IRU1120 Datasheet, PDF (5/9 Pages) International Rectifier – 2A LOW DROPOUT POSITIVE ADJUSTABLE REGULATOR

IRU1120
Thermal Protection
When the junction temperature exceeds 1508C, the in-
ternal thermal protection shuts the IRU1120 down.
Current Limit Protection
The IRU1120 provides Over Current Protection when the
output current exceeds typically 3A. The output de-
creases to limit the power dissipation.
Stability
The IRU1120 requires the use of an output capacitor as
part of the frequency compensation in order to make the
regulator stable. A minimum input capacitance of 4.7mF
and a minimum output capacitance 4.7mF Ceramic ca-
pacitor is needed for regulator stage as well as the speci-
fied minimum loads to guarantee stability.
Transient Response and PSRR
The input and output capacitors are critical in order to
ensure good transient response and PSRR. The most
important aspects of this are capacitor selection, place-
ment and trace routing. Place each capacitor as close
as physically possible to its corresponding regulator pin.
Use wide traces for a low inductance path. Couple di-
rectly to the ground and power planes as possible. The
use of low ESR capacitors is crucial to achieving good
results. Larger capacitance and lower ESR will improve
both PSRR and transient response.
Thermal Design
The IRU1120 incorporates an internal thermal shutdown
that protects the device when the junction temperature
exceeds the allowable maximum junction temperature.
Although this device can operate with junction tempera-
tures in the range of 1508C, it is recommended that the
selected heat sink be chosen such that during maxi-
mum continuous load operation the junction tempera-
ture is kept below this number. The example below shows
the steps in selecting the proper surface mount pack-
age.
Assuming, the following conditions:
VOUT = 2.7V
VIN = 3.3V
VCTRL = 5V
IOUT = 1A (DC Avg)
Calculate the maximum power dissipation using the fol-
lowing equation:
( ) ^{PD = IOUT3(VIN - VOUT)+}
IOUT
60
3(VCTRL - VOUT)
( ) PD = 13(3.3 - 2.7)+ 1 3(5 - 2.7) = 0.63W
60
For Ultra Thin-Pak^TM we have:
RTHJA = 258C/W
TA = 458C
DT = PD3RTHJA = 0.63325 = 15.758C
TJ = TA + DT = 60.758C
Layout Consideration
The IRU1120, like many other high-speed regulators,
requires that the output capacitors be close to the de-
vice for stability. For power consideration, a ground plane
pad of approximately one-inch square on the compo-
nent side must be dedicated to the device where all Gnd
pins are connected to dissipate the power. If a multilayer
board is used, it is recommended that the inner layers of
the board are also dedicated to the size of the pad for
better thermal characteristics.
Rev. 1.3
08/20/02
www.irf.com
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