IR3086AMPBF View Datasheet(PDF) - International Rectifier
Part Name
Description
Manufacturer
IR3086AMPBF Datasheet PDF : 33 Pages
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IR3086APbF
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.
RL _ MAX = RL _ ROOM ∗ [1 + 3850 *10 −6 ∗ (TL _ MAX − TROOM )]
(8)
The current sense amplifier gain 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).
GCS _ MIN = GCS _ ROOM ∗ [1 − 1470 *10 −6 ∗ (TIC _ MAX − TROOM )]
(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-.
VCS _ TOFST = VCS _ OFST + ICSIN + ∗ RCS + − ICSIN − ∗ RCS −
(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 of IR3081A Data Sheet. KP is the ratio of inductor peak current over
average current in each phase and is calculated from Equation (12).
ROCSET
= [ ILIMIT
n
∗ RL _ MAX
∗ (1 + KP ) + VCS _ TOFST ] ∗ GCS _ MIN
/ IOCSET
(11)
KP
=
(VI
−VO ) ∗VO /(L ∗VI
IO / n
∗
fSW
∗ 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 of IR3081A Data Sheet,
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.
RFB
=
RL _ MAX
∗ VO _ NLOFST − VCS _ TOFST
I FB ∗ RL _ MAX
∗ n ∗ RO
(13)
RDRP
=
RFB
∗ RL _ MAX ∗ GCS _ MIN
n ∗ RO
(14)
Body BrakingTM Related Resistors RBBFB and RBBDRP
The Body BrakingTM 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 15 of 33
9/30/04
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