Datasheet

11/25
L6229
Figure 12 shows the magnitude of the Off Time t
OFF
versus C
OFF
and R
OFF
values. It can be approximately
calculated from the equations:
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 Dead Time with:
20K
Ω ≤ R
OFF
≤ 100K Ω
0.47nF ≤ C
OFF
≤ 100nF
t
DT
= 1µs (typical value)
Therefore:
t
OFF(MIN)
= 6.6µs
t
OFF(MAX)
= 6ms
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)
.
t
RCRISE
= 600 · C
OFF
Figure 13 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 12. 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
= 20kΩ