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SILICON TOUCH TECHNOLOGY INC.
-3dB frequency
ω
3
dB
is suggested lower than the 1/100 of the PWM frequency. It
is shown as follows,
ω
3
dB
=
1
ω
PWM
100
The following example explains how to design the low pass filter. For PWM
frequency
f
PWM
=
20
kHz
, the sense resistance
R
S
=
1.5
Ω
, the desired maximum
output constant current is 100mA and V
PWM
= 3V, determine C
IN
, R
A
and R
B
.
f
3
dB
=
1
1
=
f
PWM
=
0.2
kHz
2
π
(
R
A
//
R
B
)
C
IN
100
By choosing
C
IN
=
0.1
uF
, we can get
(
R
A
//
R
B
)
=
7.96
k
Ω
.
Because to
I
max
=
10*
R
S
(
Ω
)
V
IN
,max
(
A
)
,
Then
R
A
=
(
R
A
//
R
B
)
10
×
I
max
×
R
S
V
PWM
=
15.9
k
Ω
R
B
=
15.9
k
Ω
The accuracy of output constant current is closely dependent on the PWM
frequency. The higher PWM frequency, there will be less voltage variation in IM
pin within the desired constant current. The output sense resistor
R
S
1
=
1.5
or
1.8
Ω
is recommended to have a better accuracy of output constant
current.
SP-MD115-A.005.doc
Version
：
A.005
Page
：
8
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