Easily implement complex power supply timing control

This application note explores the pros and cons of using discrete components to sequence power supplies and describes a simple but effective method for sequencing using the internal precision enable pins of the ADP5134, which combines two 1.2A buck regulators with two 300mA low dropout (LDO) regulators. This application note also describes some sequencer ICs that may be more helpful in applications that require more precise and flexible sequencing.

The application shown in Figure 1 requires the use of multiple power rails. These rails are the core power supply (V _CCINT ), I/O power supply (V _CCO ), auxiliary power supply (V _CCAUX ), and system memory power supply.

For example, Xilinx ^® Spartan-3A FPGAs integrate a power-on reset circuit that ensures that all power supplies reach their thresholds before allowing the device to be configured. The power-on reset circuit relaxes the stringent requirements for power supply sequencing; however, to minimize inrush current levels while taking into account the sequencing requirements of the circuits connected to the FPGA, the power rails must be powered up in the following sequence: V _CCINT first , then V _CCAUX , and finally V _CCO . Note that some applications require a specific sequence to be followed; therefore, always refer to the power supply requirements section of the relevant data sheet.

A simple way to sequence power supplies is to delay the signal going to the regulator’s enable pin with passive components such as resistors, capacitors, and diodes, as shown in Figure 2. When the switch is closed, D1 turns on and D2 turns off. C1 charges, and the voltage at EN2 rises at a rate determined by R1 and C1. When the switch is open, C1 discharges to ground through R2, D2, and R _{PULL . The voltage at EN2 falls at a rate determined by R2, R PULL} , and C1. Changing the values ​​of R1 and R2 changes the charge and discharge times, thus setting the regulator’s turn-on and turn-off times.

This approach can be used in applications where precise timing control is not required. Applications that only need to delay the signal may only require external resistors and capacitors. The disadvantage of using this approach with a standard regulator is that the logic threshold of the enable pin can vary greatly with voltage and temperature. In addition, the delay in the voltage ramp depends on the value and tolerance of the resistors and capacitors. Typical X5R capacitors vary by about ±15% over the temperature range of –55°C to +85°C, and the dc bias effect can add an additional ±10%, which can make the timing inaccurate and sometimes unreliable.

To achieve the stable threshold levels required for precision timing control, most regulators require an external reference voltage source. The ADP5134 overcomes this problem by integrating a precision reference voltage source, while also significantly saving cost and printed circuit board (PCB) area. Each regulator has a separate enable input pin.

When the voltage at the enable input pin rises above the ENx pin rising threshold (VIH_EN _[ 0.9V min]), the device exits shutdown mode and the management block turns on, but the regulator is not activated. The device compares the voltage at the enable input pin to a precision internal reference voltage (0.97V typical). When the voltage at the enable pin rises above the precision enable threshold, the regulator is activated and the output voltage begins to rise. The reference voltage varies only ±3% over input voltage and temperature corners. This small variation ensures accurate sequencing and eliminates issues that exist when using discrete components.

When the voltage at the enable input pin drops 80 mV (typical) below the reference voltage, the regulator is disabled. When the voltage at all enable input pins drops below the ENx falling threshold (V _{IL_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 the full temperature range.

By connecting the attenuated output of one regulator 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 sequence: from BUCK1 to BUCK2, to LDO1, and finally to LDO2. Figure 6 shows the power-up sequence after connecting EN1 to VIN1. Figure 7 shows the shutdown sequence after disconnecting EN1 from VIN1.

In some cases, achieving precise timing is more important than reducing PCB area and saving cost. For such applications, you can use a voltage monitoring and sequencer IC, such as the ADM1184 four-channel voltage monitor, which has an accuracy of ±0.8% over voltage and temperature. Another option is the ADM1186 four-channel voltage sequencer and monitor with programmable timing; this device can be used in applications that require more precise control of power-up and shutdown sequences.

For example, the ADP5034 4-channel regulator integrates two 3MHz, 1200mA buck regulators and two 300mA LDOs. A typical sequencing function can be implemented 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. As shown in Figure 8, this method can be used for regulators that do not provide a precision enable function.

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