Hot Swap Controller with Power-Up Timeout Function Simplifies Hot Swapping Boards with Multiple Power Supplies

Introduction

The LTC4212 is a Hot Swap controller that allows safe board insertion and removal from a live backplane. It features a supply range of 2.5V to 16.5V, programmable soft-start with inrush current limiting, automatic retry or latched mode operation, a high side drive for an external N-channel MOSFET and dual level overcurrent fault protection.

Unlike many Hot Swap controllers, the LTC4212 does not monitor the load side of the MOSFET directly. Instead, the LTC4212 interfaces with external power supply monitor ICs or directly with the PGOOD pin of a switching regulator. This permits the LTC4212 to monitor any number of supplies on the board. It has a power-up timeout function that disconnects a card from the backplane supply when the monitored supplies do not power-up within the programmable timeout period and a glitch filter to reject short term glitches in the monitored supplies after normal power-up.

Figure 1 shows a typical application, which uses the LTC1727-2.5V to monitor the outputs of two LDOs (LT1963) and the 5V output at the source (load) of the external N-channel MOSFET.

Figure 1. Hot Swap controller with power-up timeout and glitch filtering.

Power-Up Timeout Function

The LTC4212 has a power-up timeout function, implemented via 3 pins: PGI, PGT and PGF. The PGI (or Power Good Input) pin is a high impedance input pin that is normally connected to the RST or COMPn pins of a supply monitor IC such as the LTC1727, or to the PGOOD pin of one or more DC/DC converters. Since the RST, COMPn and PGOOD pins are typically open drain pins, the LTC4212 can monitor any number of supplies by just connecting the open drain pins together. The PGI pin requires a pull-up resistor (R4 in Figure 1) when monitoring open drain pins.

Power Good Timer

An external capacitor (CPGT) connected from the PGT (Power Good Timer) pin to ground sets the power-up timeout period, at the end of which the LTC4212 samples the PGI pin. The timeout period is 1.81s/µF and is accurate to within ±10% assuming ideal capacitors. The power good timer charges and discharges CPGT between 0.65V and 0.95V using 5µA current sources for 14 cycles to generate the timeout period. If the PGI pin is less than 1.236V when sampled, PGT trips the Electronic Circuit Breaker (ECB). This causes the GATE pin to be pulled immediately to ground to disconnect the board from the backplane supply and the FAULT pin goes low to indicate that the ECB was tripped.

Power Good Glitch Filter

Another external capacitor (CPGF) connected from the PGF (Power Good Filter) pin to ground sets the duration of the glitch filter for the PGI pin. After normal power-up, the glitch filter is enabled to reject any short-term pulses at the PGI pin. CPGF is charged by a 5µA pullup current source as long as the PGI pin is low. When a monitored supply drops out of regulation, the PGI pin goes low and VPGF starts to ramp up. When it rises above 1.236V, the glitch filter trips the ECB. The GATE pin is immediately pulled to ground and the FAULT pin goes low.

Electronic Circuit Breaker

The Electronic Circuit Breaker (ECB) can be tripped by an overcurrent fault, a power good timeout fault or by a glitch filter fault. When the ECB trips, the GATE pin is pulled down to ground immediately to disconnect the board from the backplane supply. The FAULT pin is pulled low whenever the ECB trips. In order to reconnect the board, the ON pin must be taken low for at least 120µs to reset the ECB or the VCC must be below 2.2V for more than 30µs.

Overcurrent Protection

The load current is sensed by monitoring the voltage across an external sense resistor (RSENSE in Figure 1). During power-up, a soft-start circuit limits the load current to 50mV/RSENSE. After normal power-up, 2 comparators, FASTCOMP and SLOWCOMP, monitor the load current. FASTCOMP trips the ECB if the load current exceeds 150mV/RSENSE for 500ns and protects the external MOSFET and load against fast and large overcurrent conditions. SLOWCOMP trips the ECB if the load current exceeds 50mV/RSENSE for more then 18µs.

Typical Application

Figure 1 shows LTC1727-2.5 triple supply monitor providing three comparators to monitor the voltage at the VCCA, VCC3 and VCC25 pins. Each comparator responds to a 10% overdrive in 50µs and exhibits a response time that decreases with overdrive. With a 1% overdrive, the response slows to 150µs. The maximum trip point of each comparator is –5% for VCC3 and VCC25 and varies with R5 and R6 for VCCA. Setting R5 to 11.8k and R6 to 3.01k sets the trip-point for the VCCA comparator to 4.75V or 5V–5%. When any of the three monitored supplies are below –5% for about 60µs, the corresponding open drain output COMP3, COMP25 or COMPA is pulled low. In Figure 1, all three open drain outputs are shorted to the PGI pin of the LTC4212 and share a single pull-up resistor, R4.

The LT1963-2.5 and LT1963-3.3 are fast transient response LDO regulators that can supply 1.5A of output current at voltages of 2.5V (+3%, –3.5%) and 3.3V (±3%) respectively. A minimum output capacitor of 10µF (ESR of 3Ω of less) is needed to prevent oscillations, and larger capacitors may be required to limit ripple or improve transient response with large transient loads.

Normal Power-Up Sequence

Figure 2 shows a normal power-up sequence with unloaded 2.5V, 3.3V and 5V outputs. When VCC rises above 2.2V and the ON pin is greater than 1.316V, the LTC4212 starts the 1st timing cycle. A 2µA current source charges an external capacitor (CTIMER) connected from the TIMER pin to ground. When VTIMER rises above 1.236V, the TIMER pin is pulled immediately to ground and CTIMER is discharged. The second timing cycle is started and FASTCOMP is enabled.

Figure 2. Normal power-up sequence (no load).

During the second timing cycle, a soft-start circuit in the LTC4212 servos the GATE pin to regulate the inrush current at 50mV/RSENSE. The 2µA current source is re-enabled to charge CTIMER. From the start of the 2nd cycle, the GATE rises to its final value in around 7ms. The outputs of the linear regulators rise up in 1ms. At the end of the second timing cycle (when VTIMER rises above 1.236V again), the soft-start circuit is disabled and a 10µA current source continues to pull-up the GATE pin. At the same time, SLOWCOMP is enabled, the TIMER pin is pulled back to ground and the LTC4212 starts the power good timer. At timeout, the PGI pin is sampled. In Figure 2, PGI goes high (>1.236V) well before timeout and the board remains powered up as is normal. Since the PGI pin is only sampled at timeout, any transients at the PGI pin during supply ramping are ignored.

Figure 3 shows a normal power-up sequence with the 2.5V and 3.3V outputs loaded with 1A.

Figure 3. Power-up with load.

Power-Up with Fault Sequence

Figure 4 shows power-up with a short at the 5V board supply output, VCCA. During the 2nd timing cycle, the gate voltage ramps up initially to the point where the FET just turns on. After that it flattens out due to action of the soft-start circuit to limit the load current to 50mV/RSENSE or roughly 7A. The VCC backplane supply dips 0.5V due to the 7A flowing in the interconnection to the 5V, 10A limited supply. At the end of the 2nd cycle, the soft-start circuit is replaced with a 10µA pull-up current source. This ramps the gate of the FET up and the load current rises until the SLOWCOMP trips the ECB. The gate voltage pulls down in 1µs to 2µs when the fast pull down circuit is activated. Stray inductance causes the VCC supply to spike when the load current is terminated. The power good timer and glitch filter are disabled when the ECB has been tripped.

Figure 4. Power-up with the 5V output shorted to GND.

If any of the LDO outputs are shorted to ground, the short circuit protection in the LDO reduces the fault current and the board does not latch off after the second timing cycle. As shown in Figure 5, the power good timer samples the PGI pin at timeout and trips the ECB since PGI is low. The GATE pin is pulled to ground immediately to disconnect the board from the backplane supply.

Figure 5. Power-up with the 3.3V output shorted to GND.

Figure 6 shows the response of the glitch filter to a 20µs overload at the 5V output after normal power-up. The comparators in the LTC1727-2.5 take PGI low whenever the 5V supply or any of the LDO outputs drop below their lower thresholds. The glitch filter capacitor (CPGF) is charged by a 5µA pull-up current source whenever PGI goes low. The first PGF ramp is due to the 20µs overload but VPGF does not ramp above 1.236V. The second pulse is a result of the time it takes the 5V linear regulator to recover from the over load and is long enough for VPGF to ramp above 1.236V, causing the glitch filter to trip the ECB. The GATE pin is pulled immediately to ground to disconnect the board from the backplane supply. If the PGF pin is tied to ground, the PGF pin remains permanently below 1.236V and the glitch filter is effectively disabled. Tying PGF to ground causes the LTC4212 to ignore a low PGI state after normal power-up.

Figure 6. Glitch filter operation.

Interfacing with the RST Pin

The PGI pin of the LTC4212 can be connected to the RST pin instead of the COMPx pins of the LTC1727-2.5. The RST pin is delayed by 200ms compared to the COMPn pins and CPGT must be adjusted to include the 200ms delay for the board to power-up normally. In addition, any transients on the monitored supplies that exceed the response time of the comparators in the LTC1727-2.5 will cause RST to go low for at least 200ms. This causes the glitch filter to trip the ECB.

Auto-Retry Application

Figure 7 shows an application that automatically tries to power-up the board after the ECB has been tripped due to a shorted load supply output. This circuit uses the LTC1326-2.5 supply monitor chip and ties its RST output to the PGI pin of the LTC4212. The RST signal goes high 200ms after all the monitored voltages rise above the thresholds of the VCCA, VCC3 and VCC25 comparators in the LTC1326-2.5. The ON pin is shorted to the FAULT pin and is pulled up by a 1MΩ resistor (RAUTO) to VCC. A 2µF capacitor (CAUTO) connected from the lower end of RAUTO to ground sets the auto-retry duty cycle. The LTC4212 will retry as long as the short persists. RAUTO and CAUTO must be selected to keep the duty cycle low in order to prevent overheating in the external N-channel MOSFET.

Figure 7. Auto-retry application.

Figure 8 shows the auto-retry cycle when the 5V output is shorted to ground. SLOWCOMP trips the ECB after the 2nd timing cycle. This causes the FAULT pin to be pulled low by an internal N-channel FET and CAUTO is discharged to ground. The GATE pin is pulled immediately to ground to disconnect the board. Note that in Figure 8, the time-base setting causes the first and second timing cycles to appear as a single spike. When the ON pin goes below its lower threshold of 0.455V (typical) for more than 120µs, the ECB is reset. The internal N-channel FET at the FAULT pin is switched off and RAUTO starts to charge CAUTO slowly towards VCC.

Figure 8. Auto-retry with short at 5V output.

When the ON pin rises above its upper threshold of 1.316V, the LTC4212 attempts to reconnect the board and start the first timing cycle. With a dead short at the 5V output as in Figure 8, the ECB trips after the second timing cycle when the soft-start circuit is disabled and a 10µA pull-up current source is connected to the GATE pin. The entire cycle is repeated until the short is removed. The duration of each cycle is dominated by the time needed to charge CAUTO between the lower and the upper threshold voltages of the ON pin. With RAUTO = 1MΩ and CAUTO = 2µF, the cycle time is 800ms. The switch is on for about 6ms giving a duty cycle of 0.75%.

At the end of the 2nd timing cycle, the GATE pin is about 3V due to the action of the soft-start circuit (which limits the current to 50mV/RSENSE) and the presence of the short at the 5V output. The 10µA current source takes several milliseconds to ramp up the 3.3nF of GATE pin capacitance. As a result, SLOWCOMP tends to trip the ECB instead of FASTCOMP.

When powering up with a short at the output of either LDO, the short circuit protection in LDO reduces the fault current. At the end of the Power Good timeout period, the PGI pin is still low due to the short circuit. As shown in Figure 9, the external N-channel MOSFET is turned on for a longer part of each auto-retry cycle if the overload is not enough to trip both SLOWCOMP and FASTCOMP. With RAUTO = 1MΩ and CAUTO = 2µF, the duty cycle increases to 29% causing the LT1963-2.5 LDO to heat up to a case temperature of 106°C at an ambient temperature of 25°C. A larger value of CAUTO is required when operating from higher ambient temperatures. The Si4410DY MOSFET does not heat up appreciably due to its low RDS(ON).

Figure 9. Auto-retry with short at LDO output.

If a transient short at the output of an LDO is long enough to cause the monitoring comparators in the LTC1326-2.5 (13µs typical delay) to switch, the RST output will go low for at least 200ms. As shown in Figure 10, the Power Good glitch filter trips the ECB and initiates an auto-retry cycle.

Figure 10. Auto-retry caused by a transient short at the 2.5V output.

Conclusion

The LTC4212 simplifies the design of hot swapping boards with multiple power supplies. The status outputs from power supply ICs or supply monitor ICs—such as RST, PGOOD and COMPn—are typically open drain outputs and can be connected together and monitored by the PGI pin of the LTC4212. The power good timer automatically disconnects the card from the backplane supply should any of the supplies fail to power-up within the programmed time period. After normal power-up, the glitch filter provides a means to detect out of regulation supplies while rejecting dips that last shorter than a programmable time period.

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Anthony Ng

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Y. Sim