IR3080
6
6
/
/
/
/
10
*
70
)
10
*
70
3
*
)
*
3
*
=
I
=
DEL
SS
DEL
SS
CHG
CHG
DEL
SS
DEL
SS
SSDEL
R
C
I
R
C
t
(
7)
VDAC Slew Rate Programming Capacitor
C
VDAC
and Resistor
R
VDAC
The slew rate of VDAC down-slope SR
DOWN
can be programmed by the external capacitor C
VDAC
as defined in
Equation (8), where I
SINK
is the sink current of VDAC pin as shown in Figure 15. The resistor R
VDAC
is used to
compensate VDAC circuit and is determined by Equation (9). The slew rate of VDAC up-slope SR
UP
is proportional
to that of VDAC down-slope and is given by Equation (10), where I
SOURCE
is the source current of VDAC pin as
shown in Figure15.
I
C
=
DOWN
SINK
VDAC
SR
(8)
2
15
10
2
5
VDAC
C
VDAC
R
+
=
(9)
VDAC
C
SOURCE
UP
I
SR
=
(10)
Over Current Setting Resistor
R
OCSET
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
(11), where R
L_MAX
and R
L_ROOM
are the inductor DCR at maximum temperature T
L_MAX
and room temperature
T_
ROOM
respectively.
(
10
*
3850
1
_
_
ROOM
L
MAX
L
T
R
R
+
=
The current sense amplifier gain of IR3086 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 T
IC_MAX
is calculated from Equation (12).
(
10
*
1470
1
_
_
ROOM
CS
MIN
CS
G
G
=
The total input offset voltage (V
CS_TOFST
) of current sense amplifier in phase ICs is the sum of input offset
(V
CS_OFST)
of the amplifier itself and that created by the amplifier input bias currents flowing through the current
sense resistors R
CS+
and R
CS-
.
+
=
CSIN
CS
CSIN
OFST
CS
TOFST
CS
I
R
I
V
V
_
_
The over current limit is set by the external resistor R
OCSET
as defined in Equation (14), where I
LIMIT
is
the required
over current limit. I
OCSET,
the bias current of OCSET pin, changes with switching frequency setting resistor R
OSC
and is determined by the curve in Figure 14. K
P
is the ratio of inductor peak current over average current in each
phase and is calculated from Equation (15).
I
R
)
1
[
_
+
+
=
)]
_
6
ROOM
MAX
L
T
(11)
)]
_
6
ROOM
MAX
IC
T
T
(12)
+
+
CS
R
(13)
OCSET
MIN
CS
TOFST
CS
P
MAX
L
LIMIT
n
OCSET
I
G
]
V
K
R
/
_
_
(14)
n
I
f
V
L
/
V
V
V
(
K
O
SW
I
O
O
I
P
)
/(
)
=
(15)
Page 21 of 41
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/30/04