LP3906 datasheet(18/38 Pages) NSC

18 / 38 page

SW1, SW2: Synchronous Step

Down Magnetic DC/DC Converters

(Continued)

IN, MIN =ILOAD *(RDSON, PFET +RINDUCTOR)+VOUT

•

LOAD

Load current

•

DSON, PFET

Drain to source resistance of

PFET switch in the triode region

•

INDUCTOR

Inductor resistance

FLEXIBLE POWER SEQUENCING OF MULTIPLE

POWER SUPPLIES

The LP3906 provides several options for power on sequenc-

ing. The two bucks can be individually controlled with

ENSW1 and ENSW2. The two LDOs can also be individually

controlled with ENLDO1 and ENLDO2.

If the user desires a set power on sequence, all four enables

should be tied LOW so that the regulators don’t automati-

cally enable when power is supplied. The user can then

program the chip through I

2C and raise EN_T from LOW to

HIGH to activate the power on sequencing.

POWER ON

EN_T assertion causes the LP3906 to emerge from Standby

mode to Full Operation mode at a preset timing sequence.

By default, the enables for the LDOs and Bucks are inter-

nally pulled up, which causes the part to turn ON automati-

cally. If the user wishes to have a preset timing sequence to

power on the regulators, the external regulator enables must

be tied LOW. Otherwise, simply tie the enables of each

specific regulator HIGH.

EN_T is edge triggered with rising edge signaling the chip to

power on. The EN_T input is deglitched and the default is set

at 1 ms. As shown in the next 2 diagrams, a rising EN_T

edge will start a power on sequence, while a falling EN_T

edge will start a shutdown sequence. If EN_T is high, tog-

gling the external enables of the regulators will have no

effect on the chip.

Default Power ON Sequence:

1 (ms)

2 (ms)

3 (ms)

4 (ms)

1.5

2.0

Note: LP3906 The default Power on delays can be reprogrammed at final

test or by using I2C registers to 1, 1.5, 2, 3, 6, or 11 ms.

The regulators can also be programmed through I

2C to turn

on and off. By default, the I

2C enables for the regulators are

ON.

The regulators are on following the pattern below:

Regulators on = (I

2C enable) AND (External pin enable OR

EN_T high).

20197809

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