L4992 View Datasheet(PDF) - STMicroelectronics

Part Name

Description

Manufacturer

L4992

TRIPLE OUTPUT POWER SUPPLY CONTROLLER

STMicroelectronics 

L4992

DESIGN PROCEDURE

Basically, the application circuit topology is fixed, and the design procedure concerns only the selection

of the component values suitable for the voltage and current requirements of the specific application.

The design data one needs to know are therefore:

Input voltage range: the minimum (Vinmin) and the maximum (Vinmax) voltage under which the applica-

tion is expected to operate;

Maximum load current for each of the three sections:

- Iout3 for the +3.3V section;

- Iout5 for the +5.1V section:

- Iout12 for the +12V section;

Maximum peak-to-peak ripple amplitude of the output voltage for each switching section:

- Vrpp3 for the +3.3V section;

- Vrpp5 for the +5.1V section;

The operating frequency fsw (200/300 kHz or externally synchronized).

It is worth doing some preliminary considerations. The selection of the switching frequency depends on

the requirements of the application. If the aim is to minimize the size of the external components, 300

kHz will be chosen. For low input voltage applications 200 kHz is preferred, since it leads to a higher

maximum duty cycle.

As for the switching regulators, the inductance value of the output filter affects the inductor current ripple:

the higher the inductance the lower the ripple. This implies a lower current sense resistor value (for a

given Iout), lower core losses and a lower output voltage ripple (for a given output capacitor) but, on the

other hand, more copper losses and a worse transient behaviour due to load changes. Usually the maxi-

mum ripple peak-to-peak amplitude (which occurs at Vinmax) is chosen between 15% and 50% of the full

load current. It is convenient to introduce a ripple factor coefficient, RF, that is therefore a number be-

tween 0.15 and 0.5.

As for the linear regulator, its input voltage Vinlin should not fall below 13V and therefore the auxiliary

winding should be dimensioned to get this voltage with a certain margin (say, 14V). Conversely, an

higher input voltage leads to higher losses inside the regulator, to the detriment of efficiency, and to

higher total current on the +3.3V inductor. Besides it implies a higher turns ratio and therefore a worse

magnetic coupling, which affect energy transfer during flyback.

SWITCHING REGULATORS

+5.1V Inductor

To define the inductor, it is necessary to determine firstly the inductance value. Its minimum value is

given by:

L5min

=

5.1 ⋅ (Vin max − 5.1)

Vin max ⋅ fsw ⋅ Iout5 ⋅ RF

and a value L5 > L5min should be selected.

Core geometry selection is connected to the requirements of the specific application in terms of space

utilization and other practical issues like ease of mounting, availability and so on. As to the material, the

choice should be directed towards ferrite, molypermalloy or Kool Mµ®, to achieve high efficiency. These

materials provide low core losses (ferrite in particular), so that the design can be concentrated on pre-

venting saturation and limiting copper losses.

Saturation must be avoided even at maximum peak current:

IL5pk

=

Iout5

+

5.1 ⋅ (Vin max − 5.1)

2 ⋅ fsw ⋅ L5 ⋅ Vin max

To limit copper losses, the winding DC resistance, RL, should be as low as possible (in the range of mΩ).

AC losses can usually be neglected. A practical criterion to minimize DC resistance could be to use the

largest wire that fits the selected core.

Anyway the best solution, whenever possible, is to use an off-the-shelf inductor which meets the require-

ments in terms of inductance and maximum DC current. Nowadays there is a broad range of products

13/26

Share Link: