iru1117 International Rectifier Corp., iru1117 Datasheet - Page 5

• Manufacturer Part Number: iru1117
• Description: 800ma Low Dropout Positive Adjustable Regulator
• Manufacturer: International Rectifier Corp.

Assuming the following specifications:
+5V is significantly more than required to maintain drop-
out voltage across the IRU1117 when regulating V
2.85V. The "Extra" voltage means additional power is
dissapated in the IRU1117. The diode dissapates some
of this additional power, allowing the IRU1117 to run cooler.
The steps for selecting the right package with proper
board area for heat sinking to keep the junction tem-
perature below 1358C is given as:
1) Calculate the maximum power dissipation using:
2) Calculate the maximum u
3) Select a package from the data sheet with lower u
Rev. 1.8
02/04/03
ample:
than the one calculated in the previous step.
Selecting TO-252 (D-Pak) with at least 0.5" square
of 0.062" FR4 board using 1oz. copper has 70 C/W
which is lower than the calculated number.
V
V
V
I
T
Where:
V
shown in Figure 5.
P
P
u
OUT(MAX)
JA(MAX)
A
IN
F
OUT
F
D
D
= 358C
= 0.5V
= I
= 0.8 3 (5 - 0.5 - 2.85) = 1.32W
is the forward voltage drop of the D1 diode as
= 5V
= 2.85V
OUT
=
= 0.8A
T
3 (V
J
P
- T
D
IR WORLD HEADQUARTERS: 233 Kansas St., El Segundo, California 90245, USA Tel: [phone]
A
IN
=
- V
135 - 35
F
1.32
- V
OUT
= 75.68C/W
)
JA
allowed for our ex-
OUT
www.irf.com
to
JA
Data and specifications subject to change without notice. 02/01
To set the output DC voltage, we need to select R1 and
R2:
4) Assuming R1 = 121V, 1%:
5) Calculate the power dissapation for the schottky di-
Layout Consideration
The output capacitors must be located as close to the
V
mended to use a section of a layer of the PC board as a
plane to connect the V
prevent any high frequency oscillation that may result
due to excessive trace inductance.
5V
OUT
Select R2 = 154V, 1%
ode (D1):
and select a suitable component.
R2 =
P
P
terminal of the device as possible. It is recom-
D =
D =
Visit us at www.irf.com for sales contact information
10uF
D1
Figure 5 - Final Schematic for half of the
C1
V
0.5 3 0.8 = 0.4W
(
F
3 I
V
V
OUT
REF
OUT
GTL+ termination regulator.
V
IN
-1
)
3R1 =
IRU1117
OUT
Adj
pin to the output capacitors to
(
V
2.85
1.25
OUT
TAC Fax: [phone]
-1
)
R2
154
1%
R1
121
1%
3121 = 154.8V
IRU1117
C2
22uF
2.85V
5