iru3033 International Rectifier Corp., iru3033 Datasheet - Page 9

• Manufacturer Part Number: iru3033
• Description: 8-pin Pwm Switcher And Linear Controller Ic
• Manufacturer: International Rectifier Corp.

APPLICATION INFORMATION
Introduction
The IRU3033 device is an application specific product
designed to provide an on board dual supply for the new
generation of microprocessors requiring separate Core
and I/O supplies. One of the processors fitting this re-
quirement is the new Intel P55C
cessor. Intel specifies a Core voltage of 2.8V nominal
( 100mV max) with maximum Core supply current of
6A while the I/O supply is set for 3.3V with a maximum
I/O current of 0.65A. However, in most applications the
I/O regulator also provides the voltage for other IC func-
tions such as the chip set, cache, etc. Typically a low
cost solution such as a Low Dropout (LDO) Linear Regu-
lator is selected to provide the I/O supply with the maxi-
mum designed current of 3A, keeping the power dissi-
pation and the heat sink to a reasonable size. The Core
supply regulator, however, if also selected to be a linear
regulator, will be dissipating a maximum of 12.6W [(5V-
2.8V)X5.7A] of power, which requires a substantial
amount of heat sinking and perhaps forced air cooling in
order to keep it operational. Some manufacturers sug-
gest using two regulators to current share and therefore
distribute the power dissipation equally between the regu-
lators. The problem is that, in order to equally current
share you need to sense both currents and force the
slave regulator to match the master regulator. This can
be done, but at the cost of the circuit complexity and
much higher system cost and the total power dissipa-
tion is still the same. In fact, if the task is to design a
flexible motherboard to accommodate the Cyrix 6X86L
or their future MMX processors, then the power dissipa-
tion could easily reach 20W or more. At this power dis-
sipation level the choice for a switching regulator ap-
proach becomes evident. However, the main reason that
designers have always shied away from the switching
regulators is their higher price tag and more complex
circuit design that is associated with this kind of tech-
nique.
The IRU3033 device is designed to take advantage of
the high efficiency of the switching regulator technique
for the Core supply while maintaining the low cost LDO
regulator for the I/O supply by offering both control func-
tions in a single 8-pin surface mount package. In fact,
as the typical application circuit shows, one can design
a complete flexible motherboard using the IRU3033 and
a few external components yielding a very low compo-
nent count switching regulator and with an addition of a
low cost pass transistor for the I/O supply provide a com-
plete dual supply power solution.
Rev. 1.7
07/17/02
multimedia micropro-
www.irf.com
LDO Section
The output voltage of the LDO regulator is externally pro-
grammable via 2 external resistors from 1.25V to 5V.
The internal voltage reference of the LDO regulator is set
to 1.25V and the output of the regulator can be pro-
grammed using the following formula:
The IRU3033 requires the use of an output capacitor as
part of the frequency compensation in order to be stable.
Typical designs for the microprocessor applications use
standard electrolytic capacitors with typical ESR in the
range of 50 to 100mV and an output capacitance of 500
to 1000mF. Fortunately as the capacitance increases,
the ESR decreases resulting in a fixed RC time con-
stant. The IRU3033 takes advantage of this phenomena
in making the overall regulator loop stable. For most ap-
plications a minimum of 100mF aluminum electrolytic
capacitor such as Sanyo, MVGX series, Panasonic FA
series or Nichicon PL series insures both stability and
good transient response.
An external filtering is suggested as shown in the appli-
cation circuit that reduces the switching ripple that might
show in the output of the LDO regulator.
Switching Controller Operation
The operation of the switching controller is as follows:
After the power is applied, the output drive (Drv1) goes to
100% duty cycle and the current in the inductor charges
the output capacitor causing the output voltage to in-
crease. When output reaches a pre-programmed set
point the feedback pin (V
output drive to switch low and the V
high which jumps the feedback pin higher than 1.25V
resulting in a fixed output ripple which is given by the
following equation:
V
Where:
V
R1 = Resistor connected from V
IRU3033.
R2 = Resistor connected from V
DVo = (Rt/R
Where:
Rt = Top resistor of the output divider, resistor con-
nected from V
R
from V
OUT
REF
B
= Bottom resistor of the divider, resistor connected
= 1.25V Typical
= (1 + R1/R2) 3 V
FB1
pin to V
B
)311
OUT
HYST
to the V
FB1
pin.
) exceeds 1.25V causing the
REF
FB1
pin of IRU3033.
OUT
FB2
HYST
IRU3033
to the V
pin to Gnd.
pin to switch
FB2
pin of
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