MC34708 datasheet(44/159 Pages) FREESCALE

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Analog Integrated Circuit Device Data



Freescale Semiconductor

MC34708

Functional Block Description

7.5.4

Buck Switching Regulators

Six buck switching regulators are provided with integrated power switches and synchronous rectification. In a typical application,

SW1 and SW2 are used for supplying the application processor core power domains. Split power domains allow independent

DVS control for processor power optimization, or to support technologies with a mix of device types with different voltage ratings.

SW3 is used for powering internal processor memory as well as low voltage peripheral devices and interfaces which can run at

the same voltage level. SW4A/B is used for powering external DDR memory as well as low voltage peripheral devices and

interfaces, which can run at the same voltage level. SW5 is used to supply the I/O domain for the system.

The buck regulators are supplied from the system supply BP, which is drawn from the main battery or the battery charger (when

present).

The switching regulators can operate in different modes depending on the load conditions. These modes can be set through the

SPI and include a PFM mode, PWM Pulse Skip, an Automatic Pulse Skipping mode, and a PWM mode. The previous selection

is optimized to maximum battery life based on load conditions.

Buck modes of operation are programmable for explicitly defined or load-dependent control.

When initially activated, regulators outputs will apply controlled stepping to the programmed value. The soft start feature limits

the inrush current at startup. During soft start, the regulator will be forced to PWM mode for 3.0 ms and then default to the APS

mode A built in current limiter ensures that during normal operation the maximum current through the coil is not exceeded.

Point of Load feedback is intended for minimizing errors due to board level IR drops.

7.5.4.1

General Control

Operational modes of the Buck regulators can be controlled by direct SPI programming, altered by the state of the STANDBY

pin, by direct state machine influence (i.e., entering Off or low power Off states, for example), or by load current magnitude when

so configured (Auto Pulse skip mode). Available modes include PWM with No Pulse Skipping (PWM), PWM with Pulse Skipping

(PWMPS), Pulse Frequency Mode (PFM), Automatic Pulse Skip (APS), and Off. The transition between the two modes PWMPS

and PWM can occur automatically, based on the load current (auto pulse skip mode). For light loading, the regulators should be

put into PFM mode to optimize efficiency.

SW1A/B, SW2, SW3, SW4A/B, and SW5, can be configured for mode switching with STANDBY or autonomously, based on load

current Auto pulse skip mode. Additionally, provisions are made for maintaining PFM operation in User off and Memhold modes,

to support state retention for faster startup from the Low Power Off modes for Warm Start or Warm Boot. SWxMODE[3:0] bits

will be reset to their default values defined by PUMSx settings by the startup sequencer.

Table 32 summarizes the Buck regulators programmability for Normal and Standby modes.

Table 31. Buck Operating Modes

Mode

Description

OFF

The regulator is switched off and the output voltage is discharged

PFM

The regulator is switched on and set to PFM mode operation. In this mode, the regulator

is always running in PFM mode. Useful at light loads for optimized efficiency.

PWMPS

The regulator is alternating between pulse skipping and PWM modes, depending on the

load conditions.

APS

The regulator is switched on and set to Automatic Pulse Skipping. In this mode the

regulator moves automatically between pulse skipping and full PWM mode depending

on load conditions.

PWM

The regulator is switched on and set to PWM mode. In this mode the regulator is always

in full PWM mode operation regardless of load conditions.

Table 32. Switching regulator Mode Control for Normal and Standby Operation

SWxMODE[3:0]

Normal Mode

Standby Mode

0000

Off

0001

PWM

Off

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