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参数资料
| 型号： | L6238SQT |
| 厂商： | STMICROELECTRONICS |
| 元件分类： | 运动控制电子 |
| 英文描述： | BRUSHLESS DC MOTOR CONTROLLER, 5 A, PQFP64 |
| 封装： | TQFP-64 |
| 文件页数： | 20/31页 |
| 文件大小： | 3465K |
| 代理商： | L6238SQT |
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ON during the Constant-OFF time, charging C1 to
the internal rail. At the end of the OFF time, Q1 is
turned OFF allowing current source I1 to dis-
charge the capacitor towards ground. While the
voltage on C1 is above the low input threshold of
N1, the output of N1 is low, preventing any
change of state at the output of N11 due to a high
A3 output. When the capacitor reaches the low in-
put threshold of N1, N1 chnges state allowing A3
to control the state of N11.
5.1.6 Masking/Bemf Sampling in PWM
The method of sampling the floating phase for the
bemf zero crossing defers between Linear and
PWM operation. In Linear Mode, the bemf is sam-
pled continuously after the mask time-out, until
the zero crossing is detected. Then the mask is
enabled for a time based on the commutation
phase delay plus the additional time based on the
previous period as explained earlier.
With PWM operation however, the switching
noise at turn ON (after the Constant-OFF time)
can be significant, especially at low RPMs where
the bemf is the lowest. In order to provide the
greatest noise immunity in PWM, the floating
phase is monitored only at the point where the
output is about to be turned OFF.
In operation, when the motor current reaches the
commanded level, the floating phase is first moni-
tored to determine if the bemf has crossed the
zero. The output is then turned OFF for the Con-
stant-OFF time out.
As the motor current increases through, the in-
creasing bemf causes the motor current to natu-
rally decrease. Eventually a point is reached
where the PWM is running at 100% duty cycle
and the motor current cannot reach the com-
manded level. At this time the bemf is no longer
smpled, preventing further commutation of the
output.
The PWM Limit Timer is used to set up a maxi-
mum ON time. When this limit is exceeded the
method of sensing the bemf is essentially the
same as in the case of operating in linear mode.
Figure 5-10 is an oscillograph of the controller op-
erating in PWM mode. The top trace is Aout. The
2nd trace is the voltage seen at the PWM/SLEW
pin indicating the exponential discharging of the
timing capacitor during the OFF time. Trace 3 is
the voltage appearing on the PWM Timer capaci-
tor, while trace 4 is the motor current.
Referring again to Figure 5-9, and 5-10 transistor
Q2 is turned ON at the beginning of the OFF time,
discharging the external capacitor C4 to near
ground level. At the end of the OFF-Time, Q2 is
turned off and C4 starts charging linearly via I2.
C4 is again discharged at the beginning of the
OFF time and the cycle repeats. As long as C4
does not reach the threshold of A1 (typically
3.5V), the bemf is only sampled just before turn-
off of the output. As the motor is starting up in fig-
ure 5-10, the duty cycle is roughly 50%. The
PWM limit timer is reset to ground by the start of
the OFF timer before reaching the 3.5V threshold.
In figure 5-11, as the motor spins up, the on time
of the output increases and the PWM limit timer
reaches the 3.5V. Eventually the duty cycle
reaches 100% and the sampling of the bemf is
essentially the same as in the linear mode.
The selection of components for the PWM timer is
not critical. Since the objective is to sample the
bemf only at turn OFF to maximize the signal to
noise ratio, the PWM timer slope can be set up to
convert to the full bemf sampling after a few revo-
lutions of the motor when the bemf has reached
an appropriate voltage output.
20
s/DIV
4
1A
Iout A
Aout
D95IN327
Fpwm=50KHz
Coff=120pF
Ctmr=220pF
3
500mV
2
2V
PWM/Slew
1
10v
PWM Limit
Timer
Figure 5-10
50
s/DIV
4
Aout
Iout A
D95IN328
Fpwm=12KHz
Coff=120pF
Ctmr=220pF
3
500mV
2
2V
PWM/Slew
1
10V
PWM Limit
Timer
1A
Figure 5-11
L6238S
27/31
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