L5994 STMicroelectronics, L5994 Datasheet - Page 15

• Manufacturer Part Number: L5994
• Description: IC CTRLR PS ADJ TRPL-OUT 32-TQFP
• Manufacturer: STMicroelectronics

Specifications of L5994

• Pwm Type: Current Mode
• Number Of Outputs: 3
• Frequency - Max: 345kHz
• Duty Cycle: 96%
• Voltage - Supply: 4.75 V ~ 25 V
• Buck: Yes
• Boost: No
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 140°C
• Package / Case: 32-TQFP, 32-VQFP
• Frequency-max: 345kHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

+3.3V Inductor
To define the inductor, it is necessary to determine firstly the inductance value. Its minimum value is given by:
and a value L
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 KoolM , to achieve
high efficiency. These materials provide low core losses (ferrite in particular), so that the design can be concen-
trated on preventing saturation and limiting copper losses. Saturation must be avoided even at maximum peak
current:
To limit copper losses, the winding DC resistance, R
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 requirements
in terms of inductance and maximum DC current. Nowadays there is a broad range of products offered by man-
ufacturer, also for surface mount assemblies.
+5.1V Transformer
The primary winding carries the secondary power as well, thus the total primary average current is:
where V
linear regulator. The turns ratio 1:n of the transformer is chosen so that V
turns ratio in order to minimize stray parameters, the secondary is referred to the 5.1V output, and therefore the
minimum value is given by:
where V
connected with the proper polarity (see fig. 4). The minimum primary inductance value can be expressed as:
where RF, to get positive values for L
and where V
ductance L
As to the transformer realization, the considerations regarding to the +3.3V inductor can be here repeated.
DRLIN
f
is the forward drop across the rectifier (assume 1V to be conservative). Make sure the secondary is
5P
IN
3
> L
is the voltage generated during the recirculation of the primary and fed into the input of the +12V
can be either V
> L
5PMIN
3MIN
L
5pm in
the primary peak current, which must not saturate the magnetic core, will be:
should be selected. Core geometry selection is connected to the requirements of the
=
I
5 PK
INMIN
3
-- -
4
---------------------------------------------------------------------------------------------------------------------------------------------
V
=
I
IN
L 3PK
L
or V
I
I
3M IN
5 TO T
5PMIN
TO T5
f
SW
INMAX
=
RF
MIN
, must satisfy the inequality:
+
=
I
=
OUT3
--------------------------------------------------------- -
2 f
I
, whichever gives the higher value for L
-------------------------------------------------------------------- -
V
5.1
T O T5
I
INMAX
OUT 5
=
----------------------------------------------- -
I
T OT 5
3.3
SW
V
------------------------------------------- -
+
INLIN
V
L
V
–
5.1
----------------------------------------------------- -
2 f
3.3
, should be as low as possible (in the range of m ). AC
+
RF
INMA X
L
IN
5 P
V
V
---------------------------------------- -
f
V
SW
S W
INM AX
INLIN
5.1
–
I
IN
V
OUT12
V
V
5.1
V
IN
INM AX
–
–
INM AX
IN
I
L
5.1
OUT3
5.1
5.1
–
3
+
–
–
I
5.1
OUT12
V
5.1
3.3
V
f
INM AX
–
+
2
RF
3.3
–
I
DRLIN
OUT12
V
IN
is above 13V. To reduce the
I
OUT12
5PMIN
L5994 - L5994A
. With a primary in-
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