 ambi-
ent 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
Output Capacitance and Transient Performance
The LT1121 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 that the
ESR of the capacitor is low (ceramic)
the suggested series