L6258 datasheet(9/18 Pages) STMICROELECTRONICS

9 / 18 page

In the case of VDAC equal to zero, the transcon-

ductance 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 saturation,

the two outputs OUT_A and OUT_B are simulta-

neously driven from Vs to ground ; and the differ-

ential 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 2, the transconductance loop will be positively

unbalanced respected Vr.

In this case being the error amplifier output volt-

age greater than Vr, the output of the first compa-

rator is a square wave with a duty cycle higher

than 50%, while the output of the second compa-

rator 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 tran-

sconductance 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 out-

puts 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.

Current Control Loop Compensation

In order to have a flexible system able to drive

motors with different electrical characteristics, the

non inverting input and the output of the error am-

plifier ( EA_OUT ) are available.

Connecting at these pins an external RC com-

pensation network it is possible to adjust the gain

and the bandwidth of the current control loop.

PWM CURRENT CONTROL LOOP

Open Loop Transfer Function Analysis

Block diagram : refer to Fig. 2.

Application data:

VS = 24V

Gs transconductance gain = 1/Rb

LL = 12mH

Gin transconductance gain = 1/Ra

RL = 12

Ω

Ampl. of the Tria_0_180 ref. = 1.6V

(peak to peak)

RS = 0.33

Ω

Ra = 40K

Ω

RC = to be cal-

culated

Rb = 20K

Ω

CC = to be cal-

culated

Vr = Internal reference equal to

VCC/2 (Typ. 2.5V)

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

L6258 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 variables is given by the follow-

ing formula:

Iload

⋅ RS ⋅ GS = VDAC ⋅ Gin

Iload

⋅ RS ⋅

= VDAC ⋅

Iload

= VDAC

⋅

⋅ RS =

0.5

⋅

VDAC

(A)

where:

VDAC

is the control voltage defining the

load current value

Gin

is the gain of the input transcon-

ductance amplifier ( 1/Ra )

is the gain of the sense transcon-

ductance 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 sig-

nals 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 guaranteed.

To have a very flexible system and to have the

possibility to adapt the system to any application,

the error amplifier must be compensated using an

L6258

9/18

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