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

Vo

111

1

no def

111

0

2.1

110

1

2.2

110

0

2.3

101

1

2.4

101

0

2.5

100

1

2.6

100

0

2.7

011

1

2.8

011

0

2.9

010

13

010

0

3.1

001

1

3.2

001

0

3.3

000

1

3.4

000

0

3.5

0 = Processor pin connected to ground

1 = Processor pin Open

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 #

ROW A

ROW B

1

Vin (5V)

Vin (5V)

2

Vin (5V)

Vin (5V)

3

Vin (5V)

Vin (5V)

4

Vin (12V)

Vin (12V)

5

Reserved

Reserved

6

Reserved

OUTEN

7

VID0

VID1

8

VID2

VID3

9

Reserved

PWRGD

10

Vo

GND

11

GND

Vo

12

Vo

GND

13

GND

Vo

14

Vo

GND

15

GND

Vo

16

Vo

GND

17

GND

Vo

18

Vo

GND

19

GND

Vo

20

Vo

GND

AN908 APPLICATION NOTE

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