Datasheet

PWM current control L6227
16/32 DocID9453 Rev 2
Figure 11 shows the magnitude of the Off time t
OFF
versus C
OFF
and R
OFF
values. It can be
approximately calculated from the equations:
Equation 1
t
RCFALL
= 0.6 · R
OFF
· C
OFF
t
OFF
= t
RCFALL
+ t
DT
= 0.6 · R
OFF
· C
OFF
+ t
DT
where R
OFF
and C
OFF
are the external component values and t
DT
is the internally generated
deadtime with:
Equation 2
20 K  R
OFF
 100 K
0.47 nF  C
OFF
 100 nF
t
DT
= 1 µs (typical value)
Therefore:
Equation 3
t
OFF(MIN)
= 6.6 µs
t
OFF(MAX)
= 6 ms
These values allow a sufficient range of t
OFF
to implement the drive circuit for most motors.
The capacitor value chosen for C
OFF
also affects the rise time t
RCRISE
of the voltage at the
pin RCOFF. The rise time t
RCRISE
will only be an issue if the capacitor is not completely
charged before the next time the monostable is triggered. Therefore, the on time t
ON
, which
depends by motors and supply parameters, has to 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)
.
Equation 4
t
RCRISE
= 600 · C
OFF
Figure 12 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.
t
ON
t
ON MIN
>2.5s (typ. value)=
t
ON
t
RCRISE
t
DT
–>
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