L584 datasheet(6/13 Pages) STMICROELECTRONICS

6 / 13 page

ferred, in the following formula, to IE, emitter current

of the external darlington Q2,

IE =ILOAD +Idn

because the sensing detection is on the darlington

emitter (not directly on the load).

The peak current level Ip, is set by the sensing re-

sistor, Rs, and is found from :

Ip = 0.45 / Rs (typ)

The peak value of holding current level, Ih, is set by

a voltage (Vset) applied to pin 2, giving :

Ihp =Vsetth /Rs =(Vset

± 10mV)/Rs

The peak to hold current ratio is fixed by Vset :

Ip /Ihp = 0.45 / Vsetth

Vset is fixed by an external reference and a voltage

divider (Vext, R1, R2 in fig 2) :

Vset =Vext * R2 / (R1 + R2)

Due to the particular darlington storage time and the

device reaction time not very significant differences

can be found between Ip and Ih values based on the

previous formula and the real values seen in the ap-

plications.

If the holding current function is not used, pin 2 can-

not be left floatingand it must be connectedto GND.

Figure 2 : Application Circuit Showing the Optional Components. In particular it illustrates how the holding

current level is adjusted independentlyof the peak current (with R1, R2, Vext) and how the internal

zener clamp is connected. This circuit produces the waveforms shown in Fig. 4.

Io (A)

BDX54

BDX53

BDW94

BDW93

BDV64

BDV65

Figure 3 : P.C. Board and Components Layout of the Circuit of Fig. 2 (1 : 1 scale).

L584

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