LP3907 datasheet(25/57 Pages) TI1

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EN_T

Vout Buck1

Vout Buck2

Vout LDO1

Vout LDO2

Ext_Enable

Pins

Regulator ON

Start Programmed

Timing Sequence

EN_T

LP3907

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SNVS511N – JUNE 2007 – REVISED AUGUST 2011

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, ENLDO2, 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. Otherwise, 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 external 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).

Note: The EN_T power-up sequencing may also be employed immediately after VIN is applied to the device.

However, VIN must be stable for approximately 8ms minimum before EN_T be asserted high to ensure internal

bias, reference, and the Flexible POR timing are stabilized. This initial EN_T delay is necessary only upon first

time device power on for power sequencing function to operate properly.

LP3907 Default Power-Up Sequence

Table 4. Power-On Timing Specification

Symbol

Description

Min

Typ

Max

Units

Programmable Delay from EN_T assertion to VCC_Buck1 On

1.5

Programmable Delay from EN_T assertion to VCC_Buck2 On

Programmable Delay from EN_T assertion to VCC_LDO1 On

Programmable Delay from EN_T assertion to VCC_LDO2 On

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