L6615 datasheet(12/20 Pages) STMICROELECTRONICS

12 / 20 page

L6615

12/20

Figure 12. ADJ network

To set the RADJ value it is necessary to know the tolerance required of the power supply output voltage

(VOUT±∆VO); the maximum difference between master and slave output voltage is 2*∆VO and this amount

represents the voltage that the L6615 must be able to correct.

Now two different approaches are feasible depending on whether the SMPS (whose output current must

be shared) has to be completely designed or it is an "off the shelf" component and only the current sharing

section must be designed.

In the first case, the adjustment resistor (RADJ) can be considered as a fraction of the high resistor of the

feedback divider RH (see fig.12.a): typically the first step consist of fixing the current flowing, under steady

state condition, through the feedback divider IFB; by choosing the value for R2:

we will have:

It can be an useful rule of thumb to use RADJ lower than (or equal to) one tenth of R1, considering that, in

worst case condition, it will be:

This value must not exceed the one indicated in the "Electrical characteristic section" but this is very easy

to meet, as one can easily see by using sensible values for

∆VOUT and R2.

In the second case (fig 12.b), the feedback divider has been already designed by the SMPS manufacturer

and it is not possible to modify it: the design of RADJ must be done to make the L6615 able to correct the

maximum spread without significantly shifting the SMPS regulation point. A minimum RADJ value can be

found by:

where IADJ(max) is 8mA.

Especially for low voltage output buses it is important to avoid adjustment network saturation; the design

must satisfy the following relationship:

where VADJ(MIN) can be found in the "Electrical characteristic section" for different IADJ values.

OUT

REF

Off the shelf

POWER SUPPLY

ADJ

E/A

to L6615

ADJ pin

ADJ

OUT

REF

ADJ

E/A

to L6615

ADJ pin

ADJ

OUT

REF

Off the shelf

POWER SUPPLY

ADJ

E/A

to L6615

ADJ pin

ADJ

OUT

REF

ADJ

E/A

to L6615

ADJ pin

ADJ

REF

--------------

ADJ

OUT

REF

---------------

–







⋅

ADJ max

()

OUT

∆

ADJ

------------------

ADJ min

()

OUT

∆

ADJ ma x

()

--------------------------

OUT

ADJ

() V

ADJ MIN

()

⋅

–

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