STMicroelectronics Demonstration board using a dual full-bridge L6227Q EVAL6227QR EVAL6227QR Ficha De Dados
Códigos do produto
EVAL6227QR
Circuit description
L6227Q
14/27
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 R
COFF
. 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
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 11.
t
OFF
versus C
OFF
and R
OFF
t
ON
t
ON MIN
(
)
>
2.5
µs
=
t
ON
t
TCRISE
t
DT
–
W
+
>
⎩
⎨
⎧
0.1
1
10
100
1
10
100
1.10
3
1.10
4
Coff [nF]
toff
[µ
s]
R
off
= 100k
Ω
R
off
= 47k
Ω
R
off
= 20k
Ω