PRM(R) Regulator PRM48BH480T200B00 S C NRTL US High Efficiency Remote Sense PRM Converter FEATURES DESCRIPTION (R) The half VI Chip package is compatible with standard pickand-place and surface mount assembly processes with a planar thermal interface area and superior thermal conductivity. In a Factorized Power ArchitectureTM system, the (R) PRM48BH480T200B00 and downstream VTM transformer minimize distribution and conversion losses in a high power solution. TYPICAL APPLICATIONS * * * * * * (R) The VI Chip PRM Regulator is a high efficiency converter, operating from a 38 to 55 Vdc input to generate a regulated 5 to 55 Vdc output. The ZVS Buck - Boost topology enables high switching frequency (~1 MHz) operation with high conversion efficiency. High switching frequency reduces the size of reactive components 3 enabling power density up to 1,300 W/in . * 45 V (38 to 55 VIN), non-isolated ZVS buck-boost regulator * 5 to 55 V adjustable output range * Building block for high efficiency DC-DC systems 2 * 200 W Output Power in 0.57 in footprint * 97% typical efficiency, at full load 3 3 * 1,300 W/in (81 W/cm ) Power Density * Enables a 48 V to 1.5 V, 130 A isolated, regulated 2 2 solution with total footprint of 1.7 in (11 cm ) * Flexible "Remote Sense" architecture optimizes regulation / feedback loop design to fit application requirements * Current Feedback signal allows dynamic adjustment of current limit setpoint * 4.93 MHrs MTBF (MIL-HDBK-217Plus Parts Count) An external control loop and current sensor maintain regulation and enable flexibility both in the design of voltage and current compensation loops to control of output voltages and currents. High Efficiency Server Processor and Memory Power High Density ATE system DC-DC power Telecom NPU and ASIC core power LED drivers High Density Power Supply DC-DC rail outputs Non-isolated power converters 48 V to 1.2 V, 130A Voltage Regulator Voltage Control Feedback Enable/ Disable Voltage Reference PR PC TM IM +OUT +IN -OUT SG VC -IN +IN 38 to 55 Vdc Input PRM -IN IF RE PC TM +OUT1 +OUT2 VTM VC Current Sense PRM(R) Regulator Rev 1.1 vicorpower.com Page 1 of 22 7/2015 800 927.9474 Load -OUT1 -OUT2 PRM48BH480T200B00 1.0 ABSOLUTE MAXIMUM RATINGS The ABSOLUTE MAXIMUM ratings below are stress ratings only. Operation at or beyond these maximum ratings can cause permanent damage to device. Electrical specifications do not apply when operating beyond rated operating conditions. All voltages are specified relative to SG unless otherwise noted. Positive pin current represents current flowing out of the pin. PR ................................................................................... PC ................................................................................... TM ................................................................................... +IN to -IN ....................................................................................... VS ................................................................................... SG ....................................................................................... IF ....................................................................................... RE ....................................................................................... VC to -OUT +OUT to -OUT Output Current Operating Analog IC Junction Temperature Storage Temperature ................................................................................... ....................................................................................... ....................................................................................... ....................................................................................... ....................................................................................... Min -0.3 -0.3 -0.3 -1 -0.5 -0.5 -0.3 -0.5 -1 -40 -40 Max 10.5 10 5.7 10 5.7 1 62 10.5 100 100 5.7 5 18 1.8 62 5.5 125 125 Unit V mA V mA V mA V V mA mA V V V A V A C C 2.0 ELECTRICAL CHARACTERISTICS Specifications apply over all line and load conditions, TJ = 25 C and output voltage from 20 V to 55 V, unless otherwise noted. Boldface specifications apply over the temperature range of -40 C < TJ < 125 C (T-grade). Attribute Symbol Conditions / Notes Min Typ Max Unit 38 0.001 45 55 1000 4 8.5 5.7 V V/ms W mA A F m 55 4.17 200 10 V A W s ms % See sec 10.6 % POWER INPUT SPECIFICATION Input Voltage range VIN Slew Rate No Load Power Dissipation Input Quiescent current Input Current Input Capacitance (Internal) Input Capacitance (Internal) ESR VIN dVIN/dt P NL I QC IIN_DC CIN_INT RCin 0 < VIN < 18 V PC High, VIN = 45 V PC Low, VIN = 45 V IOUT = 4.17 A, VIN = 38 V, V OUT = 48 V Effective value, V IN = 45 V (see Fig. 20) 2.6 4.5 5.5 2 3 POWER OUTPUT SPECIFICATION Output Voltage range Output Current Output Power Output Turn-ON Delay Current Sharing accuracy Efficiency Output Output Output Output Output Discharge current Voltage Ripple Inductance (Parasitic) Capacitance (Internal) Capacitance (Internal) ESR V OUT I OUT P OUT TON TOFF + TON IOUT_PS I OD VOUT_PP LOUT_PAR COUT_INT RCout 5 See Fig.16, SOA See Fig.16, SOA From PC pin release to V OUT, VIN pre-applied and TOFF already expired From VIN applied to V OUT, PC floating Equal input, output and PR voltage at full load; V IN = 45 V, V OUT = 48 V, exclusive of current limit Equal input, output and PR voltage at full load; Over line, trim, and temperature; exclusive of current limit Nominal line, full load, V OUT = 48V 50% load and VOUT = 48 V; over temperature 50% load; over temperature Section 4.0 COUT_EXT = 0 F, IOUT = 4.17 A, VIN = 45 V, V OUT = 48 V, 20 MHz BW Frequency @ 1 MHz, Simulated J-Lead model Effective value, V OUT = 48 V (see Fig. 20) 48 20 18.02 95.7 94.5 88.5 96.9 0.5 1020 2.5 2 3 1500 % % % mA mV nH F m POWERTRAIN PROTECTIONS Input Undervoltage Turn-ON Input Undervoltage Turn-OFF Input Overvoltage Turn-ON Input Overvoltage Turn-OFF Overcurrent (IF) and Input Over/Undervoltage Blanking Time Output Overvoltage Threshold Thermal Shutdown Setpoint Overtemperature, Output Overvoltage and PC Shutdown Response Time Short Circuit Vout Threshold Short Circuit Vout Recovery Threshold Short Circuit Vpr Threshold Short Circuit Vpr Recovery Threshold Short Circuit Timeout Short Circuit Fault Recovery Time Output Power Limit VIN_UVLO+ VIN_UVLOVIN_OVLO+ VIN_OVLO- Instantanous powertrain shutdown, latched after TBLANK Instantanous powertrain shutdown, latched after TBLANK TBLANK VOUT_OVLO+ TJ_OTP Instantaneous, latched shutdown Instantaneous, latched shutdown; guaranteed by design, not production tested; V TM = 4.03V 35.75 33.56 57.24 58.44 37.13 31.97 55.91 59.91 V V V V 50 120 150 s 55.25 130 56.57 59.04 V C TPROT 2 s VSC_VOUT VSC_VOUTR VSC_VPR VSC_VPRR TSC TSCR P PROT 3.0 4.0 7.2 7.1 20 0.1 V V V V ms ms W Short Circuit fault latched after V SC_VOUT and V SC_VPR thresholds persist for this time 200 PRM(R) Regulator Rev 1.1 vicorpower.com Page 2 of 22 7/2015 800 927.9474 PRM48BH480T200B00 3.0 SIGNAL CHARACTERISTICS Specifications apply over all line and load conditions, TJ = 25 C and Output Voltage from 20 V to 55 V, unless otherwise noted. Boldface specifications apply over the temperature range of -40 C < TJ < 125 C (T-grade). Primary Control PC * The PC pin enables and disables the PRM * In PRM array configurations, PC pins should be connected in order to synchronize startup. * It is a weak pull-down during any fault mode excluding short circuit. PC is a strong pull-down to SG if a Short Circuit fault is latched. Signal Type State Attribute Symbol Conditions / Notes V PC Regular PC Voltage Operation IPC_OP PC Available Current Analog Output IPC_EN After TOFF PC Source Current Startup TOFF Minimum Time to Start Section 5.0 VPC_EN Startup PC Enable Threshold Digital Input / Output VPC_DIS PC Disable Threshold Standby RPC_EXT Max Resistance to SG required to disable the PRM PC Resistance to disable IPC_SC Digital Output [Short Circuit Fault] Fault PC Sink Current to SG Short circuit, PC Voltage 1 V or above IPC_FAULT Temperature, Over- and Under-Voltage, Overcurrent Digital Output [All other Faults] Fault PC Sink Current to ~1V Voltage Source VS * Intended to power feedback components and/or auxiliary circuits. Signal Type State Attribute VS Voltage Regular VS Available Current Operation Analog Output VS Voltage Ripple Transition VS Capacitance (External) VS Fault Response Time Symbol V VS IVS VVS_PP CVS_EXT TFR_VS Conditions / Notes Min 4.7 Typ Max 5.3 1.8 mA 90 10.0 1.75 18.0 2.50 2.40 30.0 3.20 300 25 10 Min 8.55 5 Iout = 0A, Cvs_ext=0. Maximum specification includes powertrain operation in burst mode. Typ 9.00 100 From fault recognition to VS = 1.5 V Unit V A ms V V mA Max 9.45 Unit V mA 400 mV 0.04 F s Unit 30 Reference Enable RE * RE signals successful startup and powertrain ready to operate * Regulated, delayed voltage source intended to power the feedback circuit voltage reference and current monitor Signal Type State Attribute Min Typ Max V RE 3.0 3.3 3.6 V RE Available Current RE Regulation RE Voltage Ripple PC to RE Delay RE Capacitance (External) I RE %RE VRE_PP TPC_RE CRE_EXT 8.0 across load and temperature includes powertrain in burst mode Fault detected 0.1 mA % mV s F VS to RE Delay TVS_RE VS = 8.1 V to RE high, V IN > VIN_UVLO- RE Voltage Regular Operation Analog Output Transition Control Node PR * Modulator control node input * Sinks constant current when externally driven * Sources current when pulled below active range Signal Type State Analog Input Regular Operation Attribute PR Voltage Active Range PR Source Current PR Sink Current PR Resistance to SG (Internal) Symbol Symbol V PR IPR IPR_Low Conditions / Notes Conditions / Notes VPR 0.79V VPR > 0.79V 2.5 100 100 1 ms Min 0.79 Typ Max 7.40 Unit V 2 mA 250 500 750 A RPR 93.3 k Current Feedback IF * A voltage proportional to the PRM output current must be supplied externally to the IF pin in order for the device to properly protect overcurrent events and to enable output current limit (clamp) * Overcurrent protection trip will cause instantaneous powertrain disable, latched after TBLANK Signal Type Analog Input State Regular Operation Attribute Current Limit (clamp) Threshold Symbol VIF_IL Overcurrent Protection Threshold VIF_OC IF Input Impedance Current Limit Bandwidth RIF BW IL Conditions / Notes VIN = 45 V; TJ = 25 C Not Production Tested; Guaranteed by Design; TJ = 25 C PRM(R) Regulator Rev 1.1 vicorpower.com Page 3 of 22 7/2015 800 927. 9474 Min 1.90 Typ 2.00 Max 2.10 2.58 2.69 2.80 2.11 2.13 2 2.15 Unit V k kHz PRM48BH480T200B00 Temperature Monitor TM * The TM pin monitors the internal temperature of the PRM analog control IC. * "Power Good" flag to verify that the PRM is operating Signal Type Analog Output State Regular Operation Attribute Symbol V TM Conditions / Notes Full temperature range Min 2.12 Typ TM Voltage Max 4.04 TM Voltage reference VTM_AMB TJ = 27 C 2.94 3.00 3.06 TM Voltage Ripple TM Available Current VVS_PP I TM Digital Output [Fault Flag] TM Disabled Current I TM_DIS Signal Ground SG * All control signals must be referenced to this pin, with the exception of VC * SG is internally connected to -IN and -OUT Signal Type State Attribute Analog Input / Output Any Maximum Allowable Current Symbol ISG VTM Control VC * Pulsed voltage source used to power and synchronize start up of downstream VTM * If not used, must be resistively terminated to -OUT Signal Type State Attribute VC Voltage Symbol VVC Analog Output Startup VC Current Limit VC duration VC Slew Rate IVC TVC dVC/dt DC state with TM Voltage +/- 0.5V. This is a high impedance state. RVC = 68 V 350 mV A 10 mV/C 0.0 mA 100 ATM TM Gain Fault or Standby powertrain in burst mode Unit V Conditions / Notes Min -100 Typ Max 100 Unit mA Conditions / Notes Min 13 Typ Max Unit V 200 7 500 10 16 VC = 14 V, VIN > 20 V RVC = 1k PRM(R) Regulator Rev 1.1 vicorpower.com Page 4 of 22 7/2015 800 927. 9474 20 mA ms V/ s PRM48BH480T200B00 4.0 FUNCTIONAL BLOCK DIAGRAM +Vin +Vout Vcc Vcc 3.3V Linear Regulator Internal Vcc Regulator -Vin PC PR Vout Cin Cout 3.3V Q3 Q1 uC 8051 RE L -Vout 16V +Vout 9V Q4 Q2 Output Discharge (OD) 8.2V PR Modulator PR 93.3kW Enable Var. Vclamp 2.5mA Min VTM Vc Start up pulse 0.5m A 14V VC 10ms Vcc Q Q SET CLR Fault Logic TOFF delay 100uA S Instant latch R R Vout (OV) 5V 2mA max 3V RE Latch after 120us RE 3.3V Vin (OV, UV) Vs 9V 0.01uF Enable PC 10uA PC VPC_EN Overtemperature Protection TM 3 V @ 27C SG Current Limit VIF_IL Overcurrent Protection Temperature dependent voltage source IF 2130W Vref (130C) VIF_OC PRM(R) Regulator Rev 1.1 vicorpower.com Page 5 of 22 7/2015 800 927. 9474 PRM48BH480T200B00 5.0 HIGH LEVEL FUNCTIONAL STATE DIAGRAM Conditions that cause state transitions are shown along arrows. Sub-sequence activities listed inside the state bubbles. Application of Vin PC HIGH and Toff expiry STARTUP SEQUENCE PC: 1.8mA to HIGH STANDBY SEQUENCE PC: 10uA to LOW Overtemp or Output OVP Toff Timeout PC: 90uA to HIGH Powertrain Stopped Fault removed TBLNK expiry BLANKING PC: 1.8mA to HIGH Input OVP, Input UVP, or OverCurrent Prot TBLNK Timeout Powertrain Paused PC falling edge SUSTAINED OPERATION PC: 1.8mA to HIGH Powertrain Active Short Removed: Vout > VSC_VOUTR or Vpr < VSC_VPR_R Vout < 1 V And TSCR expiry PC HIGH and Ton expiry Ton timeout; VC Pulse; Powertrain Active Delayed RE Short Circuit: Vout < VSC_Vout and Vpr > VSC_Vpr SHORT CIRCUIT PC: 1.8mA to HIGH TSC Timeout Powertrain Active TSC expiry OUTPUT DISCHARGE PC: pulsed 25mA drive LOW TSCR Timeout Powertrain Stopped IOD Output Discharge PRM(R) Regulator Rev 1.1 vicorpower.com Page 6 of 22 7/2015 800 927. 9474 PC falling edge PRM48BH480T200B00 6.0 TIMING DIAGRAMS Module Inputs are shown in blue; Module Outputs are shown in brown; Timing diagrams assumes the following: Single PRM (no array) VS powers error amplifier RE powers voltage reference and output current transducer IOUT is sensed, scaled, and fed back to IF pin such that IF = 2.00 V at full load 2 1 Start up with 1.2V/ms < dVIN/dt < maximum VIN OV TOFF 3 4 Input OV recovery Quick OC Input OV (tUVLO) to the module power input and after TOFF, the PC pin will source a constant 90 A current. * Output disable: PC may be pulled down externally in order to disable the module. Pull down resistance should be less than 300 to SG. * Fault detection flag: The PC 5 V voltage source is internally turned off when a fault condition is latched. Note that aside from the Short Circuit fault condition, PC does not have significant current sinking capability. Therefore in the case of an array of PRMs with interconnected PC pins, PC does not in general reflect the fault state of all PRMs. The common PC line will not disable neighboring modules when a fault is detected except for a latched Output Short Circuit fault. Conversely any unit in the array latching a Short Circuit fault will disable the array for TSCR. Signal Ground (SG) pin provides a Kelvin connection to the PRM's internal signal ground. It should be used as the reference for PR, TM, IF, and should return all PC, VS and RE pin currents. In array configurations with common ground control circuits, a series resistor (~1 ) is recommended in order to decouple power and signal current returns. 10.2 Control circuit requirements and design procedure The PRM48BH480T200B00 is an intelligent powertrain module designed to fully exploit external output voltage feedback and current sensing sub-circuits. These two external circuits are illustrated in Figure 26, which shows an example of the PRM in a standalone application with local voltage feedback and high side current sensing. In general, these circuits include a precision voltage reference, an operational amplifier which provides closed loop feedback compensation, and a high side current sense circuit which includes a shunt and current sense IC. The following design procedures refer to the circuit shown in Figure 26. 10.2.1 Setting the output voltage level Temperature Monitor (TM) pin outputs a voltage proportional to the absolute temperature of the converter analog control IC. It can be used to accomplish the following functions: * Monitor the control IC temperature: The gain and setpoint of TM are such that the temperature, in Kelvin, of the PRM controller IC is equal to the voltage on the TM pin scaled by 100. (i.e. 3.0 V = 300 K = 27 C). * Closed loop thermal management at the system level (e.g. variable speed fans or coolant flow) * Fault detection flag: The TM voltage source is turned off as soon as a fault is detected. For system monitoring purposes (microcontroller interface) faults are detected on falling edges of TM. The output voltage setpoint is a function of the voltage reference and the output voltage sense ratio. With reference to Fig. 26, R1 and R2 form the output voltage sensing divider which provides the scaled output voltage to the negative input of the error amplifier; a dedicated reference IC provides the reference voltage to the positive input of the error amplifier. Under normal operation, the error amplifier will keep the voltages at the inverting and non-inverting inputs equal, and therefore the output voltage is defined by: Reference Enable (RE) pin outputs a regulated 3.3 V, 8 mA voltage source. It is enabled only after successful startup of the PRM powertrain (see chapters 5.0 and 6.0.) RE is intended to power the output current transducer and also the voltage reference for the control loop. Powering the reference generator with RE helps provide a controlled startup, since the output voltage of the system is able to track the reference level as it comes up. Note that the component R1 will also factor into the compensation as described in a later section. Voltage Source (VS) pin outputs a gated (e.g. mirrors PC status), non-isolated, regulated 9 V, 5 mA voltage source. It can be used to power external control circuitry; it always leads RE. VOUT = V ref R1 + R2 R2 It is important to apply proper slew rate to the reference voltage rise when the control loop is initially enabled. The recommended range for reference rise time is 1 ms to 9 ms. The lower rise time limit will ensure optimized modulator timing performance during startup, and to allow the current limit feature (through IF pin) to fully protect the device during power-up. The upper rise time limit is needed to guarantee a sufficient factorized bus voltage is provided to any downstream VTM input before the end of the VC pulse. PRM(R) Regulator Rev 1.1 vicorpower.com Page 16 of 22 7/2015 800 927. 9474 PRM48BH480T200B00 10.2.2 Setting the output current limit and overcurrent protection level The current limit and overcurrent protection set points are linked, and scale together against the current sense shunt, and the gain of the current sense amplifier. The output of the current sense IC provides the IF voltage which has VIF_IL and VIF_OC thresholds for the two functions respectively. The set points are therefore defined by: I IL = Internal output capacitance: see Figure 20 External output capacitance value In the case of ceramic capacitors, the ESR can be considered low enough to push the associated zero well above the frequency of interest. Applications with high ESR capacitor may require a different type of compensation, or cascade control. The system poles and zeros of the closed loop can then be defined as follows: Powertrain pole, assuming the external capacitor ESR can be neglected: VIF _ IL RS G CS RCOUT _ EXT << and I OC = VIF _ OC RS G CS rEQ _ OUT RLOAD rEQ _ OUT + RLOAD Main pole frequency: where GCS is the gain of the current sense amplifier. 1 F P 10.2.3 Control loop compensation requirements In order to properly compensate the control loop, all components which contribute to the closed loop frequency response should be identified and understood. Figure 25 shows the AC small signal model for the module. Modulator DC transconductance gain (GPR) and powertrain equivalent resistance (rEQ_OUT) are shown. These modeling parameters will support a design cut-off frequency up to 50 kHz. Standard Bode analysis should be used for calculating the error amplifier compensation and analyzing the closed loop stability. The recommended stability criteria are as follows: 1) Phase Margin > 45 : for the closed loop response, the phase should be greater than 45 where the gain crosses 0dB. 2) Gain Margin > 10dB : The closed loop gain should be 2 rEQ _ OUT * RLOAD rEQ _ OUT + RLOAD Compensation G MB = 20 log R * (C Mid-Band OUT [1] R1 Zero: 1 [2] 2 R 3 C1 Z1 Compensation Pole: 1 C 2 R3 C1 2 C1 + C2 lower than -10dB where the phase crosses 0. 3) Gain Slope = -20dB / decade : The closed loop gain should have a slope of -20dB / decade at the crossover frequency. FP 2 = The compensation characteristics must be selected to meet these stability criteria. Refer to Figure 27 for a local sense, voltage-mode control example based on the configuration in Figure 26. In this example, it is assumed that the maximum crossover frequency (FCMAX) has been selected to occur between B and C. Type-2 compensation (Curve IJKL) is sufficient in this case. The following data must be gathered in order to proceed: Modulator Gain GPR: See Figures 17, 18, 19 Powertrain equivalent resistance rEQ: See Figures 17, 18, 19 and for FP2>>FZ1 (C1 + C2 C1): F P2 + COUT Gain: 3 Compensation F = INT 1 2 R3 C2 PRM(R) Regulator Rev 1.1 vicorpower.com Page 17 of 22 7/2015 800 927. 9474 [3] EXT ) PRM48BH480T200B00 10.2.4 Midband (R1,R3): Gain Design 10.2.5 Compensation Zero Design (C1): With reference to Figure 27: curve EFG is the: maximum output voltage in the application minimum input voltage expected in the application minimum load in the application PRM open loop response, and is where the minimum crossover frequency FCMIN occurs. Based on stability criteria, the compensation must be in the mid-band at the minimum crossover frequency, therefore FCMIN will occur where EFG is equal and opposite of GMB. C1 can be selected using Equation [2] so that FZ1 occurs prior to FCMIN. With reference to Figure 27: curve ABC is the: minimum output voltage in the application maximum input voltage expected in the application maximum load PRM open loop response, and is where the maximum crossover frequency occurs. In order for the maximum crossover frequency to occur at the design choice FCMAX, the compensation gain must be equal and opposite of the powertrain gain at this frequency. For stability purposes, the compensation should be in the Mid-band (J-K) at the crossover. Using Equation [1], the mid-band gain can be selected appropriately. C2 C1 R3 + Vref R2 R1 F1 +IN CIN_EXT CIN_INT -IN Vref VS IF RE PR RS +OUT PRM COUT_EXT COUT_INT SG -OUT I sense IC Vref IC Figure 26 - Control circuit example PRM(R) Regulator Rev 1.1 vicorpower.com Page 18 of 22 7/2015 800 927. 9474 PRM48BH480T200B00 Open Loop Gain vs. Frequency 80 60 Gain (dB) 40 20 I Application's op-amp G*BW Compensation Gain F E PRM Open Loop Min Load B A PRM Open Loop Max Load J K FCMIN 0 L FCMAX -20 C G -40 Frequency, Log scale (y-intercept is application specific) Figure 27 - Reference asymptotic Bode plot for the considered system 10.2.6 High Frequency Pole Design (C2): Using Equation [3], C2 should be selected so that FP2 is at least one decade above FCMAX and prior to the gain bandwidth product of the operational amplifier (10 MHz for this example). For applications with a higher desired crossover frequency the use of a high gain bandwidth product amplifier may be necessary to ensure that the real pole can be set at least one decade above the maximum crossover frequency. based on the ratio of the "kick" to "droop" (as defined in Fig. 28). k Vout d 10.2.7 Verifying Stability: The preferred method for verifying stability is to use a network analyzer, measuring the closed loop response across various lines and load conditions. In the absence of a network analyzer, a load step transient response can be used in order to estimate stability. Figure 28 illustrates an example of a load step response. Equation [4] can be used to predict the phase margin time Iou t time Figure 28 - load step response example and "droop" vs. "kick" definition PRM(R) Regulator Rev 1.1 vicorpower.com Page 19 of 22 7/2015 800 927. 9474 PRM48BH480T200B00 k2 ln m 100 d2 k + 2 ln d 10.3 Burst Operation: [4] Figure 20 provides the effective internal capacitance of the module. A conservative estimate of input and output peakpeak voltage ripple at nominal line and trim is provided by equation [5]: V = Mode At light loads, the PRM will operate in a burst mode due to minimum timing constraints. An example burst operation waveform is illustrated in Figure 29. For very light loads, and also for higher input voltages, the minimum time power switching cycle from the powertrain will exceed the power required by the load. In this case the external error amplifier will periodically drive PR below the switching threshold in order to maintain regulation. Switching will cease momentarily until the error amplifier once again drives PR voltage above the threshold. QTO T CINT 0.4 I FL f SW + CEXT [5] QTOT is the total input (Fig. 15) or output (Fig. 14) charge per switching cycle at full load, while CINT is the module internal effective capacitance at the considered voltage (Fig. 20) and CEXT is the external effective capacitance at the considered voltage. 10.5 Input filter stability The PRM can provide very high dynamic transients. It is therefore very important to verify that the voltage supply source as well as the interconnecting line are stable and do not oscillate. For this purpose, the converter dynamic input impedance magnitude rEQ _ IN is provided in Figures 22, 23, 24. It is recommended to provide adequate design margin with respect to the stability conditions illustrated in 10.5.1 and 10.5.2. 10.5.1 Inductive source and local, external input decoupling capacitance with negligible ESR (i.e.: ceramic type) Figure 29 - light load burst mode of operation Note that during the bursts of switching, the powertrain frequency is constant, but the number of pulses as well as the time between bursts is variable. The variability depends on many factors including input voltage, output voltages, load impedance, and external error amplifier output impedance. In burst mode, the gain of the PR input to the plant which is modeled in the previous sections is time varying. Therefore the small signal analysis can not be directly applied to burst mode operation. 10.4 Input and Output filter design Figures 14 and 15 provide the total input and output charge per cycle, as well as switching frequency, of the PRM at full load under various input and output voltages conditions. The voltage source impedance can be modeled as a series RlineLline circuit. The high performance ceramic decoupling capacitors will not significantly damp the network because of their low ESR; therefore in order to guarantee stability the following conditions must be verified: Rline > (C Lline +C IN INT Rline << rEQ _ IN IN EXT ) r [6] EQ IN [7] It is critical that the line source impedance be at least an octave lower than the converter's dynamic input resistance, [7]. However, Rline cannot be made arbitrarily low otherwise equation [6] is violated and the system will show instability, due to under-damped RLC input network. PRM(R) Regulator Rev 1.1 vicorpower.com Page 20 of 22 7/2015 800 927. 9474 PRM48BH480T200B00 10.5.2 Inductive source and local, external input decoupling capacitance with significant RCIN_EXT ESR (i.e.: electrolytic type) In order to simplify the analysis in this case, the voltage source impedance can be modeled as a simple inductor Lline. Notice that, the high performance ceramic capacitors CIN_INT within the PRM should be included in the external electrolytic capacitance value for this purpose. The stability criteria will be temperature differences among PRMs, Vin variations, and error terms in the buffering of the error amplifier output to the PR pins. Control loop compensation procedures above will hold for an array, in general, although many parameters must be scaled against the number of PRMs in the system. Please contact Vicor Applications for assistance. 10.7 rEQ _ IN > RC Lline C IN _ EXT RC [8] IN _ EXT [9]