L295_03 datasheet(4/7 Pages) STMICROELECTRONICS

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L295

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These inputs are:

– EN chip enable (digital input, active low), enables both channels when in the low state.

– Vin1, Vin2 channel inputs (digital inputs, active high), enable each channel independently.

A channel is actived when both EN and the appropriate channel input are active.

– Vref1, Vref2 referce voltages (analog inputs), used to program the peak load currents. Peak load current

is proportional to Vref.

Since the two channels are identical, only channel one will be described. The following description applies

also the channel two, replacing FF2 for FF1, Vref for Vref1 etc.

When the channel is avtivated by low level on the EN input and a high level on the channel input, Vin2, the

output transistors Q1 and Q2 switch on and current flows in the load according to the exponential law:

where:

R1 and R2 are the resistance and inductance of the load and V is the voltage available on the load (Vs -

Vdrop - Vsense).

The current increases until the voltage on the external sensing resistor, RS1, reaches the reference volt-

age, Vref1. This peak current, Ip1, is given by:

At this point the comparator output, Vomp1, sete the RS flip-flop, FF1, that turns off the output transistor,

Q1. The load current flowing through D2, Q2, RS1, decreases according to the law:

where VA = VCEsat Q2 + Vsense + VD2

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 tran-

sistor Q2 is turned off. The load current flows through D2 and D1 according to the law:

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.

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 presence of a hysteresis circuit makes the IC work again aftera fall of

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R1t

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ref1

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 e R1t

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 e R1t

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