AN557 datasheet(47/52 Pages) STMICROELECTRONICS

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AN557 APPLICATION NOTE

best if matched in Vbe, and some other passive components. For a cost reduction Q1 can be substitute

by a simple diode 1N4148.

The divider composed by R2 and R1 fixes a voltage at the whished voltage value (for example 50 -100mV)

on the Q2 emitter.

Q2 will be reversed bias untill the emitter voltage of Q1 will raech the same value as itself.

At this point Q2 will be direct bias and will begin to absorb current from its collector; in the moment in which

Q2 will enter into conduction, a variation

∆Vsc at the current sense resistor will give a variation of the Q2

current equal to:

When the current absorbed by Q2 will reach the maximum current delivered by the error amplifier output

(or by the current of the soft-start if this pin is prefered to use), the error amplifier will fall out of regulation

and its output voltage will begin to decrease reducing, conseguentely, the duty cycle; then the regulator

will begin in this way to behave as a generator of current instead of voltage. The emitter voltage of Q2 is

fixed by the following formula:

A general criterium, is that of fixing the divider "R1R2" in such way as to make a current flow that is greater

than the necessary lowering the output voltage of the error amplifier.

The maximum current delivered by the output of the transconduttance error amplifier is 200mA; the current

that has to flow in the divider R1R2 should be around of 2-3mA to have a very precise inter-vation or

around only 1mA for slightly more soft interventions.

By varying the value of R2, the point of intervention of the current limitation will be moved. The resistor Rx

contributes to introducing a more or less accentuate foldback effect, on the output current.

In the following table suggest a few values of Rsense according to the nax output current.

The criterium used to defined the value of the sense resistor is essentially tied to the max power dissipated

by the resistance, as well as to the market availability.

If the mains objective is to maximize the efficiency when (delivering for example 10A), it is convenient to

use two current transformers instead of a dissipative resistor, one in series to the source of the internal

DMOS and one in series to the catch diode.

Using such solution, a quite simple and fine regulation of the current is possible to implement. Figure 66

shows a current generator solution with high precision on the current, using an op/amp instead of two

small signal transistors.

Higher input voltage.

Since the maximum operating input voltage of this family is 50V, when one of these devices must be sup-

plied with more elevated voltages, it is necessary to introduce a preregulator.

Fixing the output voltage of the preregulator of 45V, the power dissipation of the preregulator is:

Pd = Ii · VCE = Ii · (Vi - 45)

Rsense (mΩ)

Io (A)

Device

100

3.5

L4970A

L4977A

L4975A

L4974A

L4972A

CQ2

∆

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

EQ 2

REF

---------------------- R1

5.1 V

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





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