34708 datasheet(66/159 Pages) FREESCALE

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Analog Integrated Circuit Device Data



Freescale Semiconductor

MC34708

Functional Block Description

• Short-circuit Protection (SCP) is included on certain LDOs (see the SCP section later in this section). Exceeding the SCP

threshold will disable the regulator and generate a system interrupt. The output voltage will not sag below the specified voltage

with the rated current being drawn. For the lower current LDOs without SCP, they are less accessible to the user environment

and essentially self-limiting.

• The power tree of a given application must be scrubbed for critical use cases to ensure consistency and robustness in the

power strategy.

7.5.6.2

LDO Regulator Control

The regulators with embedded pass devices (VPLL, VGEN1, and VUSB) have an adaptive biasing scheme thus, there are no

distinct operating modes such as a Normal mode and a Low Power mode. Therefore, no specific control is required to put these

regulators in a Low Power mode.

The external pass regulator (VDAC) can also operate in a normal and low power mode. However, since a load current detection

cannot be performed for this regulator, the transition between both modes is not automatic and is controlled by setting the

corresponding mode bits for the operational behavior desired.

The regulators VUSB2, and VGEN2 can be configured for using the internal pass device or external pass device as explained in

Supplies. For both configurations, the transition between both modes is controlled by setting the VxMODE bit for the specific

regulator. Therefore, depending on the configuration selected, the automatic Low Power mode determines availability.

The regulators can be disabled and the general purpose outputs can be forced low when going into Standby (note that the

Standby response timing can be altered with the STBYDLY function, as described in the previous section). Each regulator has

an associated SPI bit for this. When the bit is not set, STANDBY is of no influence. The actual operating mode of the regulators

as a function of STANDBY is not reflected through SPI. In other words, the SPI will read back what is programmed, not the actual

state.

For regulators with internal pass devices, the previous table can be simplified by elimination of the VxMODE column.

Table 54. LDO Regulator Control (external pass device LDOs)

VxEN

VxMODE

VxSTBY

STANDBY(56)

Regulator Vx

Off

Low Power

Off

Low Power

Notes

56.

STANDBY refers to a Standby event as described earlier

Table 55. LDO Regulator Control (internal pass device LDOs)

VxEN

VxSTBY

STANDBY (57)

Regulator Vx

Off

Notes

57.

STANDBY refers to a Standby event as described earlier

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