C8051F960-B-GM Datasheet, PDF (270/492 Pages) Silicon Laboratories – Ultra Low Power 128K, LCD MCU Family

C8051F96x
20.1. Startup Behavior
The dc-dc converter is enabled by setting bit DC0EN (DC0MD.0) to logic 1. When first enabled, the M1
switch turns on and continues to supply current into the output capacitor through the inductor until the VDC
output voltage reaches the programmed level set by by the VSEL bits (DC0CF.[6:3]).
The peak transient current in the inductor is limited for safe operation. The peak inductor current is pro-
grammable using the ILIMIT bits (DC0MD.[6:4]). The peak inductor current, size of the output capacitor
and the amount of dc load current present during startup will determine the length of time it takes to charge
the output capacitor. The RDYH and RDYL bits (DC0RDY.7 and DC0DRY.6) may be used to determine
when the output voltage is within approximately 100 mV of the programmed voltage.
In order to ensure reliable startup of the dc-dc converter, the following restrictions have been imposed:
• The maximum dc load current allowed during startup is given in Table 4.20 on page 77. If the dc-dc
converter is powering external sensors or devices through the VDC pin, then the current supplied to
these sensors or devices is counted towards this limit. The in-rush current into capacitors does not
count towards this limit.
• The maximum total output capacitance is given in Table 4.20 on page 77. This value includes the
required 2.2 µF ceramic output capacitor and any additional capacitance connected to the VDC pin.

The peak inductor current limit is programmable by software as shown in Table 20.1. Limiting the peak
inductor current can allow the dc-dc converter to start up using a high impedance power source (such as
when a battery is near its end of life) or allow inductors with a low current rating to be utilized. By default,
the peak inductor current is set to 500 mA.
.
Table 20.1. IPeak Inductor Current Limit Settings
ILIMIT
001
010
011
100
101
Peak Current (mA)
200
300
400
500
600
The peak inductor current is dependent on several factors including the dc load current and can be esti-
mated using following equation:
efficiency = 0.80
inductance = 0.68 µH
frequency = 2.4 MHz
I_PK
=
----------2----^-----^I--^L---^O---^A----^D-----^----^---^V----^D-----^C------–----^V----^B-----^A----^T----^D-----^C-----^---------
efficiency  inductance  frequency
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