Ideal Diode and Hot Swap Controller Enables Supply Redundancy and Isolates Faults Chew Lye Huat Schottky diodes are used in a variety of ways to implement multisource power systems. For instance, high availability electronic systems—such as µTCA network and storage servers—employ power Schottky diode-OR circuits in redundant power systems. Diode ORing is also used in systems with alternate power sources, such as an AC wall adapter and a backup battery feed. The problem is that the Schottky diodes consume power due to the forward voltage drop—the resulting heat must be dissipated with dedicated copper area on the PCB, or by heat sinks bolted to the diode, both of which require significant space. The family of products comprising the LTC4225, LTC4227 and LTC4228 minimize power loss by using external N-channel MOSFETs for pass elements, minimizing the voltage drop from the supply to the load when the MOSFETs are turned on. When an input source voltage drops below the output common supply voltage, the appropriate MOSFET is turned off, thereby matching the function and performance of an ideal diode. As shown in Figure 1, by adding a current sense resistor and configuring two MOSFETs back-to-back with separate gate control, the LTC4225 enhances the ideal diode performance with inrush current limiting and overcurrent protection. This allows the boards to be safely inserted and removed from a live backplane without damaging the connector. The LTC4227 can be used with the current sense resistor and the Hot Swap MOSFET added 24 | January 2013 : LT Journal of Analog Innovation Figure 1. An overview of different configurations with sense resistor and external N-channel MOSFETs for the LTC4225, LTC4227 and LTC4228 VOUT1 VIN1 IN1 SENSE1 DGATE1 HGATE1 OUT1 LTC4225* IN2 SENSE2 DGATE2 HGATE2 OUT2 VIN2 VOUT2 VIN1 VOUT VIN2 IN1 DGATE1 IN2 DGATE2 SENSE+ SENSE– HGATE OUT LTC4227* VIN1 VOUT1 IN1 DGATE1 SENSE1+ SENSE1– HGATE1 OUT1 IN2 DGATE2 SENSE2+ SENSE2– HGATE2 OUT2 LTC4228* VIN2 VOUT2 *ADDITIONAL DETAILS OMITTED FOR CLARITY after the parallel-connected ideal diode MOSFET to save one MOSFET. By configuring the sense resistor between the ideal diode and Hot Swap MOSFET, the LTC4228 improves on the LTC4225 by recovering more quickly from input brownouts to preserve the output voltage. The LTC4225-1, LTC4227-1 and LTC4228-1 feature a latchoff circuit breaker, while the LTC4225-2, LTC4227-2 and LTC4228-2 provide automatic retry after a fault. Both options are available in 24-pin, 20-pin and 28-pin 4mm × 5mm QFN and SSOP packages for LTC4225, LTC4227 and LTC4228, respectively. IDEAL DIODE CONTROL The LTC4225 and LTC4228 function as an ideal diode by monitoring the voltage between IN and OUT pins (IN and SENSE+ pins for LTC4227) with an internal gate drive amplifier, which drives the DGATE pin. The amplifier quickly pulls up the DGATE pin, turning on the MOSFET for ideal diode control, when it senses a large forward voltage drop (Figure 2). An external capacitor connected between the CPO and IN pins provides the charge needed to quickly turn on the ideal diode MOSFET. An internal charge pump charges up this capacitor at device power-up. design features The LTC4225, LTC4227 and LTC4228 minimize power loss by using external N-channel MOSFETs for pass elements, minimizing the voltage drop from the supply to the load when the MOSFETs are turned on. When an input source voltage drops below the output common supply voltage, the appropriate MOSFET is turned off, thereby matching the function and performance of an ideal diode. ON 5V/DIV CPO 10V/DIV HGATE 10V/DIV OUT 10V/DIV DGATE 10V/DIV OUT 10V/DIV PWRGD 10V/DIV 20ms/DIV HOT SWAP CONTROL 50ms/DIV Figure 2. Ideal diode controller CPO and DGATE pull up when IN supply turns on Figure 3. Hot Swap controller HGATE starts up and PWRGD pulls low after 100ms delay when ON toggles high The DGATE pin sources current from the CPO pin and sinks current into the IN and GND pins. The gate drive amplifier controls DGATE to servo the forward voltage drop across the sense resistor and the two external N-channel MOSFETs to 25mV. Figure 4. The LTC4225 in a µTCA application to supply 12V power to two µTCA slots VIN1 12V If the load current causes more than 25mV of voltage drop, the gate voltage RS1 0.004Ω BULK SUPPLY BYPASS CAPACITOR R2 137k R1 20k CF1 10nF R4 137k CF2 10nF CCP1 0.1µF IN1 SENSE1 DGATE1 LTC4225 ON2 CPO2 IN2 SENSE2 DGATE2 CCP2 0.1µF VIN2 12V HGATE1 INTVCC GND BULK SUPPLY BYPASS CAPACITOR HGATE2 RH2 10Ω RS2 0.004Ω PLUG-IN CARD 1 MH1 MD1 Si7336ADP Si7336ADP RH1 10Ω CPO1 ON1 C1 0.1µF R3 20k Pulling the ON pin high and the EN pin low initiates a 100ms debounce timing cycle. After this timing cycle, a 10µ A current from the charge pump ramps up the HGATE pin. When the Hot Swap MOSFET turns on, the inrush current is limited at a level set by an external sense resistor connected between the IN and SENSE pins for LTC4225 (SENSE+ and MH2 MD2 Si7336ADP Si7336ADP 12V 7.6A RHG1 47Ω CHG1 15nF OUT1 FAULT1 PWRGD1 EN1 TMR1 TMR2 EN2 PWRGD2 FAULT2 OUT2 RHG2 47Ω CHG2 15nF + CL1 1600µF VIN1 R5 100k R6 100k CT1 47nF CT2 47nF R7 100k PLUG-IN CARD 2 R8 100k VIN2 + IN 10V/DIV rises to enhance the MOSFET used for ideal diode control. In the case of an input supply short-circuit when the MOSFETs are conducting, a large reverse current starts flowing from the load toward the input. The gate drive amplifier detects this failure condition as soon as it appears and turns off the ideal diode MOSFET by pulling down the DGATE. CL2 1600µF 12V 7.6A BACKPLANE January 2013 : LT Journal of Analog Innovation | 25 If the main supply loses power, the controller reacts quickly to turn off the ideal diode MOSFET in the main supply path and turn on the MOSFET in the redundant supply path, providing a smooth supply switchover to the output load. The Hot Swap MOSFETs remain on so they do not affect the supply switchover. SENSE– pins for LTC4227 and LTC4228). An active current limit amplifier servos the gate of the MOSFET so that 65mV appears across the current sense resistor. If the sense voltage exceeds 50mV for more than a fault-filter delay configured at the TMR pin, a circuit breaker trips and pulls HGATE low. Inrush current can be further reduced, if desired, by adding a capacitor from HGATE to GND. When the MOSFET’s gate overdrive (HGATE to OUT voltage) exceeds 4.2V, the PWRGD pin pulls low (Figure 3). powering down the system. The LTC4225 and LTC4228, which both include dual ideal diode and Hot Swap controllers, are ideal for these applications—they provide smooth supply switchover between two supplies and overcurrent protection. If the main supply loses power, the controller reacts quickly to turn off the ideal diode MOSFET in the main supply path and turn on the MOSFET in the redundant supply path, providing a smooth supply switchover to the output load. The Hot Swap MOSFETs remain on so they do not affect the supply switchover. The controller turns off a Hot Swap MOSFET when the respective ON pin is pulled low or EN pin is pulled high. When an overcurrent fault is detected at the output, the gate of the Hot Swap MOSFET is pulled down quickly, COMBINING THE IDEAL DIODE AND HOT SWAP CONTROL In a typical µTCA application with redundant supplies (Figures 4 and 9), the outputs are diode-ORed at the backplane, so cards can be removed or inserted without Figure 5. LTC4225 for 2-channel power prioritizer with IN1 as the prioritizing input 5V PRIMARY SUPPLY RS1 0.006Ω INPUT 1 R1 20k C1 R4 0.1µF 41.2k In a traditional diode-ORed multisupply system, the input supply with the higher voltage is passed to the output, while the lower voltage supply is shut out. This simple solution satisfies the needs of applications where the priority of the supplies is not simply a matter of the higher voltage supply winning. Figure 5 shows a backup supply system where the 5V primary supply (INPUT1) is passed to the output whenever it is available, while the 12V backup MH1 SiR466DP RH1 10Ω CCP1 0.1µF OUT1 FAULT1 ON1 PWRGD1 IN1 SENSE1 DGATE1 HGATE1 INTVCC TMR1 TMR2 LTC4225 GND ON2 PWRGD2 FAULT2 EN2 CPO2 + Z2 SMAJ13A BV=14.4V R3 3.92k 26 | January 2013 : LT Journal of Analog Innovation IN2 SENSE2 DGATE2 HGATE2 CCP2 0.1µF INPUT 2 12V BACKUP SUPPLY RHG1 47Ω CHG1 33nF EN1 CPO1 CF1 0.1µF PRIORITIZING A POWER SUPPLY + Z1 SMAJ13A BV=14.4V R2 49.9k MD1 SiR466DP after which the output is regulated in current limit until the fault filter delay set by the TMR pin capacitor times out. The Hot Swap MOSFET is turned off and the FAULT pin is latched-low to indicate a fault. The electronic circuit breaker is reset by pulling the ON pin below 0.6V. RS2 0.006Ω D1 LS4148 MD2 SiR466DP MH2 SiR466DP OUT2 CT2 47nF CL 470µF CT1 47nF VOUT 5A design features In a typical µTCA application with redundant supplies, the outputs are diodeORed at the backplane, so cards can be removed or inserted without powering down the system. The LTC4225 and LTC4228, which both include dual ideal diode and Hot Swap controllers, are ideal for these applications—they provide smooth supply switchover between two supplies and overcurrent protection. RS1 0.006Ω BULK SUPPLY BYPASS CAPACITOR RH1 10Ω CPO1 PWREN2 IN1 LTC4225 GND IN2 SENSE2 HGATE2 supply (INPUT2) is called on only when the primary supply fails to deliver. As long as INPUT1 is above the 4.3V UV threshold set by the R1-R2 divider at the ON1 pin, MH1 is turned on, connecting INPUT1 to the output. When MH1 is on, PWRGD1 goes low, which in turn pulls ON2 low and disables the IN2 path by turning MH2 off. If the primary supply fails and INPUT1 drops below 4.3V, ON1 turns off MH1 and PWRGD1 goes high, allowing ON2 to turn on MH2 and connect the INPUT2 to the output. The ideal diode MOSFETs MD1 and MD2 prevent backfeeding of one input to the other under any condition. RH2 10Ω OUT1 FAULT1 PWRGD1 EN1 TMR1 TMR2 CT2 47nF CT1 47nF PLUG-IN CARD 2 RHG2 47Ω CHG2 15nF ZH2 ZD2 BULK SUPPLY BYPASS CAPACITOR RS2 0.006Ω CL1 1000µF EN2 PWRGD2 FAULT2 DGATE2 OUT2 ON2 CCP2 0.1µF VIN2 12V DGATE1 SENSE1 HGATE1 INTVCC CPO2 Figure 6. LTC4225 for application with the Hot Swap MOSFET on the supply side and the ideal diode MOSFET on the load side + RHG1 47Ω CHG1 15nF ON1 C1 0.1µF 12V 5A ZH1 ZD1 CCP1 0.1µF PWREN1 PLUG-IN CARD 1 MD1 SiR466DP + VIN1 12V MH1 SiR466DP MH2 SiR466DP MD2 SiR466DP CL2 1000µF 12V 5A BACKPLANE ZH1, ZD1, ZH2, ZD2: CMHZ4706, BV=19V SWAPPING THE DIODE AND HOT SWAP FET ON SUPPLY AND LOAD SIDE The LTC4225 allows applications with back-to-back MOSFETs to be configured with the MOSFET on the supply side as the ideal diode and the MOSFET on the load side as the Hot Swap control (Figure 4) or vice versa (Figure 6). In Figure 6, an external Zener diode clamp may be required between the GATE and SOURCE pins of the MOSFET to prevent it from breaking down if the MOSFET’s gate-to-source voltage is rated for less than 20V. In either arrangement, LTC4225 smoothly switches between supplies with its ideal diode ORing between the IN and OUT pins. DUAL IDEAL DIODE AND SINGLE HOT SWAP CONTROL Figure 7 shows a LTC4227 application where the sense resistor is placed after dual supply ideal diode MOSFETs connected in parallel, which is then followed by a single Hot Swap MOSFET. Here, the LTC4227 regulates an overloaded output at 1× the current limit before fault timeout, instead of 2×, as in the LTC4225 diode-OR application. As a result, power dissipation is reduced during an overload condition. The LTC4227 also features the D2ON pin, which allows the IN1 supply to be easily prioritized. For example, Figure 8 shows a simple resistive divider connecting IN1 to the D2ON pin, so that the January 2013 : LT Journal of Analog Innovation | 27 Tight 5% circuit breaker threshold accuracy and fast acting current limit protect the supplies against overcurrent faults. The LTC4228’s fast recovery from input brownouts preserves the output voltage in the face of such events. Figure 7. LTC4227 for card-resident diode-OR application with Hot Swap control MD1 SiR462DP VIN1 12V Z1 SMAJ13A BV=14.4V VIN2 12V CCP1 0.1µF MD2 SiR462DP RS 0.006Ω MH Si7336ADP + Z2 SMAJ13A BV=14.4V RH 10Ω CCP2 0.1µF CL 680µF 12V 7.6A RHG 47Ω CHG 15nF R2 137k R1 20k CF 10nF CPO1 ON INTVCC OUT D2ON R3 100k R4 100k FAULT PWRGD TMR GND CT 0.1µF C1 0.1µF CARD CONNECTOR FASTER OUTPUT RECOVERY FROM INPUT COLLAPSE IN1 supply is prioritized until IN1 falls below 2.8V, wherein MD2 is turned on and the diode-OR output is switched from the main 3.3V supply at IN1 to the auxiliary 3.3V supply at IN2. SENSE– HGATE LTC4227 EN BACKPLANE CONNECTOR IN2 DGATE2 SENSE+ IN1 DGATE1 CPO2 while the other supply is not available, HGATE is pulled low to turn off the Hot Swap MOSFET as the IN supply drops below the undervoltage lockout threshold. When the input supply recovers, HGATE is allowed to start up to In the LTC4225 µTCA application shown in Figure 4, if one of the input supplies collapses to ground momentarily Figure 8. Plug-in card IN1 supply controls the IN2 supply turn-on via D20N of LTC4227 MD1 SiR462DP VMAIN 3.3V Z1 SMAJ7A BV=7.78V VAUX 3.3V CCP1 0.1µF Z2 SMAJ7A BV=7.78V R2 22.1k R1 20k CPO1 CF1 0.1µF CARD CONNECTOR 28 | January 2013 : LT Journal of Analog Innovation R6 28.7k R5 20k MH Si7336ADP + IN1 DGATE1 CPO2 IN2 DGATE2 SENSE+ SENSE– HGATE OUT FAULT PWRGD ON LTC4227 D2ON INTVCC C1 0.1µF CF2 10nF RS 0.008Ω CCP2 0.1µF EN BACKPLANE CONNECTOR MD2 SiR462DP GND TMR CT 0.1µF R3 10k R4 10k CL 100µF 3.3V 5A design features The LTC4225, LTC4227 and LTC4228 enable ideal diode and Hot Swap functions for two power rails by controlling external N-channel MOSFETs. They feature fast reverse turn-off, smooth supply switchover, active current limit and status and fault reporting. turn on the MOSFET. As it takes a while to charge up HGATE and the depleted output capacitance, the output voltage may brown out during this period. This prevents the SENSE+ voltage from entering into undervoltage lockout and turning off the Hot Swap MOSFET. As the input supply recovers, it charges up the depleted load capacitance and instantly provides power to the downstream load, since the Hot Swap MOSFET remains on. In this situation, the LTC4228 offers an advantage over the LTC4225 by recovering more quickly to preserve the output voltage. As shown in Figure 9, the sense resistor is placed in between the ideal diode and Hot Swap MOSFET, allowing the SENSE+ pin voltage to be held up by the output load capacitance temporarily when the input supply collapses. supply switchover, active current limit and status and fault reporting. Their tight 5% circuit breaker threshold accuracy and fast acting current limit protect the supplies against overcurrent faults. The LTC4228’s fast recovery from input brownouts preserves the output voltage in the face of such events. n CONCLUSION The LTC4225, LTC4227 and LTC4228 enable ideal diode and Hot Swap functions for two power rails by controlling external N-channel MOSFETs. They feature fast reverse turn-off, smooth Figure 9. LTC4228 for µTCA application to supply 12V power to two µTCA slots MD1 Si7336ADP BULK SUPPLY BYPASS CAPACITOR RH1 10Ω CCP1 0.1µF CPO1 IN1 DGATE1 R1 20k R3 20k R4 137k CF2 10nF INTVCC LTC4228 GND OUT1 VSENSE1+ R5 100k R6 100k CL1 1600µF R7 100k CT1 47nF CT2 47nF + PLUG-IN CARD 2 EN2 PWRGD2 FAULT2 STATUS2 ON2 CPO2 IN2 DGATE2 SENSE2+ SENSE2– HGATE2 RH2 10Ω CCP2 0.1µF VIN2 12V RHG1 47Ω CHG1 15nF STATUS1 FAULT1 PWRGD1 EN1 TMR1 TMR2 ON1 C1 0.1µF 12V 7.6A SENSE1+ SENSE1– HGATE1 R2 137k CF1 10nF PLUG-IN CARD 1 MH1 Si7336ADP BULK SUPPLY BYPASS CAPACITOR MD2 Si7336ADP RS2 0.004Ω MH2 Si7336ADP OUT2 RHG2 47Ω CHG2 15nF R8 100k R9 100k R10 100k + VIN1 12V RS1 0.004Ω VSENSE2+ CL2 1600µF 12V 7.6A BACKPLANE January 2013 : LT Journal of Analog Innovation | 29