
Analog Integrated Circuit Device Data
Freescale Semiconductor
59
PC34708
Functional Block Requirements and Behaviors
Buck Switchers Block Description and Application Information
The switcher can operate in different modes depending on the load conditions. These modes can be set through 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, switcher outputs will apply controlled stepping to the programmed value. The soft start feature limits the
inrush current at startup. During soft start, the switchers will be forced to PWM mode for 3.0 ms and then default to the APSKIP
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.
7.3.2.1
Switching Frequency
A PLL generates the switcher system clocking from the 32.768 kHz crystal oscillator reference. The switching frequency can be
programmed to 2.0 MHz or 4.0 MHz by setting the PLLX SPI bit as shown in
Table 57.
The clocking system provides a near instantaneous activation when the switchers are enabled or when exiting PFM operation
for PWM mode. The PLL can be configured for continuous operation with PLLEN = 1.
7.3.2.2
SW1
SW1 is fully integrated synchronous Buck PWM voltage mode control DC/DC regulator. It can be operated in single phase/dual
phase mode. The operating mode of the switcher is configured by the SW1CFG pin. The SW1CFG pin is sampled at startup.
Table 56. 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.
APSKIP
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 57. Buck Switcher Frequency
PLLX
Switching Frequency (Hz)
0
2 000 000
1
4 000 000
Table 58. SW1 Configuration
SW1CFG
SW1A/B Configuration Mode
VCOREDIG
Single Phase Mode
Ground
Dual Phase Mode