L6258EX ,PWM CONTROLLEDFEATURESFigure 1. Package■ ABLE TO DRIVE BOTH WINDINGS OF A BIPOLAR STEPPER MOTOR OR TWO DC MOTORS■ ..
L6258EXTR ,PWM CONTROLLEDAbsolute Maximum RatingsSymbol Parameter Value UnitV Supply Voltage 45 VsV Logic Supply Voltage 7 V ..
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LC338128M-80 ,1 MEG (131072 words x 8 bit) pseudo-SRAMPin AssignmentAuA1:A7A5"A:"A2Al"1/0:1/021/0:END 1DIP32, SOP32VccA15Ax:1/051/07TADSvns1/04Top viewAu ..
LC33832M-10 ,256K (32768word x 8bit) Pseudo-SRAMFeatures3133-DIP28• 32768 words · 8 bits configuration• Single 5 V ±10% power supply[LC33832S, SL]• ..
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L6258EX-L6258EXTR
PWM CONTROLLED
1/23
L6258EXFebruary 2005
1FEATURES ABLE TO DRIVE BOTH WINDINGS OF A
BIPOLAR STEPPER MOTOR OR TWO DC
MOTORS OUTPUT CURRENT UP TO 1.5A EACH
WINDING WIDE VOLTAGE RANGE: 12V TO 40V FOUR QUADRANT CURRENT CONTROL,
IDEAL FOR MICROSTEPPING AND DC
MOTOR CONTROL PRECISION PWM CONTROL NO NEED FOR RECIRCULATION DIODES TTL/CMOS COMPATIBLE INPUTS CROSS CONDUCTION PROTECTION THERMAL SHUTDOW
DESCRIPTIONL6258EX is a dual full bridge for motor control appli-
cations realized in BCD technology, with the capabil-
ity of driving both windings of a bipolar stepper motor
or bidirectionally control two DC motors.
L6258EX and a few external components form a
complete control and drive circuit. It has high efficien-
cy phase shift chopping that allows a very low current
ripple at the lowest current control levels, and makes
this device ideal for steppers as well as for DC mo-
tors.The power stage is a dual DMOS full bridge ca-
pable of sustaining up to 40V, and includes the
diodes for current recirculation.The output current ca-
pability is 1.5A per winding in continuous mode, with
peak start-up current up to 2A. A thermal protection
circuitry disables the outputs if the chip temperature
exceeds the safe limits.
PWM CONTROLLED - HIGH CURRENT
DMOS UNIVERSAL MOTOR DRIVER
Figure 2. Block DiagramRev. 4
L6258EX
Table 2. Absolute Maximum Ratings(1) This current is intended as not repetitive start up current for max. 1 second.
Figure 3. Pin Connection (Top view)
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L6258EX
Table 3. Pins FunctionNote: The number in parenthesis shows the relevant Power Bridge of the circuit. Pins 18, 19, 1 and 36 are connected together.
L6258EX
Figure 4. Thermal Characteristics
5/23
L6258EX
ELECTRICAL CHARACTERISTICS(VS = 40V; VDD = 5V; Tj = 25°; unless otherwise specified.)Note 1: This is true for all the logic inputs except the disable input.
(*) Chopping frequency is twice fosc value.
L6258EX FUNCTIONAL DESCRIPTIONThe circuit is intended to drive both windings of a bipolar stepper motor or two DC motors.
The current control is generated through a switch mode regulation.
With this system the direction and the amplitude of the load current are depending on the relation of phase and
duty cycle between the two outputs of the current control loop.
The L6258EX power stage is composed by power DMOS in bridge configuration as it is shown in figure 5, where
the bridge outputs OUT_A and OUT_B are driven to Vs with an high level at the inputs IN_A and IN_B while are
driven to ground with a low level at the same inputs .
The zero current condition is obtained by driving the two half bridge using signals IN_A and IN_B with the same
phase and 50% of duty cycle.
In this case the outputs of the two half bridges are continuously switched between power supply (Vs) and
ground, but keeping the differential voltage across the load equal to zero.
In figure 5A is shown the timing diagram of the two outputs and the load current for this working condition.
Following we consider positive the current flowing into the load with a direction from OUT_A to OUT_B, while
we consider negative the current flowing into load with a direction from OUT_B to OUT_A.
Now just increasing the duty cycle of the IN_A signal and decreasing the duty cycle of IN_B signal we drive pos-
itive current into the load.
In this way the two outputs are not in phase, and the current can flow into the load trough the diagonal bridge
formed by T1 and T4 when the output OUT_A is driven to Vs and the output OUT_B is driven to ground, while
there will be a current recirculation into the higher side of the bridge, through T1 and T2, when both the outputs
are at Vs and a current recirculation into the lower side of the bridge, through T3 and T4, when both the outputs
are connected to ground.
Since the voltage applied to the load for recirculation is low, the resulting current discharge time constant is high-
er than the current charging time constant during the period in which the current flows into the load through the
diagonal bridge formed by T1 and T4. In this way the load current will be positive with an average amplitude
depending on the difference in duty cycle of the two driving signals.
In figure 5B is shown the timing diagram in the case of positive load current
On the contrary, if we want to drive negative current into the load is necessary to decrease the duty cycle of the
IN_A signal and increase the duty cycle of the IN_B signal. In this way we obtain a phase shift between the two
outputs such to have current flowing into the diagonal bridge formed by T2 and T3 when the output OUT_A is
driven to ground and output OUT_B is driven to Vs, while we will have the same current recirculation conditions
of the previous case when both the outputs are driven to Vs or to ground.
So, in this case the load current will be negative with an average amplitude always depending by the difference
in duty cycle of the two driving signals.
In figure 5C is shown the timing diagram in the case of negative load current .
Figure 2 shows the device block diagram of the complete current control loop.
3.1 Reference VoltageThe voltage applied to VREF pin is the reference for the internal DAC and, together with the sense resistor val-
ue, defines the maximum current into the motor winding according to the following relation:
where Rs = sense resistor value MAX
0.5V REF⋅S
------ --------------------- 1-----V REFS------------⋅==
7/23
L6258EX
Figure 5. Power Bridge Configuration
L6258EX
Figure 6. Current Control Loop Block Diagram
3.2 Input Logic (I0 - I1 - I2 - I3)The current level in the motor winding is selected according to this table:
Table 4.
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L6258EX
3.3 Phase Input ( PH )The logic level applied to this input determines the direction of the current flowing in the winding of the motor.
High level on the phase input causes the motor current flowing from OUT_A to OUT_B through the load.
3.4 Triangular GeneratorThis circuit generates the two triangular waves TRI_0 and TRI_180 internally used to generate the duty cycle
variation of the signals driving the output stage in bridge configuration.
The frequency of the triangular wave defines the switching frequency of the output, and can be adjusted by changing
the capacitor connected at TR1_CAP pin :
where : K = 1.5 x 10-5
3.5 Charge Pump CircuitTo ensure the correct driving of the high side drivers a voltage higher than Vs is supplied on the Vboot pin. This
boostrap voltage is not needed for the low side power DMOS transistors because their sources terminals are
grounded. To produce this voltage a charge pump method is used. It is made by using two external capacitors;
one connected to the internal oscillator (CP) and the other (Cboot) to storage the overvoltage needed for the
driving the gates of the high side DMOS. The value suggested for the capacitors are:
Table 5.
3.6 Current Control LOOPThe current control loop is a transconductance amplifier working in PWM mode.
The motor current is a function of the programmed DAC voltage.
To keep under control the output current, the current control modulates the duty cycle of the two outputs OUT_A
and OUT_B, and a sensing resistor Rs is connected in series with the motor winding in order to produce a volt-
age feedback compared with the programmed voltage of the DAC .
The duty cycle modulation of the two outputs is generated comparing the voltage at the outputs of the error am-
plifier, with the two triangular wave references .
In order to drive the output bridge with the duty cycle modulation explained before, the signals driving each out-
put ( OUTA & OUTB ) are generated by the use of the two comparators having as reference two triangular wave
signals Tri_0 and Tri_180 of the same amplitude, the same average value (in our case Vr), but with a 180° of
phase shift each other.
The two triangular wave references are respectively applied to the inverting input of the first comparator and to
the non inverting input of the second comparator.
The other two inputs of the comparators are connected together to the error amplifier output voltage resulting
by the difference between the programmed DAC. The reset of the comparison between the mentioned signals
is shown in fig. 7.ref K----=
L6258EX
Figure 7. Output comparator waveformsIn the case of VDAC equal to zero, the transconductance loop is balanced at the value of Vr, so the outputs of
the two comparators are signals having the same phase and 50% of duty cycle .
As we have already mentioned, in this situation, the two outputs OUT_A and OUT_B are simultaneously driven
from Vs to ground ; and the differential voltage across the load in this case is zero and no current flows in the
motor winding.
With a positive differential voltage on VDAC (see Fig 6, the transconductance loop will be positively unbalanced
respected Vr.
In this case being the error amplifier output voltage greater than Vr, the output of the first comparator is a square
wave with a duty cycle higher than 50%, while the output of the second comparator is a square wave with a duty
cycle lower than 50%.
The variation in duty cycle obtained at the outputs of the two comparators is the same, but one is positive and
the other is negative with respect to the 50% level.
The two driving signals, generated in this case, drive the two outputs in such a way to have switched current
flowing from OUT_A through the motor winding to OUT_B.
With a negative differential voltage VDAC, the transconductance loop will be negatively unbalanced respected Vr.
In this case the output of the first comparator is a square wave with a duty cycle lower than 50%, while the output
of the second comparator is a square wave with a duty cycle higher than 50%.
The variation in the duty cycle obtained at the outputs of the two comparators is always of the same.
The two driving signals, generated in this case, drive the the two outputs in order to have the switched current
flowing from OUT_B through the motor winding to OUT_A.
3.7 Current Control Loop CompensationIn order to have a flexible system able to drive motors with different electrical characteristics, the non inverting
input and the output of the error amplifier ( EA_OUT ) are available.
Connecting at these pins an external RC compensation network it is possible to adjust the gain and the band-
width of the current control loop.
11/23
L6258EX PWM CURRENT CONTROL LOOP
4.1 Open Loop Transfer Function AnalysisBlock diagram : refer to Fig. 6.
Table 6. Application data:these data refer to a typical application, and will be used as an example during the analysis of the stability of the
current control loop.
The block diagram shows the schematics of the L6258EX internal current control loop working in PWM mode;
the current into the load is a function of the input control voltage VDAC , and the relation between the two vari-
ables is given by the following formula:
Iload · RS · GS = VDAC · Gin
where:
VDAC is the control voltage defining the load current value
Gin is the gain of the input transconductance amplifier ( 1/Ra ) is the gain of the sense transconductance amplifier ( 1/Rb ) is the resistor connected in series to the output to sense the load current
In this configuration the input voltage is compared with the feedback voltage coming from the sense resistor,
then the difference between this two signals is amplified by the error amplifier in order to have an error signal
controlling the duty cycle of the output stage keeping the load current under control.
It is clear that to have a good performance of the current control loop, the error amplifier must have an high DC
gain and a large bandwidth .
Gain and bandwidth must be chosen depending on many parameters of the application, like the characteristics
of the load, power supply etc..., and most important is the stability of the system that must always be guaran-
teed.
To have a very flexible system and to have the possibility to adapt the system to any application, the error am-
plifier must be compensated using an RC network connected between the output and the negative input of the
same.
For the evaluation of the stability of the system, we have to consider the open loop gain of the current control
loop: LOAD RSb
-------⋅⋅ V DACa
-------⋅= LOAD V DACbaRs⋅--- ---------------⋅ 0.5 V DACS
----- ----------A()⋅==
L6258EX Aloop = ACerr · ACpw · ACload · ACsense
where AC... is the gain of the blocks that refers to the error, power and sense amplifier plus the attenuation of
the load block.
The same formula in dB can be written in this way:
AloopdB = ACerrdB + ACpwdB + ACloaddB + ACsensedB
So now we can start to analyse the dynamic characteristics of each single block, with particular attention to the
error amplifier.
4.2 Power AmplifierThe power amplifier is not a linear amplifier, but is a circuit driving in PWM mode the output stage in full bridge
configuration.
The output duty cycle variation is given by the comparison between the voltage of the error amplifier and two
triangular wave references Tri_0 and Tri_180. Because all the current control loop is referred to the Vr refer-
ence, the result is that when the output voltage of the error amplifier is equal to the Vr voltage the two output
Out_A and Out_B have the same phase and duty cycle at 50%; increasing the output voltage of the error am-
plifier above the Vr voltage, the duty cycle of the Out_A increases and the duty cycle of the Out_B decreases
of the same percentage; on the contrary decreasing the voltage of the error amplifier below the Vr voltage, the
duty cycle of the Out_A decreases and the duty cycle of the Out_B increases of the same percentage.
The gain of this block is defined by the amplitude of the two triangular wave references; more precisely the gain
of the power amplifier block is a reversed proportion of the amplitude of the two references.
In fact a variation of the error amplifier output voltage produces a larger variation in duty cycle of the two outputs
Out_A and Out_B in case of low amplitude of the two triangular wave references.
The duty cycle has the max value of 100% when the input voltage is equal to the amplitude of the two triangular
references.
The transfer function of this block consist in the relation between the output duty cycle and the amplitude of the
triangular references.
Vout = 2 · VS · (0.5 - DutyCycle)
Moreover, having the two references Tri_0 and Tri_180 a triangular shape it is clear that the transfer function of
this block is a linear constant gain without poles and zeros.
4.3 Load AttenuationThe load block is composed by the equivalent circuit of the motor winding (resistance and inductance) plus the
sense resistor.
We will considered the effect of the Bemf voltage of the motor in the next chapter.
The input of this block is the PWM voltage of the power amplifier and as output we have the voltage across the
sense resistor produced by the current flowing into the motor winding. The relation between the two variable is :
ACpwdB 20 Vout∆in∆---------------log⋅ 2VS⋅
Triangular Amplitude--------- ---------- -------------- ----------------------==
ACpwdB 10 224⋅
1.6-- ------------log⋅ 29.5dB== senseoutL RS+---- -----------------RS⋅=
