AN1625 STMicroelectronics, AN1625 Datasheet - Page 14

• Manufacturer Part Number: AN1625
• Description: L6235 THREE PHASE BRUSHLESS DC MOTOR DRIVER
• Manufacturer: STMicroelectronics

AN1625 APPLICATION NOTE
2.11.1 Off-time Selection and minimum on-time
Figure 14 also shows the lower limit for the On Time t
has to be said that t
smaller than t
stant.
So, small C
switching frequency), but, the smaller is the value for C
performance.
Figure 14. Off-time selection and minimum on-time.
2.11.2 Slow Decay Mode
Figure 15 shows the operation of the bridge in the Slow Decay mode during the Off Time. At any time only two
legs of the three-phase bridge are active, therefore only the two active legs of the bridge are shown in the figure
and the third leg will be off. At the start of the Off Time, the lower power MOS is switched off and the current
14/39



t
RCRISE
t
t
O N
O N
OFF
t
t
RCRISE
= 600 · C
O N MIN
RCRISE
value gives more flexibility for the applications (allows smaller On Time and, therefore, higher
ON
1 10
1 10
- t
–
DT
OFF
is always bigger than t
=
t
100
100
DT
. In this last case the device continues to work but the Off Time t
4
3
10
10
1.5 s (typ. value)
1
1
0.1
0.1
1
1
ON(MIN)
Coff [nF]
Coff [nF]
ON
because the device imposes this condition, but it can be
OFF
for having a good PWM current regulation capacity. It
, the more influential will be the noises on the circuit
10
10
100
100
R = 100 k
R = 47 k
R = 20 k
OFF
is not more con-