L6280 데이터 시트보기 (PDF) - STMicroelectronics

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L6280

THREE CHANNELS MULTIPOWER DRIVER SYSTEM

STMicroelectronics 

Pwm Current Control Loop

The current control is achieved big a cycle of

charge (TON) and discharge (TOFF) 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 TON. FF1

is setted by the clock pulse and the transistor QA

is ON. At the moment Q1is ON the current expo-

nentially increases until RS x IP equals VREF.

A reset pulse is produced, QA is switched OFF

and Q2 is switched ON (Fig. 9). Since the mag-

netic flux 0MA = NA IP cannot suddenly change

and since the coil tourus number in the discharge

loop is doubled, the peak current IP modifies it

self into IP/2. The OFF time is characteryzed by a

slow recirculation of the current IP/2 that de-

creases until a new clock pulse sets a new TON

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 TON + TOFF period of each phase, is

halved by FF3 that drives ON G1 and G2 alter-

nately. During TOFF of one winding, for instance

MA (and QA 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 QB.

Fig. 11 shows a simplified timing before and dur-

ing the phase change from AB to AB (CCW, full

Figure 8 - TON Configuration: Motor Windings

MA (A; A).

L6280

step). It can be seen that before the time t1, IA

and IB are alternately controlled in a chopping pe-

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 (fosc =

80KHz).

Afther the time t1, as soon as IA 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 IB chopping only at the

next clock pulse (Fig 10; Fig 11).

This means that the chopping time becomes Tch2

= 6 oscillator pulses, the frequency decreases to

16.6KHz (fosc = 80 KHz) and the time sharing be-

comes of 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

TOFF of the unchanged phase B and then a small

increase of the ripple of the current IB (see ∆IB1

<∆IB2 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 DC motor could be, the time shar-

ing is automatically switched OFF and the PWM

frequency becomes fosc/4 = 20KHz. Table 1

shows how the reference voltage can be modified

with a three bits DAC to allow microstepping op-

erations (see below).

Figure 9 - TOFF Configuration: Motor Windings

MA (A: A).

9/26

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