ir3840 International Rectifier Corp., ir3840 Datasheet - Page 22

• Manufacturer Part Number: ir3840
• Description: Integrated 12a Synchronous Buck Regulator
• Manufacturer: International Rectifier Corp.

Detailed calculation of compensation TypeIII
02/09/09
Desired
C
C
F
F
Select
F
Select
Calculate
R
Select
Calculate
R
R
Select
R
Z
P
P
4
3
10
8
3
9
2
2
3
=
=
=
=
=
=
=
=
=
V
2
2
2
2
F
0.5
F
2
π
π
π
π
:
:
:
:
o
π
o
o
V
C
R
F
R
*
*
*
-
*
ref
7
*
8
Z
*
V
F
C
F
3
F
1
1
Phase
1
1
1
1
C
F
1
P
ref
=
o
Z
1
=
R
=
R
−
+
7
+
−
1
=
s
1
3
7
C
*
10
2.2
3
1.87
*
3.92
sin
sin
sin
sin
*
*
*
,
0
=
L
*
7
F
,
.
R
R
F
C
o
R
5
Z
300
*
R
nF
P
3
2
Θ
Θ
V
Θ
Θ
3
3
8
*
*
2
8
;
;
kΩ
C
k
;
F
in
-
R
Margin
and
C
;
C
Ω
=
=
Z
R
and
o
9
3
2
4
kHz
R
10
17.63
567.1
*
=
=
=
=
10
V
;
C
283.7
2.49
osc
9.65
R
8.82
R
=
4
8
9
128
:
;
kHz
Θ
kHz
=
R
:
kΩ
nF,
3.97
3
=
kHz
pF
, Ω
70
=
,
1.85
Select
Select
o
kΩ
Select
Select
and
,
kΩ
:
:
R
:
:
C
C
R
9
4
3
10
=
=
=
2.49
=
10
220
130
nF
kΩ
pF
Ω
Setting the Power Good Threshold
Power Good threshold is internally set at 88% of
Vref. When the voltage at the FB pin exceeds the
threshold, PGood is asserted.
The PGood is an open drain output. Hence, it is
necessary to use a pull up resistor R
PGood pin to Vcc.
resistor must be chosen such as to limit the
current flowing into the PGood pin, when the
output voltage is not in regulation, to less than 5
mA. A typical value used is 4.7kΩ.
Programming the Current-Limit
The Current-Limit threshold can be set by
connecting a resistor (R
to the OCSet pin. The resistor can be calculated
by using equation (4). This resistor R
be placed close to the IC.
The
coefficient and it should be considered for the
worst case operation.
I
SET
(50%
I
R
R
I
OCSet
SET
DS
OCSet
=
R
(
I
on
≅
L
DS(on)
(
=
)
critical
I
over
=
o
=
59.07
(
LIM
2.696
5
9 .
)
)
=
=
mΩ
nominal
has
R
μA
12
kΩ
OCSet
*
R
A
1.5
(at
DS
*
Select
IR3840MPbF
a
The value of the pull-up
1.5
∗
(
on
=
F
I
OCSET
OCSet
s
)
8.85
output
=
positive
=
18
600
R
) from the SW pin
A
mΩ
..........
7
kHz)
=
current
2.8
..........
temperature
OCSET
kΩ
)
PG
...
(32)
must
from
22