ir3084u International Rectifier Corp., ir3084u Datasheet - Page 28

• Manufacturer Part Number: ir3084u
• Description: Xphasetm Vr10, Vr11 & Opteron/athlon64 Control Ic
• Manufacturer: International Rectifier Corp.

No Load Output Voltage Setting Resistor R
An external resistor, R
output voltage offset, V
However, the converter’s output voltage will be set by the combination of VSETPT plus some contribution from
the VDRP pin. At no load, both pins of the Error Amplifier are at VDAC – VSETPT while the VDRP pin is at
VDAC + V
the VDRP pin is at a higher voltage than the FB pin of the Error Amplifier, VDRP will contribute to the no load
offset through the RDRP and RFB resistors. The design approach is to choose a value for the feedback resistor,
RFB, from 100 to 2K and then calculate RDRP and R
The total input offset voltage (V
(V
sense resistors R
Finally, calculate the no-load setpoint resistor using Equation (7) and the droop resistor using Equation (8);
CS_OFST)
Page 28 of 47
VSETPT
V
RVSETPT =
RDRP
CS_TOFST
CS_OFST
of the amplifier itself plus that created by the amplifier input bias currents flowing through the current
Where:
Where:
B
A
=
=
=
V
•G
CS+
Io
=
RFB
=
O_NLOFST
CSA
*
V
(A
and R
R
CS_OFST
A
VSETPT
VSETPT
(V
O_NLOFST
L
n
+
⋅
*
I
R
G
n is the number of phases
V
Ro is the desired load line slope (ohms)
V
I
VSETPT is calculated by Equation (5).
I
*
O
VSETPT
VSETPT
VSETPT
D
CS_OFST
O_NLOFST
CS_TOFST
L
D
B
CSA
G
is the full load output current of the converter
+
which is a function of R
is the DCR of the output inductor
CS−
−
−
a positive number for VR10 and VR11, a negative number for AMD
, connected between the VDAC pin and the VSETPT pin is used to set the no load
CSA
−
Io
+
is the gain of the current sense amplifiers
VSETPT
C
C
,
is the current into the VSETPT pin at the switching frequency,
(
as shown in Equation (6).
*
CS_TOFST
+
which is the difference between the V
I
−
*
and G
Ro
CSIN
V
is the no load offset voltage below the DAC setting,
is the total offset voltage of the current sense amplifiers, see below.
B
D)
+
CS
+
C
_
∗
CSA
TOFST
R
) of the current sense amplifier in the phase IC is the sum of input offset
CS
are the input offset and gain of the current sense amplifiers). Because
*
+
) (
VSETPT
G
−
CSA
I
OSC
CSIN
, Feedback Resistor RFB, and AVP Resistor R
VSETPT
. See Figure 15 on page 25.
−
∗
R
to provide the required no load offset voltage.
CS
−
)
C
D =
DAC
=
V
V
voltage and output voltage at no load.
CS
O_NLOFST
_
TOFST
*
G
CSA
IR3084U
(5)
(6)
(7)
(8)
9/14/2005
DRP