L295 datasheet(5/8 Pages) STMICROELECTRONICS

5 / 8 page

If the oscillator pin (9) is connected to ground the

load current falls to zero as shown in fig. 1.

At this time t2 the channel 1 is disabled, by taking

the inputs Vin1 low and/or EN high, and the output

transistor Q2 is turned off. The load current flows

through D2 and D1 according to the law:

= (

+ IT2 ) e

− R1 t

−

where VB = VS + VD1 + VD2

IT2 = current value at the time t2.

Fig. 2 in shows the current waveform obtained with

an RC network connected between pin 9 and

ground. From to t1 the current increases as in fig.

1. A difference exists at the time t2 because the

current starts to increase again. At this time a pulse

is produced by the oscillator circuit that resets the

flip.flop, FF1, and switches on the outout transistor,

Q1. The current increases until the drop on the

sensing resistor RS1 is equal to Vref1 (t3) and the

cycle repeats.

SIGNAL WAVEFORMS

The switching frequency depends on the value R

and C, as shown in fig. 4 and must be chosen in

the range 10 to 30 KHz.

It is possible with external hardware to change the

reference voltage Vref in order to obtain a high peak

current Ip and a lower holding current Ih (see fig. 3).

The L295 is provided with a thermal protection that

switches off all the output transistors when the

junction temperature exceeds 150

°C. The pres-

ence of a hysteresis circuit makes the IC work again

aftera fall of the junction temperature of about

°C.

The analog input pins (Vref1 , Vref2) can be left open

or connected to Vss; in this case the circuit works

with an internal reference voltage of about 2.5V and

the peak current in the load is fixed only by the value

of Rs:

2.5

Figure 1. Load current waveform with pin 9

connected to GND.

Figure 2. Load current waveform with external

R-C network connected between pin 9 and

ground.

5/8

L295

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