TDA7272A datasheet(6/16 Pages) STMICROELECTRONICS

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The speed n of a k pole motor results :

10.435

CT kRP

and becomes independent of the resistor RT

which only determines the current level and the

duty cycle which should be 1 : 1 at the nominal

speed for minimum torque ripple.

The second fast loop consists of a voltage to cur-

rent converter which is driven at pin 8 by the low

pass filter RL,CL. The output current at this stage

is injected by a PNP current mirror into the inner

resistor RB. So the driving voltage of the output

stage consists of the integrator output voltage

plus the fast loop voltage contribution across RB.

The power output stage realizes different modes

depending on the logic status at pin 18 and 19.

- Normal operation for left and right mode :

each upper TR of the bridge is used as

voltage follower whereas the lower acts as

a switch.

- Stop mode where the upper half is open

and the lower is conductive.

- High impedance status where all power

elements are switched-off.

The high impedance status is also generated

when the supply voltage overcomes the 5 V to 20

V operating range or when the chip temperature

exceeds 150

°C.

A short circuit protection limits the output current

at 1.5 A. Integrated diodes clamp spikes from the

inductive load both at VCC and ground.

The reference voltages are derived from a com-

mon bandgap reference. All blocks are widely

supplied by an internal 3.5 V regulator which pro-

vides a maximum supply voltage rejection.

PIN FUNCTION AND APPLICATION INFORMA-

TION

PIN 1

Trigger input. Receives a proper voltage which

contains the information of the motor speed. The

waveform can be derived directly by the motor

current (fig. 3). The external resistor generates a

proper voltage drop. Together with the input resis-

tance at pin 1 [RIN(1) = 500

Ω ] the external ca-

pacitor CD realize a high pass filter which differen-

tiates the commutation spikes of the motor

current. The trigger level is 0V.

The biasing of the pin 1 is 20 mV with a hystere-

sis of 10 mV. So the sensing resistance must be

chosen high enough in order to obtain a negative

spike of the least 30 mV on pin 1, also with mini-

mum variation of motor current :

≥

30mV

∆IMOT min.

Such value can be too much high for the preregu-

lation stage V-I and it could be necessary to split

them into 2 series resistors RS =RS1 +RS2 (see

fig. 4) as explained on pin 8 section.

The information can be taken also from an exter-

nal tachogenerator. Fig. 5 shows various sources

connections :

the input signal mustn’t be lower than 0.7 V.

Figure 3.

Figure 4.

TDA7272A

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