L6258 datasheet(8/18 Pages) STMICROELECTRONICS

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Charge Pump Circuit

To ensure the correct driving of the high side driv-

ers 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 con-

nected 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:

Cboot

Storage

Capacitor

100

Pump

Capacitor

Current Control LOOP

The current control loop is a transconductance

amplifier working in PWM mode.

The motor current is a function of the pro-

grammed DAC voltage.

To keep under control the output current, the cur-

rent control modulates the duty cycle of the two

outputs OUT_A and OUT_B, and a sensing resis-

tor Rs is connected in series with the motor wind-

ing in order to produce a voltage feedback com-

pared 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 amplifier, with the two triangular wave

references .

In order to drive the output bridge with the duty

cycle modulation explained before, the signals

driving each output ( OUTA & OUTB ) are gener-

ated 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 respec-

tively 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 con-

nected together to the error amplifier output volt-

age resulting by the difference between the pro-

grammed DAC. The reset of the comparison

between the mentioned signals is shown in fig. 3.

Tri_0

Tri_180

Error Ampl.

Output

First Comp.

Output

Second Comp.

Output

Figure 3. Output comparator waveforms

L6258

8/18

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