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L292

SWITCH-MODE DRIVER FOR DC MOTORS

STMicroelectronics 

L292

Pin 12 of this chip is the main summing point of the system where TACHO and the D/A converter output

are compared.

The input to D/A converter consists of 5 bit word plus a sign bit supplied by the microprocessor. The sign

bit represets the direction of motor rotation. The (analogue) output of the D/A conveter -DAC/OUT - is com-

pared with the TACHO signal and the risulting error signal is amplified by the error amplifier, and subse-

quently appears on pin 1. The ERRV sognal (from pin 1 , L291) is fed to pin 6 of the final chip, the L292

H-bridge motor-driver. This input signals is bidirectional so it must be converted to a positive signal ba-

cause the L292 uses a single supply voltage. This is accomplished by the first stage - the level shifter,

which uses an internally generated 8 V reference.

This same reference voltage supplies the triangle wave oscillator whose frequency is fixed by the external

RC network (R20, C17 - pins 11 and 10) where:

1fosc

=

-----1-------

2RC

(with R ≥ 8.2kΩ)

The oscillator determines the switching frequency of the output stage and should be in the range 1 to 30

KHz.

Motor current is regulated by an internal loop in the L292 which is performed by the resistors R18, R19

and the differential current sense amplifier, the output of which is filtered by an external RC network and

fed back to the error amplifier.

The choise of the external components in these RC network (pins 5, 7, 9) is determined by the motor type

and the bandwidth requirements. The values shown in the diagram are for a 5Ω, 5 MH motor. (See L292

Transfer Function Calculation in Application Information).

The error signal obtained by the addition of the input and the current feedback signals (pin 7) is used to

pulse width modulate the oscillator signal by means of the comparator. The pulse width modulated signal

controls the duty cycle of the Hbridge to give an output current corresponding to the L292 input signal.

The interval between one side of the bridge switching off and the other switching on, t, is programmed by

C17 in conjuction with an internal resistor Rt. This can be foud from:

τ = Rτ · Cpin10. (C17 in the diagram)

Since Rτ is approximately 1.5 KΩ and the recommended t to avoid simultaneous conduction is 2.5µs Cpin

10 should be around 1.5 nF.

The current sense resistors R18 and R19 should be high precision types (maximum tolerance ± 2 %) and

the recommended value is given by:

Rmax· Io max ≤ 0.44V

It is possible to synchronize two L292 ’s, if desired, using the network shown in fig. 2.

Finally, two enable inputs are provited on the L292 (pins 12 and 13-active low and high respectively). Thus

the output stage may be inhibited by taking pin 12 high or by taking pin 13 low. The output will also be

inhibited if the supply voltage falls below 18V.

The enable inputs were implemented in this way because they are intended to be driven directly by a mi-

croprocessor. Currently available microprocessors may generates spikes as high as 1.5V during power-

up. These inputs may be used for a variety of applications such as motor inhibit during reset of the logical

system and power-on reset (see fig. 3).

5/13

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