Ⅰ. What is Soft Start?
In a DC-DC Buck converter, if the converter is fully turned on immediately upon power-up, the output capacitor will be charged instantaneously as if it were short-circuited. This leads to excessive input inrush current, which may damage the MOSFET/diode or cause voltage drop in the upstream power supply.
Ⅱ. Why Adjust the Soft Start Time?
- If the time is too short: The current surge will be large, which may trigger overcurrent protection.
- If the time is too long: The power-up speed will be slow, which may cause system power-up delay and even affect other power rails that start synchronously.
The soft start function is generally integrated into the switching regulator IC. We will take the Microchip MIC24066 switching Buck regulator as an example.
The core function of soft start is to control the output voltage to rise slowly, thereby reducing the inrush current during circuit startup. This prevents damage or impact to the power supply, regulator IC, and downstream loads due to excessive current at startup. At the same time, it ensures the output voltage smoothly enters the normal operating state (without overshoot or sudden voltage rise).
The MIC24066 is specifically equipped with a programmable soft start pin (SS pin). Users can adjust the soft start time through external components to adapt to the startup requirements of different loads (e.g., inductive loads and large-capacitance loads require longer soft start time to suppress surges).
Ⅲ. How to Set the Soft Start Time of the MIC24066
The soft start time of the MIC24066 is achieved by connecting a capacitor (denoted as Css) between the SS pin and AGND (analog ground). The capacitance directly determines the length of the soft start time. The specific setting steps and calculation method are as follows:
1. Hardware Connection Principle
The MIC24066 integrates a fixed internal soft start current source (Iss). When the IC starts up, the internal current source charges the external capacitor Css connected to the SS pin, and the capacitor voltage rises gradually. This in turn controls the output voltage Vout to rise slowly at the same rate — the larger the capacitance, the longer the charging time, and thus the longer the soft start time.
(Image source: Microchip)
Core connection requirement: Only a ceramic capacitor that meets the calculated capacitance value needs to be soldered between the SS pin of the MIC24066 and AGND; no additional resistors or other components are required.
2. Soft Start Time Calculation Formula
The formula for the relationship between soft start time and capacitance is:
C_{ss} = \dfrac{I_{SS} \times t_{SS}}{V_{REF}}
Where:
- Css = Capacitance value connected between the SS pin and AGND
- Iss = Internal soft start charging current provided by the IC (typical value = 1.3 µA)
- tss = Target soft start time
- VREF = Internal reference voltage (typical value = 0.6 V)
If the target soft start time tss = 5 ms, substitute into the formula to calculate Css:
C_{ss} = \dfrac{1.3µA \times 5ms}{0.6 V} = \dfrac{1.3\times 10^{-6} \times 5\times 10^{-3}}{0.6 } \approx 10.8nF
Related Products
- Switching Regulator: MIC24066T-E/QNA
- Switching Regulator Development Board: EV69G45A (non-isolated Buck output)
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