IRU1502-33
8
Rev. 1.0
07/17/03
www.irf.com
APPLICATION INFORMATION
Introduction
The IRU1502-33 regulator is a 3-terminal device offered
in a fixed output of 3.3V and it is designed specifically to
provide an extremely low dropout voltage.
The IRU1502-33 is designed to meet the fast current
transient needs as well as providing an accurate initial
voltage, reducing the overall system cost with the need
for fewer number of output capacitors.
Thermal Protection
When the junction temperature exceeds 1358C, the in-
ternal thermal protection shuts the IRU1502-33 down.
Current Limit Protection
The IRU1502-33 provides Over Current Protection when
the output current exceeds typically 1.4A. The output
decreases to limit the power dissipation.
Stability
The IRU1502-33 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 Ce-
ramic capacitor is needed for regulator stage as well as
the specified 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 IRU1502-33 incorporates an internal thermal shut-
down that protects the device when the junction tem-
perature exceeds the allowable maximum junction tem-
perature. Although this device can operate with junction
temperatures in the range of 1508C, it is recommended
that the selected heat sink be chosen such that during
maximum continuous load operation the junction tem-
perature is kept below 1258C. The following shows the
typical thermal design.
Figure 17 - Thermal resistor diagram for IRU1502-33.
Where:
ΘJC is the thermal resistance from junction to case.
ΘCH is the thermal resistance from case to heat sink
if applicable.
ΘHA is the thermal resistance from heat sink to am-
bient.
ΘJA(through plastic cover) is the thermal resistance from junc-
tion to the ambient through plastic cover. Typically it
is very large and can be neglected. Therefore, over-
all junction-to-ambient thermal resistance can be rep-
resented as:
Where ΘJA is the junction to ambient thermal
resistance.
ΘJA ≅ ΘJC+ΘCH+ΘHA
Power consumption
AmbientJunction
Θ
JA(through plastic cover)
Θ
JC
Θ
CH
Θ
HA
