iru3055 International Rectifier Corp., iru3055 Datasheet - Page 20

• Manufacturer Part Number: iru3055
• Description: 5-bit Programmable 3-phase Synchronous Buck Controller Ic -
• Manufacturer: International Rectifier Corp.

IRU3055
Introduction to Intel Specification
According to the Intel spec, the output voltage is depen-
dent on the load current. When the current goes up, the
voltage goes down. The characteristic can be modeled
by the following:
Rearrangement results in:
For Intel spec:
Implementation of Voltage Droop with IRU3055
With a single single-ended OPAMP, the IRU3055 can
achieve voltage droop function as shown in Figure 22.
The voltage Vc is a constant voltage such as 2V or 5V.
The signal Vo+Rs3Io can be from inductor current sens-
ing. The real application circuit is shown in Figure 20.
20
Figure 21 - The Intel specification for the load line.
IRU3055
Vo = V
Where V
slope of load line.
V
V
K
V
SET
OFFSET
LOAD
OFFSET
= Vo + V
Figure 22 - Implement voltage droop
= 98mV/45A = 2.18mV
Vo
SET
= 25mV
OFFSET
- V
V
V
FB
S E T
with a single OPAMP.
OFFSET
OFFSET
is the offset voltage and K
- K
+ K
R1
V
LOAD
LOAD
S E T
V
O(MIN)
3Io
V
3Io
O(MAX)
e x R1
c x R1
R2/d
R2
(constant voltage)
V
Vc
---(10)
Vo
Vo+(Rs x Io)
O(TYP)
---(11)
LOAD
Io
is the
www.irf.com
With this simple circuit, the output voltage will linearly
decrease as load current increases. The output voltage
will fall in Intel spec. The resistor ratio “c” will determine
the slope of the voltage-current load line. The resistor
ratios “d” and “e” determine the offset voltage.
In an ideal case, these parameters can be calculated
by:
For a 3-phase converter with inductor current sensing:
In practice, the resistor ratios “c” and “d” have to be tuned
in order to take some parasitic parameters such as PCB
layout trace into account.
R7, R8, R9 and capacitor C15 configures a inductor cur-
rent losses sensing network to sense the load current.
(Attn: The C15 and R11 must connect directly to the
output terminal.) The RC networks that sense the induc-
tor current have to satisfy the following:
For example, in the application circuit in Figure 20, the
inductor is 1mH and the DC resistance is 1.6mV. If the
filter capacitor C15 is chosen to be 1mF, then the cur-
rent sensing resistors R7, R8 and R9 are:
Because the given inductor is larger at zero current (it is
1.3mH at 0 current). A large resistor has to be taken.
In the application circuit in Figure 20, R7,R8 and R9=2.2K.
Select R17 (referring to R2 in Figure 22) to be 1MV if we
consider the input bias of OPAMP LM324. Select R10
(referring to R1 in Figure 22) to be 1KV.
Connect the voltage Vc to 2V reference voltage shown in
Figure 20.
The implementation circuit is shown in Fig.20, Resistor
Component selection guide
c =
d =
e =
Rs =
(R/3)3C = L/R
R = 33L/R
R = 331mH/1.6mV/1mF = 1.87K
R10=1K and R17=1MV
Vc=2V
Where Rs is equivalent current sensing resistors.
Where R
K
V
K
Rs
LOAD
OFFSET
LOAD
R
Vc
3
L
Rs
L
3
- Rs
is the DC resistance of the inductor.
L
/C
V
OFFSET
Vc
L
08/13/02
Rev. 1.4