LP3906 Datasheet, PDF (18/38 Pages) National Semiconductor (TI) – Dual High-Current Step-Down DC/DC and Dual Linear Regulator with I2C Compatible Interface

SW1, SW2: Synchronous Step
Down Magnetic DC/DC Converters
(Continued)
V_{IN, MIN} = I_LOAD * (R_{DSON, PFET} + R_INDUCTOR) + V_OUT
• I_LOAD
• R_{DSON, PFET}
Load current
Drain to source resistance of
PFET switch in the triode region
• R_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 I²C 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.
t₁ (ms)
1.5
Default Power ON Sequence:
t₂ (ms)
2.0
t₃ (ms)
3
t₄ (ms)
6
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 I²C to turn
on and off. By default, the I²C enables for the regulators are
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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