AN908 Datasheet, PDF (3/9 Pages) STMicroelectronics – APPLICATION NOTE

AN908 APPLICATION NOTE
Supply Voltages
The VRM is supplied by the 5V and the 12V
standard outputs available from the conventional
PC power supplies. The 12V is used here to sup-
ply the L4990A and the 5V power bus is used to
supply the output power on the regulated DC out-
put. An input inductor, L3, is necessary to limit the
dI/dt of the input current during load transients
conditions.
This value must not exceed 0.1 A/us to prevent
the intervention of protections on the main power
supply.
Control Inputs
Enable: The module accept an open collector sig-
nal that disable the power supply in the low state.
This function is realized by discharging the soft
start capacitor.
This solution allows the module to restart with
the soft start sequence.
Voltage Identification: The module is able to ac-
cept 4 open collectors input signals coming from
the microprocessor.
Table 1 shows the VID0-3 code. The voltage
identification is implemented with R23-26, R9 and
R14 resistors, to allow to programm the con-
verter output voltage at a value lower than the
L4990A internal feedback reference, of 2.5V.
The four voltage identification bit are buffered
with a 3-State buffer, 74HCT125.
Table 1.
VID3
VID2
VID1
VID0
1
1
1
1
1
1
1
0
1
1
0
1
1
1
0
0
1
0
1
1
1
0
1
0
1
0
0
1
1
0
0
0
0
1
1
1
0
1
1
0
0
1
0
1
0
1
0
0
0
0
1
1
0
0
1
0
0
0
0
1
0
0
0
0
0 = Processor pin connected to ground
1 = Processor pin Open
Vo
no def
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
3
3.1
3.2
3.3
3.4
3.5
Output Control
Power Good Output: A PWRG signal is available.
It’s in the Low State when the programmed out-
put voltage is inside the +/-10% of the nominal
voltage, and in the High state outside the men-
tioned voltage range. This function is realized by
using two comparators inside the quad LM339.
Internal Controls
Current limiting: The current limiting intervention
is fixed at 13A to guarantee a good functionality
at 12A continuous output current.
Behind 13A, the controller is going into pulse-by-
pulse constant limiting current. A foldback type
limiting current is also available.
In case a hiccup limiting current is accepted, the
L4990A controller has this function on board.
Over Voltage Protection: a circuit is detecting an
Overvoltage condition when output voltage is
18% above the nominal output voltage. In this
case, the VRM is disabled.
Connector description
The connector is standard 40 pin, with a current
capability of 3A/pin (type AMPMOD2 part number
532956-7 or equivalent) and the pinout is shown
in Tab2.
Table 2. Connector pinout.
Pin #
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
ROW A
Vin (5V)
Vin (5V)
Vin (5V)
Vin (12V)
Reserved
Reserved
VID0
VID2
Reserved
Vo
GND
Vo
GND
Vo
GND
Vo
GND
Vo
GND
Vo
ROW B
Vin (5V)
Vin (5V)
Vin (5V)
Vin (12V)
Reserved
OUTEN
VID1
VID3
PWRGD
GND
Vo
GND
Vo
GND
Vo
GND
Vo
GND
Vo
GND
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