This application note proposes a circuit that accomplishes the power supply sequence without using any dedicated power supply sequence IC, by using general purpose power supply ICs that do not have the Power Good output or an output discharge function required for the sequence control.
Power-supply sequencing is required for microcontrollers, FPGAs, DSPs, ADCs, and other devices that operate from multiple voltage rails. These applications typically require that the core and analog blocks be powered up before the digital I/O rails, although some designs may require other sequences.
Example of a circuit that accomplishes the power supply sequence Figure 2-11 shows an example of a circuit that accomplishes the power supply sequence. The DCDCs in the 3 systems are assumed to be switching regulators (DC/DC converters) or linear regulators (LDO). The DCDCs feature an enable (EN) pin that can control the ON/OFF of the output.
Note that some applications require specific sequences, so always read the power requirements section of each data sheet. A simple way to sequence power supplies is to delay the signal going to a regulator’s enable pin with passive components such as resistors, capacitors, and diodes, as shown in Figure 2.
If this order is not followed, the equipment may not be turned ON normally or devices may be damaged. The order of turning ON/OFF the multiple power supply systems is referred to as a power supply sequence. A power supply sequence IC may be used as a dedicated device to control the power supply sequence.
This article discusses the advantages and disadvantages of using discrete components to sequence the power supplies, and describes a simple, yet effective, method of achieving sequencing by using the internal precision enable pins of the ADP5134, which combines two 1.2-A buck regulators with two 300-mA LDOs.