L584 datasheet(11/13 Pages) STMICROELECTRONICS

11 / 13 page

Figure 9 : Application circuit showing how two separate supplies can be used.

In this application it is assumed that the 5V supply

for L584 is taken from a logic supply, which is al-

ready protected, against load dump transients and

vol-tage reversal.

Pin 1 must be left open, as shown in fig. 9, if VS is

always lower than 46V even during the voltage tran-

sients.

Note that toff is also related to the required current

ripple

∆I on the peak or on the holding current level

by :

(Io –

∆I) RL +Voff

toff =–

Io RL +Voff

Where : Io is the initial current valuein OFF condition

(equal to Ip or IH in accordance to the current level

considered),

VOFF =VDIODE +VCEQ1

RL is the series resistance value of the induc-

tance L :

Therefore C2 can be dimensioned directly by :

IREF Lln

(Io –

∆I) RL +VOFF

C2 =

Io RL +VOFF

Note that toff is the same for both the peak and hold-

ing current.

ton time is given by :

LVon – R(I1 –

∆I)

ton =ln

RVon – RI1

where : I1 is the final current value in ON condition

(equal to Ip or IH in accordance to the current level

considered),

R= RL +RSENSE

Von =VS –VCEsatQ2

If the constant times are respectively

>20 toff

and

> 20 ton

it is possible to consider a purely inductive load and

therefore :

∆I

toff =L

; ton =L

Von

L584

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