How Soft-Start Circuits Work

Some of the simplest techniques for controlling the power-on rate of a power supply involve the use of passive components. However, building filters, snubbers, and feedback loops that can provide exactly the power-up characteristics you want can be difficult. Another tool that some designs do not always consider is soft-start, either because it is integrated onto the die or it requires an external circuit that is not intuitive.

To add soft-start functionality to a power regulator is simple, it just involves a few components that determine the startup transition time. If your DC/DC converter controller does not contain one of these circuits integrated into the die, then you can easily add one with the guidelines below.

Soft-Start Circuits in Power Regulation

At a high level, soft-start circuits control the startup sequence for a switching regulator or an LDO regulator. When the regulator is first powered on, the regulator’s voltage output slowly increases as the input current is drawn from the power source. The consequence of this controlled turn-on functionality is that inrush current into the regulator is limited, and there is much less chance that the load experiences damage during the power-up process.

Soft-start circuits typically appear in the control loop for an LDO or switching regulator, as shown in the example below.

Example soft-start circuit added to a power regulator. Some PMICs will have this circuit built into the regulator chip.

The soft-start circuit is essentially an RC circuit with a diode (the R portion is the resistive feedback loop). The ramp-up time for the output voltage is proportional to the RC time constant of the circuit, so the turn-on time can be easily controlled with just two parameters in the feedback loop. In PMICs with an integrated soft-start circuit, a designer can use an external capacitor to adjust the startup time for the regulator.

As long as the R term is large enough and there is negligible inductance in the feedback loop path, the transient response will always be underdamped and the turn-on rate will be slow enough to prevent overshoot at the load.

How Soft-Start Circuits Work

When power is first applied, the control loop will hold the output voltage low. As time passes, the control loop will gradually allow the output voltage to increase. This is often achieved by slowly charging a capacitor, which controls the feedback in the control loop. As the capacitor charges, the output voltage is allowed to increase. There are four stages in the ramp-up process:

• Initialization: When power is first applied to the regulator, the soft-start capacitor is initially at a low voltage. The soft-start capacitor begins charging once the regulator is powered on via an internal current source connected to this capacitor. The voltage across the capacitor starts to rise gradually.
• Output voltage ramp-up: The voltage at the soft start pin is used to control the ramp-up of the output voltage. As the voltage at the soft start pin increases, the output voltage of the regulator also increases. This gradual increase in output voltage helps to prevent inrush current and the associated problems.
• Reaching equilibrium: Once the capacitor is fully charged, the soft-start capacitor reaches its maximum voltage. At this point, the output voltage of the regulator has reached its final value, and the regulator is in its steady-state operation.
• Fault conditions: If a fault condition occurs (such as an overload or overheating), the regulator may shut down. In this case, the capacitor in the soft-start circuit discharges and shuts down the output voltage. When the fault condition clears, the start-up sequence begins again until the capacitor is fully charged.

Soft-Start vs. Power-Good

If you have a soft-start (SS) or power-good (PGOOD or PG) pin on a power regulator, these pins will output a voltage that can be measured with another IC. It’s common to use regulators with PG outputs in elaborate power monitoring and management strategies. Because the output from a soft-start pin is effectively a measure of the output voltage, how does this compare with a PG pin on a power regulator? Both provide an indication of the power output from a power regulator, but in different ways:

• SS Pin: As the output voltage rises gradually from zero to its final value, the capacitor on the SS pin begins charging as it receives current from an internal source. Once equilibrium is reached the SS capacitor will also stop charging.
• PG Pin: When the output voltage has reached its intended stable value, the regulator asserts the PG signal to its logical high value. If the output voltage falls out of the acceptable range, then PG is de-asserted.

In short, PG pins output what is effectively a single bit, which will indicate whether the regulator has reached its steady ON or OFF state. SS outputs provide a gauge of the progress of the power-on transition, but it should not be used as a proxy for a direct measurement of the output voltage. If this measurement is needed, measure from the feedback resistor divider to a high-impedance input.

No matter how you want to control power sequencing and on-off timing for your devices, you can create and evaluate your circuits with the complete set of simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.

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