Functional description L6258EA
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2 Functional description
The 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 L6258EA power stage is composed by power DMOS in bridge configuration as it is
shown in Figure 4, where the bridge outputs OUT_A and OUT_B are driven to V
s
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 (V
s
) and ground, but keeping the differential voltage across the load equal to zero.
In Figure 4 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 positive 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 V
s
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 higher 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 4 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 4 is shown the timing diagram in the case of negative load current.
Figure 5 shows the device block diagram of the complete current control loop.
