LP3907 Datasheet, PDF (20/44 Pages) National Semiconductor (TI) – Dual High-Current Step-Down DC/DC and Dual Linear Regulator with I2C Compatible Interface

FLEXIBLE POWER SEQUENCING OF MULTIPLE POWER
SUPPLIES
The LP3907 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 con-
trolled with ENLDO1 and ENLDO2.
If the user desires a set power on sequence, he can program
the chip through I²C and raise EN_T from LOW to HIGH to
activate the power on sequencing.
POWER UP SEQUENCING USING THE EN_T FUNCTION
EN_T assertion causes the LP3907 to emerge from Standby
mode to Full Operation mode at a preset timing sequence. By
default, the enables for the LDOs and Bucks (ENLDO1, ENL-
DO2, EN_T, ENSW1, ENSW2) are 500K internally pulled
down, which causes the part to stay OFF until enabled. If the
user wishes to use the preset timing sequence to power on
the regulators, transition the EN_T pin from Low to High. Oth-
erwise, simply tie the enables of each specific regulator HIGH
to turn on automatically.
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 1ms. 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, toggling the ex-
ternal enables of the regulators will have no effect on the chip.
The regulators can also be programmed through I²C to turn
on and off. By default, I²C enables for the regulators on ON.
The regulators are on following the pattern below:
Regulators on = (I²C enable) AND (External pin enable OR
EN_T high).
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