參數(shù)資料
型號： L6280
廠商： 意法半導(dǎo)體
英文描述： THREE CHANNELS MULTIPOWER DRIVER SYSTEM
中文描述： 三通道移動電站驅(qū)動系統(tǒng)
文件頁數(shù)： 9/26頁
文件大?。?/td> 388K
代理商： L6280
PwmCurrent Control Loop
The current control is achieved big a cycle of
charge (T
ON
) and discharge (T
OFF
) of the energy
stored in each couple of windings of the driven
motor (MA and MB). Fig. 8 shows the windings
MA of an unipolar stepper motor during T
ON
. FF1
is setted by the clock pulse and the transistor Q
A
is ON. At the moment Q
1
is ON the current expo-
nentiallyincreases until R
S x
I
P
equalsV
REF
.
A reset pulse is produced, Q
A
is switched OFF
and Q
2
is switched ON (Fig. 9). Since the mag-
netic flux 0
MA
= N
A
I
P
cannot suddenly change
and since the coil tourus number in the discharge
loop is doubled, the peak current I
P
modifies it
self into I
P
/2. The OFF time is characteryzedby a
slow recirculation of the current I
P
/2 that de-
creases until a new clock pulse sets a new T
ON
configuration. To control the current in two sepa-
rate windings MA and MB with just one sense re-
sistor RS and one comparator, a special PWM
control loop based on a ”time sharing” technique
(Patented)is used(Fig. 10).
In this configuration the chopping frequency, that
defines the T
ON
+ T
OFF
period of each phase, is
halved by FF3 that drives ON G1 and G2 alter-
nately. During T
OFF
of one winding, for instance
MA (and Q
A
is OFF), its current does not flow
throught the sensing resistor that can be used to
monitor the current that flows through the second
winding MB, allowed by the ON-status of Q
B
.
Fig. 11 shows a simplified timing before and dur-
ing the phase change from AB to AB (CCW, full
step). It can be seen that before the time t
1
, I
A
and I
B
are alternatelycontrolled in a choppingpe-
riod Tch1 of 4 oscillator periods or two clock peri-
ods. The time sharing is 50% - 50% and the
chopping frequency is typically of 20KHz (f
osc
=
80KHz).
Afther the time t
1
, as soon as I
A
is sensed, a dif-
ferent time sharing is generated. In fact since a
Reset pulse is last after one clock pulse, FF2 can
drive FF3 to change for I
B
chopping only at the
next clock pulse(Fig 10; Fig 11).
Thismeans that the chopping time becomes Tch2
= 6 oscillator pulses, the frequency decreases to
16.6KHz (f
osc
= 80 KHz) and the time sharing be-
comesof 67% - 33%.
At the end of the phase change period tphc the
time sharing comes back to 50% - 50% again. It
can be noted that this behaviour allows a faster
phase change and then a higher speed of the
motor. The cost of that, is the increase of the
T
OFF
of the unchangedphase B and then a small
increase of the ripple of the current I
B
(see
I
B1
<
I
B2
in Fig. 11).
This time sharing current control method is also
used when two indipendent load are driven by
one single channel. when only one load is pre-
sent,such as a DCmotor could be, the timeshar-
ing is automatically switched OFF and the PWM
frequency becomes f
osc
/4 = 20KHz. Table 1
shows how the referencevoltage can be modified
with a three bits DAC to allow microstepping op-
erations (seebelow).
Figure8 -
T
ON
Configuration:Motor Windings
MA (A; A).
Figure9 -
T
OFF
Configuration:Motor Windings
MA (A:A).
L6280
9/26
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