How to Simplify and Implement Complex Power Supply Sequencing

　　IntroductionPower

       supply sequencing is a necessary function for microcontrollers, FPGAs, DSPs, ADCs, and other devices that require multiple voltage rails. These applications typically require the core and analog blocks to be powered up before the digital I/O rails, but some designs may require other sequences. Regardless, proper power-up and power-down sequencing can prevent both immediate damage from latch-up and long-term damage from ESD. In addition, power supply sequencing can stagger inrush currents during power-up, a technique that is useful for applications powered from current-limited supplies.

　　This article discusses the pros and cons of using discrete components for power sequencing and introduces a simple and effective method for sequencing using the precision enable pins of the ADP5134, which has two 1.2-A buck regulators and two 300-mA LDOs. It also lists a range of ICs that can be used in applications that require more precise and flexible sequencing.

　　An application that requires multiple power rails is shown in Figure 1. These rails are the core power supply (VCCINT), I/O power supply (VCCO), auxiliary power supply (VCCAUX), and system memory power supply.

　　Figure 1. Typical power supply method for processors and FPGAs

　　For example, the Xilinx Spartan-3A FPGA has a built-in power-on reset circuit that ensures that all supplies reach their thresholds before allowing the device to be configured. This helps to reduce power supply sequencing requirements, but to achieve minimum inrush current levels and comply with the sequencing requirements of the circuits connected to the FPGA, the power rails should be powered up in the following sequence: VCC_INT → VCC_AUX → VCCO. Please note: Some applications require a specific sequence, so be sure to read the power supply requirements section of the data sheet.

　　Simplify Power Supply Sequencing Using Passive Delay Networks

　　A simple way to sequence power supplies is to use passive components such as resistors, capacitors, and diodes to delay the signal going to the regulator’s enable pin, as shown in Figure 2. When the switch is closed, D1 conducts while D2 remains open. Capacitor C1 charges and the voltage at EN2 rises at a rate determined by R1 and C1. When the switch is open, capacitor C1 discharges to ground through R2, D2, and RPULL. The voltage at EN2 decreases at a rate determined by R2, RPULL, and C2. Changing the values ​​of R1 and R2 changes the charge and discharge times, thus setting the regulator’s turn-on and turn-off times.

　　Figure 2. A simple method for power supply sequencing using resistors, capacitors, and diodes.

　　This method can be used in applications that do not require precise timing control, and some applications that only need to delay the signal and may only require external R and C. The disadvantage of using this method for standard regulators is that the logic threshold of the enable pin can vary greatly due to voltage and temperature. In addition, the delay in the voltage ramp depends on the resistor and capacitor values ​​and tolerances. Typical X5R capacitors vary by about ±15% over the temperature range of –55°C to +85°C, and another ±10% due to dc bias effects, making the timing control imprecise and sometimes unreliable.

　　Precision enables easy timing control

　　To achieve stable threshold levels for precise timing control, most regulators require an external reference voltage source. The ADP5134 solves this problem by integrating a precision reference voltage source, significantly saving cost and PCB area. Each regulator has an independent enable pin. When the voltage at the enable input rises above VIH_EN (minimum 0.9 V), the device exits shutdown mode and the management module is turned on, but the regulator is not activated. The voltage at the enable input is compared to a precision internal reference voltage (typically 0.97 V). Once the voltage at the enable pin rises above the precision enable threshold, the regulator is activated and the output voltage begins to increase. The reference voltage varies only ±3% at the input voltage and temperature transitions. This small range of variation ensures precise timing control and solves various problems encountered when using discrete components.

　　When the voltage at the enable input drops 80 mV (typical) below the reference voltage, the regulator is disabled. When the voltage on all enable inputs drops below VIL_EN (0.35 V maximum), the device enters shutdown mode. In this mode, the current consumption drops to less than 1 μA. Figure 3 and Figure 4 show the accuracy of the ADP5134 precision enable threshold for Buck1 over temperature.

　　Figure 3. Precision enable turn-on threshold over temperature (10 samples)

　　Figure 4. Precision enable shutdown threshold over temperature (10 samples) 　　Simplifying power supply sequencing using resistor dividers

　　By connecting an attenuated version of the regulator output to the enable pin of the next regulator to be powered up, a multichannel power supply can be sequenced as shown in Figure 5, where the regulators are turned on or off in the following order: Buck1 → Buck2 → LDO1 → LDO2. Figure 6 shows the power-up sequence when EN1 is connected to VIN1. Figure 7 shows the shutdown sequence when EN1 is disconnected from VIN1.

　　Figure 5. Simple timing control using the ADP5134.

　　Figure 6. ADP5134 startup sequence.

　　Figure 7. ADP5134 shutdown sequence.

　　Sequencer ICs Improve Timing Accuracy

　　In some cases, achieving precise timing is more important than reducing PCB area and cost. For these applications, you can use a voltage monitoring and sequencer IC, such as the ADM1184 quad-channel voltage supervisor, which has an accuracy of ±0.8% over voltage and temperature. Alternatively, for applications that require more precise power-up and power-down sequence control, you can use the ADM1186 quad-channel voltage sequencer and supervisor with programmable timing control.

　　The ADP5034 quad regulator integrates two 3-MHz, 1200-mA step-down regulators and two 300 mA LDOs. A typical sequencing function can be implemented by using the ADM1184 to monitor the output voltage of one regulator and provide a logic high signal to the enable pin of the next regulator when the monitored output voltage reaches a certain level. This method (shown in Figure 8) can be used for regulators that do not have a precision enable function.

　　Figure 8. Sequencing the ADP5034 quad regulator using the ADM1184 quad voltage monitor.

　　in conclusion

　　Sequencing is simple and easy using the ADP5134 precision enable input, requiring only two external resistors per channel. More precise sequencing can be achieved with the ADM1184 or ADM1186 voltage supervisors.