When you flip the power switch on your device, your power regulator will begin to turn on and will eventually reach equilibrium. A similar cycle occurs when power is switched off, where stored energy (capacitors) discharges and eventually the output voltage and current drop to zero. There will always be some inductance in these circuits that can result in transient overshoot, which can sometimes be so large that the circuit damages or destroys the load it is intended to power.
To solve this problem, the regulator startup and shutdown has to be controlled. There are some simple ways to do this, some of which may be built into your power regulator IC. Additionally, some simple component choices will help reduce transient responses seen at the inputs and outputs of regulator circuits.
On-Off Transition Rates
Without any specialized features like soft-start and sequencing (see below), there are two aspects of power regulator circuits that determine the turn-on and turn-off times in power regulators:
- Filtering at the input and output stages
- Discharge of the output stage in to the feedback loop
Turn-on Time With Filtering
The rate at which a power regulator turns on corresponds to the transient response of the regulator circuit when the input power is switched on. This is determined by the total reactance along the current paths in the regulator circuit. This reactance (and resistance) is modified when a filter is added to the circuit at the input and/or output.
Typically the parasitics in the switching stage and the PCB layout will dominate the transient response, which may induce an underdamped transient response leading to strong overshoot at the load. The example below shows such responses without filtering applied
Transient response example at a power regulator input (blue), output (red), and inductor current (green).
When a filter circuit is added at the input or output, the transient response can be converted to a critical or underdamped response. To learn more about filter design for power converter circuits, read this article.
Turn-off Time in Feedback Loops
The turn-off time is partially determined by the presence of feedback in a power regulator, as well as the capacitance on the output stage. The output capacitance stage, the resistor divider in the feedback loop, and the load collectively form an RC circuit.
These feedback resistors will impact the turn-off time by modifying the discharge rate of the output capacitor stage.
When the input power is cut from the regulator, the capacitors begin discharging their power into the load and the feedback loop. As a result, the regulator attempts to compensate for the drop in the output voltage as long as there is enough residual power to run the regulator’s internal oscillator. This is usually very short, so the turn-off time is dominated by this discharge rate. To slow down the turn-off time, a larger load and larger resistors in the feedback loop will increase the RC time constant and thus increase the time required to turn off the regulator.
Soft Start Circuit
Some power regulators will come supplied with a soft-start pin, which is connected to an internal soft-start circuit. Some regulators do not include a soft-start pin or circuit, but the circuit can be added internally if soft-start functions are required on the regulator. Soft-start pins are normally connected to a shunt capacitor, and the capacitor controls the turn-on rate of the regulator once the regulator is enabled.
Example soft-start circuit added to a power regulator. Some PMICs will have this circuit built into the regulator chip.
The soft-start pin provides a measure of the output voltage scaled down to some lower level (see the output voltage range in your datasheet). Once the soft-start capacitor is fully charged, the pin reaches its maximum voltage and the output voltage of the regulator has reached its final value. This corresponds to steady-state operation.
These ICs are typically used to turn on a set of regulators in a system in a specific sequence by slowly toggling enable pins or directly toggling input power to a power regulator. In some components, this happens automatically and may not require digital programming, although more advanced sequencers may be programmable. With this kind of automated sequencing, the designer or user does not need to manually power on any of the power components in the system.
While these components are not normally used to intentionally slow down the power-on action of a regulator, they are capable of doing this. The turn-on action is similar to soft-start functionality when measured over time (see below).
Example power sequencing diagram that would be to power on an embedded system.
Some sequencers can use the PGOOD outputs or direct output voltage measurement from a set of regulators to time each regulator in the sequence. The PGOOD output or the voltage output are fed back into a monitoring pin, and when these pass a threshold the next regulator in the sequence is toggled ON. This slowly cascades the regulators slowly into their ON states in the correct order. As long as the transitions are smooth (there is enough capacitance), then you should not expect glitches on each output regulator.
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.