IRU1117 Просмотр технического описания (PDF) - International Rectifier
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IRU1117 Datasheet PDF : 10 Pages
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IRU1117
Assuming the following specifications:
VIN = 5V
VF = 0.5V
VOUT = 2.85V
IOUT(MAX) = 0.8A
TA = 358C
Where:
VF is the forward voltage drop of the D1 diode as
shown in Figure 5.
+5V is significantly more than required to maintain drop-
out voltage across the IRU1117 when regulating VOUT to
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.
To set the output DC voltage, we need to select R1 and
R2:
4) Assuming R1 = 121V, 1%:
( ) ( ) R2 =
VOUT
VREF
-1
3R1 =
2.85 -1
1.25
3121 = 154.8V
Select R2 = 154V, 1%
5) Calculate the power dissapation for the schottky di-
ode (D1):
PD = VF 3 IOUT
PD = 0.5 3 0.8 = 0.4W
and select a suitable component.
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:
D1
5V
C1
10uF
V IN
V OUT
IRU1117
2.85V
C2
22uF
1) Calculate the maximum power dissipation using:
PD = IOUT 3 (VIN - VF - VOUT)
PD = 0.8 3 (5 - 0.5 - 2.85) = 1.32W
Adj
R1
121
1%
R2
154
1%
2) Calculate the maximum uJA allowed for our ex-
ample:
uJA(MAX)
= TJ - TA
PD
=
135 - 35
1.32
=
75.68C/W
3) Select a package from the data sheet with lower uJA
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.
Figure 5 - Final Schematic for half of the
GTL+ termination regulator.
Layout Consideration
The output capacitors must be located as close to the
VOUT terminal of the device as possible. It is recom-
mended to use a section of a layer of the PC board as a
plane to connect the VOUT pin to the output capacitors to
prevent any high frequency oscillation that may result
due to excessive trace inductance.
Rev. 1.8
02/04/03
IR WORLD HEADQUARTERS: 233 Kansas St., El Segundo, California 90245, USA Tel: (310) 252-7105
TAC Fax: (310) 252-7903
Visit us at www.irf.com for sales contact information
Data and specifications subject to change without notice. 02/01
www.irf.com
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