Si1000/1/2/3/4/5
16.1. Startup Behavior
On initial power-on, the dc-dc converter outputs a constant 50% duty cycle until there is sufficient voltage
on the output capacitor to maintain regulation. The size of the output capacitor and the amount of load cur-
rent present during startup will determine the length of time it takes to charge the output capacitor.
During initial power-on reset, the maximum peak inductor current threshold, which triggers the overcurrent
protection circuit, is set to approximately 125 mA. This generates a “soft-start” to limit the output voltage
slew rate and prevent excessive in-rush current at the output capacitor. 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 4.15 on page 65. If the dc-dc
converter is powering external sensors or devices through the VDD_MCU/DC+ pin or through GPIO
pins, 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 4.15 on page 65. This value includes the
required 1 µF ceramic output capacitor and any additional capacitance connected to the
VDD_MCU/DC+ pin.
Once initial power-on is complete, the peak inductor current limit can be increased by software as shown in
Table 16.1. Limiting the peak inductor current can allow the device to start up near the battery’s end of life.
.
Table 16.1. IPeak Inductor Current Limit Settings
SWSEL
ILIMIT
Peak Current (mA)
1
0
1
0
0
0
1
1
100
125
250
500
The peak inductor current is dependent on several factors including the dc load current and can be esti-
mated using following equation:
2 ILOADVDD/DC+ – VBAT
efficiency inductance frequency
IPK
=
-------------------------------------------------------------------------------------------
efficiency = 0.80
inductance = 0.68 µH
frequency = 2.4 MHz
166
Rev. 1.0
SILICON [ SILICON ]
