L296 datasheet(19/22 Pages) STMICROELECTRONICS

19 / 22 page

Figure 43 : 5.1V/2A Power Supply using External

Limiting Current Resistor and Crow-

bar Protection on the Supply Voltage

(L296P only)

SOFT-START AND REPETITIVE POWER-ON

When the device is repetitivelypowered-on,the soft-

start capacitor, CSS, must be discharged rapidly to

ensurethat each start is ”soft”. This can be achieved

economicallyusing the reset circuit, as shownin Fig-

ure 44.

In this circuit the divider R1, R2 connectedto pin 12

determines the minimum supply voltage, below

which the open collector transistor at the pin 14 out-

put discharges CSS.

Figure 44

Figure 45

Figure 46

The approximate discharge times obtainedwith this

circuit are :

CSS (

µF)

tDIS (

µs)

2.2

4.7

200

300

600

If thesetimes are still too long,an externalPNPtran-

sistor may be added, as shown in Figure 45 ; with

this circuit discharge times of a few microseconds

may be obtained.

HOW TO OBTAIN BOTH RESET AND

POWER FAIL

Figure 46 illustrateshowit is possibleto obtainat the

same time both the power fail and reset functions

simply by adding one diode(D) and one resistor (R).

In this case the Reset delay time (pin 13) can only

start when the output voltage is VO

≥ VREF - 100mV

and the voltage accross R2 is higher than 4.5V.

With the hysteresis resistor it is possible to fix the in-

put pin 12 hysteresis in order to increase immunity

to the 100Hz ripple present on the supply voltage.

Moreover, the power fail and reset delay time are

automatically locked to the soft-start. Soft-start and

delayed reset are thus two sequential functions.

The hysteresis resistor should be In the range of

aboit 100k

Ω and the pull-up resistor of 1 to 2.2kΩ.

L296 - L296P

19/22

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