Power Supply Isolation Controller Simplifies Hot Swapping the CompactPCI Bus for 5V-/3.3-Only Applications – Design Note 253 Andy Gardner and extraction and reports on the state of the supply voltages via the HEALTHY# signal. Although ±12V supplies are provided on the CompactPCI backplanes, many plug-in boards only require 5V and 3.3V. The LTC ®1646 is the ideal power supply isolation controller for these applications. LTC1646 Feature Summary The LTC1646 features can be summarized as follows: The LTC1646 Hot Swap™ controller is designed to meet the power supply isolation requirements found in the CompactPCI hot swap specification PICMG 2.1 for plug-in boards requiring 5V and/or 3.3V supplies. The chip will turn a board’s supply voltages on and off in a controlled manner, allowing the card to be safely inserted or removed without causing glitches on the power supplies and causing other boards in the system to reset. It also protects against short circuits, precharges the bus I/O connector pins during insertion COMPACT PCI CIRCUIT CARD CONNECTOR (FEMALE) • Dual level, programmable circuit breakers: this feature is enabled after power-up is complete. If either supply exceeds current limit for more than 20μs, the circuit breaker will trip and the chip latches off. In the event that either supply exceeds three times the set current limit, all supplies are disabled and the chip latches off without delay. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and Hot Swap is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. R2 0.007Ω Q1 1% 1 2 IRF7413 Z2 Z1 5V R1 0.005Ω 1 1% 2 LONG 5V 3.3V LONG 3.3V V(I/O) R16 1.2k 5% BD_SEL# V(I/O) R14 3k 5% HEALTHY# PCI_RST# R18 10Ω 5% R19 10Ω 5% C5 0.1μF 3 R15 3k, 5% 11 5VSENSE FAULT 4 PWRGD 16 RESETIN CL (3VOUT) R5 1k, 5% R6 2 10k, 5% RESETOUT PRECHARGE 13 R13 10Ω, 5% 1 R7 3k 5% DRIVE 14 R10 C3 18Ω, 5% 4.7nF R11 10k 5% C2 0.1μF 3VOUT LTC1646 GROUND R9 1k, 5% Q3 PRECHARGE OUT MMBT2222A 1V ±20% IOUT = ±55mA DATA LINE EXAMPLE R8 12Ω 5% 3VIN Figure 1. CompactPCI Application with Only 3.3V and 5V Supplies 3VIN 5VIN 3.3V 5V RESET# I/O DATA BUS Z1, Z2: BZX84C6V2 03/01/253_conv C1 0.01μF 5 5VOUT TIMER R12 18Ω, 5% DATA BUS 12 5VIN OFF/ON 3 3.3V 7.6A R4 10Ω 5% 9 10 7 3VSENSE GATE 3VOUT GND 6 I/O PIN 1 CL (5VOUT) Q2 IRF7413 R3 10Ω 5% 8 3VIN 5V 5A 4 4 C4 0.1μF R17 1k, 5% 15 3 LOCAL_PCI_RST# COMPACT PCI BACKPLANE CONNECTOR (MALE) • Controls 5V and/or 3.3V CompactPCI supplies. DN253 F01 PCI BRIDGE (21154) • Current limited power-up: the supplies are allowed to power up in current limit. This allows the chip to power up boards with widely varying capacitive loads without tripping the circuit breaker. The maximum allowable power-up time is programmable using the TIMER pin and an external capacitor. • Programmable foldback current limit: a programmable analog current limit with a value that depends on the output voltage. If the output is shorted to ground during the power-up cycle, the current limit drops to keep power dissipation and supply glitches to a minimum. • Precharge output: on-chip reference and amplifier provide 1V for biasing bus I/O connector pins during card insertion and extraction. • BD_SEL#, HEALTHY#, PCI_RST# and LOCAL_PCI_RST# signals are supported. • Space saving 16-pin SSOP package. Typical Application Figure 1 shows a typical application using the LTC1646. The main 3.3V and 5V inputs to the LTC1646 come from the medium-length power pins. The long 3.3V and 5V connector pins are shorted to the medium length 5V and 3.3V connector pins on the plug-in card and provide early power for the LTC1646’s precharge circuitry, the V(I/O) pull-up resistors and the PCI bridge chip. The BD_SEL# signal is connected to the OFF/ON pin while the PWRGD pin is connected to the HEALTHY# signal. The HEALTHY# signal is combined on chip with the PCI_RST# signal to generate the LOCAL_PCI_RST# signal which is available at the RESETOUT pin. The power supplies are controlled by placing external N-channel pass transistors Q1 and Q2 in the 3.3V and 5V power paths. Resistors R1 and R2 sense overcurrent conditions, and R5 and C1 provide current control-loop compensation. Resistors R3 and R4 prevent high frequency oscillations in Q1 and Q2. Power-Up Sequence Figure 2 shows a typical power-up sequence. When the CompactPCI card is inserted, the long 5V, 3.3V and GND connector pins make contact first. The Data Sheet Download www.linear.com Linear Technology Corporation LTC1646’s precharge circuit biases the bus I/O pins to 1V during this stage of the insertion. The 5V and 3.3V medium length pins make contact during the next stage of the insertion, but the slot power is disabled as long as the OFF/ON pin is pulled high by the 1.2k pull-up resistor to V(I/O). During the final stage of the board insertion, the BD_SEL# short connector pin makes contact and the OFF/ON pin can be pulled low. This enables the pass transistor to turn on and a 5μA current source is connected to the TIMER pin. Each supply is then allowed to power up at the rate dV/dt = 13μA/ C1 or as determined by the current limit and the load capacitance, whichever is slower. Current-limit faults are ignored while the TIMER pin voltage is less than 1.25V. Once both supplies are within tolerance, the HEALTHY# signal is pulled low and the LOCAL_PCI_RST# signal goes high. GATE 10V/DIV 5VOUT 3VOUT 5V/DIV TIMER 5V/DIV BD_SEL# 5V/DIV HEALTHY# 5V/DIV LCL_PCI_RST# 5V/DIV PRECHARGE 5V/DIV 20ms/DIV DN253 F02 Figure 2. Normal Power-Up Sequence Conclusion Using the LTC1646, a CompactPCI board can be made hot swappable so the system power can remain uninterrupted while the board is being inserted or removed. With the LTC1646, safe hot swapping becomes as easy as hooking up an IC, a couple of power FETs and a handful of resistors and capacitors. For applications help, call (408) 432-1900 dn253f_conv LT/TP 0301 345K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2001