L6238S datasheet(27/31 Pages) STMICROELECTRONICS

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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.

µs/DIV

Iout A

Aout

D95IN327

Fpwm=50KHz

Coff=120pF

Ctmr=220pF

500mV

PWM/Slew

10v

PWM Limit

Timer

Figure 5-10

µs/DIV

Aout

Iout A

D95IN328

Fpwm=12KHz

Coff=120pF

Ctmr=220pF

500mV

PWM/Slew

10V

PWM Limit

Timer

Figure 5-11

L6238S

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