Datasheet

11/22
L6227
be bigger than t
RCRISE
for allowing a good current regulation by the PWM stage. Furthermore, the on time t
ON
can not be smaller than the minimum on time t
ON(MIN)
.
t
RCRISE
= 600 · C
OFF
Figure 10 shows the lower limit for the on time t
ON
for having a good PWM current regulation capacity. It has to
be said that t
ON
is always bigger than t
ON(MIN)
because the device imposes this condition, but it can be smaller
than t
RCRISE
- t
DT
. In this last case the device continues to work but the off time t
OFF
is not more constant.
So, small C
OFF
value gives more flexibility for the applications (allows smaller on time and, therefore, higher
switching frequency), but, the smaller is the value for C
OFF
, the more influential will be the noises on the circuit
performance.
Figure 9. t
OFF
versus C
OFF
and R
OFF
t
ON
t
ON MIN()
>2.5µs (typ. value)=
t
ON
t
RCRISE
t
DT
–>
0.1 1 10 100
1
10
100
1
.
10
3
1
.
10
4
Coff [nF]
toff [µs]
R
off
= 100kΩ
R
off
= 47kΩ
R
off
= 20k
Ω