AN1625 STMicroelectronics, AN1625 Datasheet - Page 15

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

recirculates around the upper half of the bridge. Since the voltage across the coil is low, the current decays slow-
ly. After the Dead Time the upper power MOS is operated in the synchronous rectification mode reducing the
impedance of the freewheeling diode and the related conducting losses. When the monostable times out, upper
MOS that was operating the synchronous mode turns off and the lower power MOS is turned on again after
some delay set by the Dead Time to prevent cross conduction.
Figure 15. Slow Decay Mode Output Stage Configurations
In some conditions (short off-time, very low regulated current, high motor winding L / R) the system may need
an on-time shorter than 1.5 µs. In these cases the PWM current controller can loose the regulation.
Figure 16 shows the operation of the circuit in this condition. When the current first reaches the threshold, both
the high-side are turned on for a fixed time and the current decays.
During the following on-time current increases above the threshold, but the bridge cannot be turned off until the
minimum on-time expires. Since current increases more in each on-time than it decays during the off-time, it
keeps growing during each cycle, with steady state asymptotic value set by duty-cycle and load DC resistance:
the resulting peak current will be I
is the load resistance.
Figure 16. Minimum on-time can cause the PWM controller to loose the regulation.
A) ON TIME
D01IN1336
B) 1 s DEAD TIME
pk
= V
S
× D / R
LOAD
, where D = t
V
ref
C) SYNCHRONOUS
/ R
SENSE
RECTIFICATION
ON
less than 1.5 µs
/ (t
is about 1.5 µs
needed t
minimum t
ON
AN1625 APPLICATION NOTE
+t
OFF
ON
ON
is
) is the duty-cycle and R
D) 1 s DEAD TIME
15/39
LOAD