L6223 datasheet(16/33 Pages) STMICROELECTRONICS

16 / 33 page

Figure 19: Dot Matrix Printer Motor Driver schematic diagram (See also fig. 10, 16).

External RC Network (pin 14)

The external RC network provides the higher of

the two possible chopping frequencies. The dis-

charge time of the capacitor represents the mini-

mum t(ON) available in closed loop. In open loop it

is possible to select a smaller t(ON), (see Fig. 20).

The t(ON) min defines the min current the IC can

supply to the motor, as well as the protection

”window”. This window is necessary to mask the

spike generated at the beginning of each chop-

ping period (see Fig. 21a).

Figure 20: Oscillator waveform and timing.

The window must be large enough to mask this

spike, without penalizing excessively the min cur-

rent control. The capacitor C mainly defines the

value of the window. The mathematical formulas

to calculate the approximate values are:

f(osc) = 1/(0.84

• RC)

for R > 10k

Ω

tON(min) =t(window) = 0.84

• C • R • Rin/(R+Rin)

for Rin = 1.2k

Ω

where Rin is the resistor internal to the IC for the

capacitor discharging.

L6223

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L6223 Datasheet(PDF) 16 Page - STMicroelectronics