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IR3087MPBF Datasheet(PDF) 16 Page - International Rectifier

Part # IR3087MPBF
Description  XPHASETM PHASE IC WITH OPTI-PHASETM, OVP, AND OVERTEMP DETECT
PDF  35 Pages
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Manufacturer  IRF [International Rectifier]
Direct Link  http://www.irf.com
Logo IRF - International Rectifier

IR3087MPBF Datasheet(HTML) 16 Page - International Rectifier

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IR3087PBF
Over Current Setting Resistor ROCSET
The inductor DC resistance is utilized to sense the inductor current. The copper wire of inductor has a constant
temperature coefficient of 3850 ppm/°C, and therefore the maximum inductor DCR can be calculated from Equation
(8), where RL_MAX and RL_ROOM are the inductor DCR at maximum temperature TL_MAX and room temperature
T_ROOM respectively.
)]
(
10
*
3850
1
[
_
6
_
_
ROOM
MAX
L
ROOM
L
MAX
L
T
T
R
R
−
∗
+
∗
=
−
(8)
The current sense amplifier gain of IR3087 decreases with temperature at the rate of 1470 ppm/°C, which
compensates part of the inductor DCR increase. The phase IC die temperature is only a couple of degrees Celsius
higher than the PCB temperature due to the low thermal impedance of MLPQ package. The minimum current sense
amplifier gain at the maximum phase IC temperature TIC_MAX is calculated from Equation (9).
)]
(
10
*
1470
1
[
_
6
_
_
ROOM
MAX
IC
ROOM
CS
MIN
CS
T
T
G
G
−
∗
−
∗
=
−
(9)
The total input offset voltage (VCS_TOFST) of current sense amplifier in phase ICs is the sum of input offset
(VCS_OFST) of the amplifier itself and that created by the amplifier input bias currents flowing through the current
sense resistors RCS+ and RCS-.
−
−
+
+
∗
−
∗
+
=
CS
CSIN
CS
CSIN
OFST
CS
TOFST
CS
R
I
R
I
V
V
_
_
(10)
The over current limit is set by the external resistor ROCSET as defined in Equation (11), where ILIMIT is the required
over current limit. IOCSET, the bias current of OCSET pin, changes with switching frequency setting resistor ROSC
and is determined by the curve in Figure 14. KP is the ratio of inductor peak current over average current in each
phase and is calculated from Equation (12).
OCSET
MIN
CS
TOFST
CS
P
MAX
L
LIMIT
OCSET
I
G
V
K
R
n
I
R
/
]
)
1
(
[
_
_
_
∗
+
+
∗
∗
=
(11)
n
I
f
V
L
V
V
V
K
O
SW
I
O
O
I
P
/
)
2
/(
)
(
∗
∗
∗
∗
−
=
(12)
No Load Output Voltage Setting Resistor RFB and Adaptive Voltage Positioning Resistor RDRP
A resistor between FB pin and the converter output is used to create output voltage offset VO_NLOFST, which is the
difference between VDAC voltage and output voltage at no load condition. Adaptive voltage positioning further
lowers the converter voltage by RO*IO, where RO is the required output impedance of the converter.
RFB is not only determined by IFB, the current flowing out of FB pin as shown in Figure 14, but also affected by the
adaptive voltage positioning resistor RDRP and total input offset voltage of current sense amplifiers. RFB and RDRP
are determined by (13) and (14) respectively.
MAX
L
FB
O
TOFST
CS
NLOFST
O
MAX
L
FB
R
I
R
n
V
V
R
R
_
_
_
_
∗
∗
∗
−
∗
=
(13)
O
MIN
CS
MAX
L
FB
DRP
R
n
G
R
R
R
∗
∗
∗
=
_
_
(14)
Body Braking
TM Related Resistors RBBFB and RBBDRP
The body braking
TM during Dynamic VID can be disabled by connecting BBFB pin to ground. If the feature is
enabled, Resistors RBBFB and RBBDRP are needed to restore the feedback voltage of the error amplifier after
Dynamic VID step down. Usually RBBFB and RBBDRP are chosen to match RFB and RDRP respectively.
Page 16 of 35
1/31/05



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