L6285 datasheet(11/16 Pages) STMICROELECTRONICS

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Charge Pump

The charge pump circuitry generates the overvol-

tage needed to drive the gate of the high side out-

put DMOS power transistors.It is realized by us-

ing two external capacitors (C1 and C2) and two

integrated diodes that operate as a full wave recti-

fier (see Fig. 7). The oscillator peak to peak out-

put voltage is stored by C2 and summed to the

Power Supply Voltage Vp..

The voltage present at the pin BOOT1, is then the

overvoltage needed to supply the gate of the high

side DMOS drivers.

A0TH

A1TH

THERMAL PROTECTION

CIRCUIT STATUS

PREALARM

ALARM

NOT POSSIBLE

OPERATING

THERMAL SHUTDOWN

THERMAL PROTECTION

The thermal protection shuts down the chip be-

fore it can reach a dangerous temperature.

Additional informations to the microprocessor are

available at the A0TH, A1TH pins.

Figure 7: Charge Pump Circuit

APPLICATION INFORMATION

A typical application circuit is shown in Fig.8. By

this application it is possible to drive two unipolar

stepper motors (M1,M2) and one DC motor (M3).

As it can be seen, only two external Zener diodes

(D1,D2) are needed to clamp the voltage tran-

sients generated by the stray inductance of the

motor windings. This is recommended when the

peak current is not more than three to four hun-

dred mAmps. For a power supply voltage of

VP=24V

±10%, D1=D2 must be 30V ±5%-1W

(1N4751A or equivalent). Both the VP and the VS

pins need bypass capacitors (C1,C2,C3); to sup-

ply the high-side DMOS (Source Transistors) at

pin.15 ,only two external capacitors (C4,C5) com-

plete the charge pump circuitry. The oscillator fre-

quency, that is twice the chopping frequency for

M1 and M2, is mainly defined by the network

R6C6:

fosc = [0.69 (Rch +Rdsc)Cosc ]

-1 , where

Rch =R6 ;

Rdsc = 600 ohm typ.

At the same time, the lockout duration (or protec-

tion window) needed for a correct chopping be-

havior, is given by :

Tlockout = 0.69 Rdsc Cosc

The shown values (fig.8) give a nominal fre-

quency a little bit more than 41KHz and a protec-

tion window of 1.4

µs roughly. The Schottky diode

D3 and the pull-up resistor R5 driven via an open-

collector transistor can generate the Reset func-

tion. The chopping current is sensed across R1

A/B; R2 A/B that must be of a not inductive type.

The DC motor PWM Open Loop Voltage Control

operates at a frequency defined by C7, charged

with a typical constant current source (I = 240

µA), up to V1 = 0.67 VS. Since the discharge

time is very short, it can be written :

fosc = I/Cosc V1, where Cosc = C7.

Tha values indicated in figure give a typical fre-

quency of about 22 KHz.

L6285

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