Accurate Power Supply Sequencing Prevents System Damage Design Note 384 Jeff Heath and Akin Kestelli Introduction Many complex systems—such as telecom equipment, memory modules, optical systems, networking equipment, servers and base stations—use FPGAs and other digital ICs that require multiple voltage rails that must start up and shut down in a specific order, otherwise the ICs can be damaged. The LTC®2924 is a simple and compact solution to power supply sequencing in a 16pin SSOP package (see Figures 1 and 2). How it Works Four power supplies can be easily sequenced using a single LTC2924 and multiple LTC2924s can be just as easily cascaded to sequence any number of power supplies. With slightly reduced functionality, six power supplies can be sequenced with a single LTC2924. The LTC2924 controls the start-up and shutdown sequence and ramp rates of four power supply channels via output pins (OUT1 to OUT4). Each OUT pin uses a 10µA current source connected to an internal charge pump and a low resistance switch to GND. This combination makes the outputs flexible enough to connect them directly to many power supply shutdown pins or to external N-channel MOSFET switches. The LTC2924 monitors the output voltage of each sequenced power supply via four input pins (IN1 to IN4). These inputs use precision comparators and a trimmed bandgap voltage reference to provide better than 1% accuracy. The power ON and power OFF voltage thresholds are set using resistive dividers for each of the four channels. The power ON threshold and the power OFF threshold are individually selectable on a channel by channel basis (see “Selecting the Hysteresis Current and IN Pin Feedback Resistors” in the data sheet for details). The LTC2924 timer pin (TMR) is used to provide an optional delay between the completion of start-up of one supply and the start-up of the next power supply. The delay time is selected by placing a capacitor between the TMR pin and ground (delay = 200ms/μF), whereas floating the TMR pin removes any delay. The start-up delay can be different than the shutdown delay. Figure 3 shows a simple circuit where the shutdown delay is half the start-up delay. , LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. VON = 2.64V VOFF = 1.98V 3.3V SHDN Q2 1V VON = 0.93V VOFF = 0.915V 0.1μF 5V TO 5V SHDN 5V 0.1μF VON = 3.97V VOFF = 2.97V 0.1μF Q1 3V VON = 2.79V VOFF = 2.73V 0.1μF VCC 5V EARLY 10k 10k OUT1 IN1 OUT2 IN2 OUT3 IN3 OUT4 IN4 ON LTC2924 SYSTEM CONTROLLER 6.04k 100k 1.62k 66.5k 1.69k 18.2k 3.09k 20k DONE FAULT TMR PGT 150nF 150nF HYS/CFG GND 49.9k Q1-Q2: IRL3714S ALL RESISTORS 1% DN384 F01 Figure 1. Typical Application with External N-Channel MOSFETs 03/06/384_conv POWER SUPPLY 1 POWER SUPPLY 2 POWER SUPPLY 3 5V EARLY* VON 3.01V VOFF 2.68V 3.3V SHDN VON 4.49V VOFF 3.99V 5V SHDN VON 1.43V VOFF 1.27V 1.6V SHDN VON 2.25V VOFF 2V 2.5V SHDN POWER SUPPLY 4 0.1μF 24.9k 15.8k 49.9k 33.2k 9.31k 11.81k 7.87k 8.45k VCC OUT1 10k 10k OUT2 LTC2924 IN1 IN2 OUT3 IN3 OUT4 IN4 ON SYSTEM CONTROLLER DONE FAULT TMR *5V EARLY MUST BE ON BEFORE SEQUENCING SUPPLIES 150nF PGT HYS/CFG GND 49.9k 150nF DN384 F02 Figure 2. Four Power Supply Sequencer Using Shutdown Pins The LTC2924 also includes a power good timer (PGT). The LTC2924 starts the PGT as each individual power supply is enabled. If any power supply fails to reach its nominal specified voltage within the allotted time interval, a power ON fault is detected. no software and designs can be fine tuned during system integration simply by changing resistor and capacitor values. Ease of design, low component cost, and a small footprint make the LTC2924 an excellent choice for power supply sequencing and monitoring. VCC Conclusion The LTC2924 fits into a wide variety of power supply sequencing and monitoring applications. With very few external components and a 16-pin narrow SSOP, an LTC2924-based sequencing solution requires very little board space. The power supply enable pins require no configuration by the designer, yet are versatile enough to directly drive shutdown pins or external N-channel MOSFETs. Softstart of power supplies can be achieved simply by adding a capacitor. If the sequencing of more than four power supplies is required, the LTC2924 can be cascaded to sequence a virtually unlimited number of power supplies. Tailoring the LTC2924 to a specific application requires Data Sheet Download www.linear.com Linear Technology Corporation 0.1μF VCC IN1 OUT1 IN2 OUT2 IN3 IN4 LTC2924 VCC FAULT TMR 150nF OUT4 PGT ON 150nF OUT3 GND 10k DONE 2N7002 DN384 F03 POWER ON TIMER DELAY = 30ms POWER OFF TIMER DELAY = 15ms Figure 3. Programming Different ON/OFF Delays For applications help, call (408) 432-1900 dn384f_conv LT/TP 0306 409K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2005