C8051F93X Datasheet, PDF (179/330 Pages) Silicon Laboratories – Pipelined intstruction architecture executes 70 of instruction in 1 or 2 system clocks

C8051F93x-C8051F92x
16.5. Minimizing Power Supply Noise
To minimize noise on the power supply lines, the GND and GND/DC- pins should be kept separate, as
shown in Figure 16.2; one or the other should be connected to the pc board ground plane. For applications
in which the dc-dc converter is used only to power internal circuits, the GND pin is normally connected to
the board ground.
The large decoupling capacitors in the input and output circuits ensure that each supply is relatively quiet
with respect to its own ground. However, connecting a circuit element "diagonally" (e.g. connecting an
external chip between VDD/DC+ and GND, or between VBAT and GND/DC-) can result in high supply
noise across that circuit element. For applications in which the dc-dc converter is used to power external
analog circuitry, it is recommended to connect the GND/DC– pin to the board ground and connect the bat-
tery’s negative terminal to the GND pin only, which is not connected to board ground.
To accommodate situations in which ADC0 is sampling a signal that is referenced to one of the external
grounds, we recommend using the Analog Ground Reference (P0.1/AGND) option described in Section
5.12. This option prevents any voltage differences between the internal chip ground and the external
grounds from modulating the ADC input signal. If this option is enabled, the P0.1 pin should be tied to the
ground reference of the external analog input signal. When using the ADC with the dc-dc converter, we
also recommend enabling the SYNC bit in the DC0CN register to minimize interference.
These general guidelines provide the best performance in most applications, though some situations may
benefit from experimentation to eliminate any residual noise issues. Examples might include tying the
grounds together, using additional low-inductance decoupling caps in parallel with the recommended ones,
investigating the effects of different dc-dc converter settings, etc.
16.6. Selecting the Optimum Switch Size
The dc-dc converter has two built-in switches (the diode bypass switch and duty cycle control switch). To
maximize efficiency, one of two switch sizes may be selected. The large switches are ideal for carrying
high currents and the small switches are ideal for low current applications. The ideal switchover point to
switch from the small switches to the large switches varies with the programmed output voltage. At an out-
put voltage of 2 V, the ideal switchover point is at approximately 4 mA total output current. At an output
voltage of 3 V, the ideal switchover point is at approximately 8 mA total output current.
16.7. DC-DC Converter Clocking Options
The dc-dc converter may be clocked from its internal oscillator, or from any system clock source, select-
able by the CLKSEL bit (DC0CF.0). The dc-dc converter internal oscillator frequency is approximately
2.4 MHz. For a more accurate clock source, the system clock, or a divided version of the system clock may
be used as the dc-dc clock source. The dc-dc converter has a built in clock divider (configured using
DC0CF[6:5]) which allows any system clock frequency over 1.6 MHz to generate a valid clock in the range
of 1.6 to 3.2 MHz.
When the precision internal oscillator is selected as the system clock source, the OSCICL register may be
used to fine tune the oscillator frequency and the dc-dc converter clock. The oscillator frequency should
only be decreased since it is factory calibrated at its maximum frequency. The minimum frequency which
can be reached by the oscillator after taking into account process variations is approximately 16 MHz. The
system clock routed to the dc-dc converter clock divider also may be inverted by setting the CLKINV bit
(DC0CF.3) to logic 1. These options can be used to minimize interference in noise sensitive applications.
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