AN557 datasheet(44/52 Pages) STMICROELECTRONICS

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AN557 APPLICATION NOTE

44/52

The current, flowing in backward, to have 0V it will be:

∆VR2 = R2 · 0.21 = 4.7k · 0.21 = 1V

Therefore, when the cursor "P" reaches Vref + 1V the output voltage goes to zero.

At this point we are able to define as well the values of P1 and R3.

When the "P" cursor is completely moved to high, there should be 6V of dropping to "P", and in this way

0.6mA will flow.

The current flowing in R3, considering that the voltage at pin 15 has a typical value of 12V, it will be of

0.8mA.

In this case the R3 value will be 7K5ohm.

3.3V / 10A DC-DC CONVERTER

When an output voltage lower to the reference voltage of 5.1V must be stabilized with a good result from

stability and regulation point of view, and not having available the not-inverting input of the error amplifier,

it is possible to use an external reference.

In this case a TL431C reference has been chosen, which is cheaper and widespread used. In this case

more than a simple reference, it is a true shunt regulator, containing a reference, an error amplifier and a

transistor capable of absorbing a max current of 100mA.

Such component can be compensated like a common OP/AMP, and therefore in our application can sub-

stitute both the internal reference and the error amplifier.

The fig. 61 represents the electrical diagram of the application at 3.3V.

The operating input voltage is between 12V (due to the internal UVLO) and 35V, with a minimum operating

switching frequency of 100KHz.

The maximum operating input voltage is limited only 35V because the minimum "ON" time, which should

not be reduced below 1 microsecond. At input voltage of 35V, output voltage of 3.3V and fs=100KHz the

Ton time is already about of 1 microsecond.

Figure 61.

ref

----------

5.1 V

24K

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

0.21 m A

L4970A

22K

TL431

0.1

µF

330

Ω

STPS

1545

0.22

µF

µH

EKR

1000

µF

10V

Vo=3.3V

Io=10A

2200

µF

12V <Vi <35V

CORE TYPE 58071 MAGNETICS

D93IN001

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