Easily implement complex power supply timing control

Introduction

Power sequencing is a necessary function for microcontrollers, FPGAs, DSPs, ADCs, and other devices that require multiple voltage rails. These applications typically require that the core and analog blocks be powered 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 sequencing can stagger inrush currents during power-up, a technique that is useful for applications that are 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 within the ADP5134, which contains two 1.2-A buck regulators and two 300-mA LDOs. The article also lists a range of ICs that can be used in applications that require more accurate and flexible sequencing.

Figure 1 shows an application that requires multiple power rails. These rails are the core supply (VCCINT), I/O supply (VCCO), auxiliary supply (VCCAUX), and system memory 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 sequencing requirements, but to achieve minimum inrush current levels and comply with sequencing requirements for circuits connected to the FPGA, the power rails should be powered up in the following sequence: VCC_INT → VCC_AUX → VCCO. Note: Some applications require a specific sequence, so be sure to read the power requirements section of the data sheet.

Simplifying Power Sequencing with Passive Delay Networks

A simple way to implement power sequencing is to delay the signal going to the regulator enable pin using passive components such as resistors, capacitors, and diodes, as shown in Figure 2. When the switch is closed, D1 conducts and 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 sequencing, and in some applications where only a delay of the signal is sufficient and may only require external R and C. The disadvantage of 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 can vary by an additional ±10% due to dc bias effects, making sequencing imprecise and sometimes unreliable.

Precision Enables Easy Sequencing

To achieve stable threshold levels for precise sequencing, most regulators require an external reference. The ADP5134 solves this problem by integrating a precision reference, saving significant 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 turns 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 by only ±3% over input voltage and temperature. This small range ensures precise timing control, solving 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)

Using Resistor Dividers to Simplify Power Supply Sequencing

Multichannel power supplies can be sequenced by connecting an attenuated version of the regulator output to the enable pin of the next regulator to be powered up, 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, a voltage monitoring and sequencer IC can be used, such as the ADM1184 quad voltage monitor, which has ±0.8% accuracy over voltage and temperature. Alternatively, for applications that require more precise power-up and shutdown sequence control, the ADM1186 quad voltage sequencer and monitor with programmable timing control can be used. The ADP5034 quad regulator

integrates two 3-MHz, 1200-mA buck regulators and two 300 mA LDOs. A typical timing control 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 precise enable function.

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

Conclusion Sequencing

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