Application Note
14
2011-07-06
Figure 5
Current sense circuit
Assuming capacitive current divider is adopted as current sense circuit.
So
1
cs
C
is chosen to be far less
than
r
C
,
e. g, around
100
/
r
C
, say 470pF.
1
cs
R
is normally of a few hundred
Ω
for filtering purpose, say
200
Ω
.
We can obtain the following equation considering
1
cs
C
and
r
C
as current divider:
r
cs
ocp
r
cs
cs
ocp
C
C
C
I
C
C
C
I
I
cs
1
1
1
1
≈
+
=
[21]
One major design criterion for the current sense is to ensure Over-Current Protection (OCP).
Accordingly,
we can also obtain:
2
8
.
0
*
2
2
2
1
cs
R
C
R
I
I
cs
cs
π
π
=
=
[22]
where 0.8V is the OCP first level.
Then we get:
Ω
=
=
=
−
−
70
10
*
470
10
*
66
*
47
.
2
*
2
8
.
0
*
*
2
8
.
0
12
9
1
2
π
π
cs
r
ocp
cs
C
C
I
R
[23]
Rcs2 is chosen as 68
Ω
.
2
cs
C
is selected so that the current loop speed is fast enough and the ripple on CS pin is around 20% of
the average value.
2
2
*
cs
cs
C
R
is around
min
1
f
.
nF
f
R
C
cs
cs
490
10
*
30
*
68
1
*
1
3
min
2
2
=
=
≈