L6258 datasheet(5/18 Pages) STMICROELECTRONICS

Part #	L6258

Description	PWM CONTROLLED - HIGH CURRENT DMOS UNIVERSAL MOTOR DRIVER

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

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5 / 18 page

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 L6258 power stage is composed by power

DMOS in bridge configuration as it is shown in fig-

ure 1, 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 1A 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 sig-

nal 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 di-

agonal bridge formed by T1 and T4 when the out-

put 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 recircula-

tion 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 de-

pending on the difference in duty cycle of the two

driving signals.

In figure 1B is shown the timing diagram in the

case of positive load current

On the contrary, if we want to drive negative cur-

rent 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 ob-

tain 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 recircula-

tion 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 sig-

nals.

In figure 1C 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.

Reference Voltage

The voltage applied to VREF pin is the reference

for the internal DAC and, together with the sense

resistor value, defines the maximum current into

the motor winding according to the following rela-

tion:

IMAX

=

0.5

⋅ VREF

RS

=

1

FI

⋅

VREF

RS

where Rs = sense resistor value

L6258

5/18

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