AN901 Datasheet, PDF (12/25 Pages) STMicroelectronics – Electromagnetic compatibility

AN901
3.1. External Soft-Start and Switching Frequency Calibration
The Si886xx has two additional external pins compared to Si884xx for setting switching frequency and adjusting
soft start time, SH_FC and SS. The capacitor C6 is connected between pin SS and GNDA and sets the soft start
time. The resistor R13 is connected between pin SH_FC and GNDA when the dc-dc is operating. Si886xx supports
switching frequencies from 200 kHz to 900 kHz, and is set by:
A practical C6 value for soft start is:
T_sw  ^R----₁-¹--₀--³--₂--^-₅----_.C-₅----⁶--
Equation 31.
C6 = 470nF
Equation 32.
With C6 = 470 nF, R13 range to set acceptable T_sw is 2.42 kΩ to 10.9 kΩ. For any given T_sw, soft start time may be
increased or decreased by increasing or decreasing C6 while adjusting R13 to maintain the same R13 x C6 time
constant.
The time spent in calibration mode is approximately the time constant created by R13 and C6.
3.2. Soft Start
In soft start mode, the dc-dc peak current limit is gradually increased to limit the sudden demand of current needed
from the primary supply. This mode of operation guarantees that V_OUT monotonically increases and minimizes the
probability of a voltage overshoot. Once 90% of the final V_OUT is reached, soft start mode ends, and Proportional
(P) Mode starts. The total duration of soft start is load dependent as it affects how many switching cycles are
required for V_OUT to reach 90% of final value. In this mode of operation, the voltage feedback loop is inactive, and
hence, loop stability is not a concern.
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