NOT RECOMMENDED FOR NEW DESIGNS. REPLACE WITH IR3088A Data Sheet No. PD94712 IR3088 XPHASETM PHASE IC WITH FAULT AND OVERTEMP DETECT DESCRIPTION The IR3088 Phase IC combined with an IR XPhaseTM Control IC provides a full featured and flexible way to implement power solutions for the latest high performance CPUs and ASICs. The “Control” IC provides overall system control and interfaces with any number of “Phase” ICs which each drive and monitor a single phase of a multiphase converter. The XPhaseTM architecture results in a power supply that is smaller, less expensive, and easier to design while providing higher efficiency than conventional approaches. The IR3088 is intended for the following application conditions: • Excessive impedance between converter and load • Output voltage exceeding the control IC reference/VID voltage FEATURES • 2.5A Average Gate Drive Current • Loss-Less Inductor Current Sense • Internal Inductor DCR Temperature Compensation • Programmable Phase Delay • Programmable Feed-Forward Voltage Mode PWM Ramp • Sub 100ns Minimum Pulse Width supports 1MHz per-phase operation • Current Sense Amplifier drives a single wire Average Current Share Bus • Current Share Amplifier reduces PWM Ramp slope to ensure sharing between phases • Body BrakingTM disables Synchronous MOSFET for improved transient response and prevents negative output voltage at converter turn-off • Phase Fault Detection • Programmable Phase Over-Temperature Detection • Control FET driver’s 25V input voltage capability simplifies boot-strap supply design • Small thermally enhanced 20L MLPQ package CSIN- 16 17 18 PHSFLT CSIN+ GATEH PGND GATEL 14 13 12 11 VCC LGND VCCL 15 10 6 Page 1 of 34 PWMRMP ISHARE 9 VRHOT SCOMP 5 HOTSET EAIN 4 IR3088 PHASE IC RMPIN- 8 3 DACIN 20 BIASIN 2 VCCH RMPIN+ 7 1 19 PACKAGE PINOUT 9/30/04 IR3088 ORDERING INFORAMATION Device Order Quantity IR3088MTR 3000 per reel * IR3088M 100 piece strips * Samples only ABSOLUTE MAXIMUM RATINGS Operating Junction Temperature……………..150oC Storage Temperature Range………………….-65oC to 150oC ESD Rating………………………………………HBM Class 1C JEDEC standard PIN # PIN NAME VMAX VMIN ISOURCE ISINK 1 2 3 4 5 6 RMPIN+ RMPINHOTSET VRHOT ISHARE SCOMP 20V 20V 20V 20V 20V 20V -0.3V -0.3V -0.3V -0.3V -0.3V -0.3V 1mA 1mA 1mA 1mA 5mA 1mA 1mA 1mA 1mA 30mA 5mA 1mA 7 8 9 10 11 EAIN PWMRMP LGND VCC VCCL 20V 20V n/a 24V 27V -0.3V -0.3V n/a -0.3V -0.3V 1mA 1mA 50mA n/a n/a 1mA 20mA n/a 50mA 3A for 100ns, 200mA DC 12 GATEL 27V -0.3V DC, -2V for 100ns 3A for 100ns, 200mA DC 3A for 100ns, 200mA DC 13 PGND 0.3V -0.3V n/a 14 GATEH 27V -0.3V DC, -2V for 100ns 3A for 100ns, 200mA DC 3A for 100ns, 200mA DC 15 VCCH 27V -0.3V n/a 16 17 CSIN+ CSIN- 20V 20V -0.3V -0.3V 1mA 1mA 3A for 100ns, 200mA DC 1mA 1mA 18 19 20 PHSFLT DACIN BIASIN 20V 20V 20V -0.3V -0.3V -0.3V 1mA 1mA 1mA 20mA 1mA 1mA Page 2 of 34 3A for 100ns, 200mA DC 9/30/04 IR3088 ELECTRICAL SPECIFICATIONS Unless otherwise specified, these specifications apply over: 8.4V ≤ VCC ≤ 14V, 6V ≤ VCCH ≤ 25V, 6V ≤ VCCL ≤ 14V, 0 oC ≤ TJ ≤ 125 oC, CGATEH = 3.3nF, CGATEL = 6.8nF PARAMETER Gate Drivers GATEH Rise Time GATEH Fall Time GATEL Rise Time GATEL Fall Time GATEL low to GATEH high delay GATEH low to GATEL high delay Disable Pull-Down Current Current Sense Amplifier CSIN+ Bias Current CSIN- Bias Current Input Offset Voltage Gain at TJ = 25 oC Gain at TJ = 125 oC Slew Rate Differential Input Range Common Mode Input Range Rout at TJ = 25 oC Rout at TJ = 125 oC Ramp Discharge Clamp Clamp Voltage Clamp Discharge Current Page 3 of 34 TEST CONDITION VCCH = 12V, Measure 2V to 9V transition time VCCH = 12V, Measure 9V to 2V transition time VCCL = 12V, Measure 2V to 9V transition time VCCL = 12V, Measure 9V to 2V transition time VCCH = VCCL = 12V, Measure the time from GATEL falling to 1V to GATEH rising to 1V VCCH = VCCL = 12V, Measure the time from GATEH falling to 1V to GATEL rising to 1V Force GATEH or GATEL = 2V with BIASIN = 0V CSIN+ = CSIN- = DACIN. Measure input referred offset from DACIN MIN TYP MAX UNIT 22 50 ns 22 50 ns 50 75 ns 50 75 ns 10 25 50 ns 10 25 50 ns 15 25 40 µA -0.5 -0.5 -3 -0.25 -0.25 0.5 0 0 5 µA µA mV 32 27 34 29 12.5 36 31 V/V V/V Current Sense Amplifier output is an internal node. Slew rate at the ISHARE pin will be set by the internal 10kΩ resistor and any stray external capacitance Force I(PWMRMP) = 500µA. Measure V(PWMRMP) – V(DACIN) -20 0 7.9 9.3 V/µs 10.5 12.4 100 4 13.1 15.5 mV V kΩ kΩ -10 5 20 mV 4 8 mA 9/30/04 IR3088 PARAMETER TEST CONDITION MIN TYP MAX Ramp Comparator Input Offset Voltage 20 40 80 Hysteresis Note 1 -10 0 10 RMPIN+, RMPIN- Bias Current -1 -0.5 1 Propagation Delay VCCH = 12V. Measure time from RMPIN 100 150 240 input (50mV overdrive) to GATEL transition to <11V. PWM Comparator PWM Comparator Input Offset -5 5 15 Voltage EAIN & PWMRMP Bias Clamp and Current Share Adjust OFF -1 -0.4 1 Current Propagation Delay VCCH = 12V. Measure time from 70 150 PWMRMP input (50mV overdrive) to GATEH transition to < 11V. Common Mode Input Range Exceeding the Common Mode input 5 range results in 100% duty cycle Share Adjust Error Amplifier Input Offset Voltage 10 20 30 Input Voltage Range EAIN – PWMRMP, Note 1 -3.5 3.5 PWMRMP Adjust Current 4 8 Transconductance I(PWMRMP) = 3.5mA, Note 1 0.9 1.6 2.3 SCOMP Source/Sink Current Note 1 20 30 40 SCOMP Activation Voltage Amount SCOMP must increase from its 60 150 300 minimum voltage until the Ramp Slope Adjust current equals = 10µA PWMRMP Min Voltage 150 225 350 I(PWMRMP) = 500µA 0% Duty Cycle Comparator Threshold Voltage Compare to V(DACIN) 88 91 94 Propagation Delay VCCL = 12V. Measure time from EAIN < 100 150 0.9 x V(DACIN) (200mV overdrive) to GATEL transition to < 11V. Note 1. Phase Fault Comparator Threshold Voltage Compare to V(DACIN) 88 91 94 Output Voltage I(PHSFLT) = 4mA 300 400 PHSFLT Leakage Current V(PHSFLT) = 5.5V 0 10 VRHOT Comparator HOTSET Bias Current -2 -0.5 1 Output Voltage I(VRHOT) = 29mA 150 400 VRHOT Leakage Current V(VRHOT) = 5.5V 0 10 o Threshold Hysteresis TJ ≥ 85 C 3.0 7.0 9.0 MIN TYP MAX Threshold Voltage TJ ≥ 85 oC 4.73mV/ oC x TJ + 4.73mV/ oC x TJ + 4.73mV/ oC x TJ + 1.176V 1.241V 1.356V Page 4 of 34 9/30/04 UNIT mV mV µA ns mV µA ns V mV V mA A/V µA mV mV % ns % mV µA µA mV µA o C V IR3088 PARAMETER General VCC Supply Current VCCL Supply Current VCCH Supply Current TEST CONDITION MIN TYP MAX UNIT -5 -2 10 2.5 5.5 6.5 -2.5 -0.5 14 5 8 10 2 1 mA mA mA mA µA µA 6V ≤ VCCH ≤ 14V 14V ≤ VCCH ≤ 25V BIASIN Bias Current DACIN Bias Current Note 1: Guaranteed by design, but not tested in production PIN DESCRIPTION PIN# 1 2 3 PIN SYMBOL RMPIN+ RMPINHOTSET 4 VRHOT 5 ISHARE 6 SCOMP 7 EAIN 8 PWMRMP 9 10 11 12 13 14 15 16 17 18 LGND VCC VCCL GATEL PGND GATEH VCCH CSIN+ CSINPHSFLT 19 DACIN 20 BIASIN Page 5 of 34 PIN DESCRIPTION Non-inverting input to Ramp Comparator Inverting input to Ramp Comparator Inverting input to VRHOT comparator. Connect resistor divider from VBIAS to LGND to program VRHOT threshold. Diode or thermistor may be substituted for lower resistor for enhanced/remote temperature sensing. Open Collector output of the VRHOT comparator which drives low if IC junction temperature exceeds the user programmable limit. Connect external pull-up. Output of the Current Sense Amplifier and input to the Share Adjust Error Amplifier. Voltage on this pin is equal to V(DACIN) + 34 * [V(CSIN+) – V(CSIN-)]. Connecting ISHARE pins together creates a Share Bus enabling current sharing between Phase ICs. The Share bus is also used by the Control IC for voltage positioning and OverCurrent protection. Compensation for the Current Share control loop. Connect a capacitor to ground to se the control loop’s bandwidth. PWM comparator input from the error amplifier output of Control IC. Both Gate Driver outputs drive low if the voltage on this pin is less than 91% of V(DACIN). PWM comparator ramp input. Connect a resistor from this pin to the converter input voltage and a capacitor to LGND to program the PWM ramp. Signal ground and IC substrate connection Power for internal circuitry Power for Low-Side Gate Driver Low-Side Gate Driver Output and input to GATEH non-overlap comparator Return for Gate Drivers High-Side Gate Driver Output and input to GATEL non-overlap comparator Power for High-Side Gate Driver Non-inverting input to the Current Sense Amplifier Inverting input to the Current Sense Open Collector output of the Phase Fault comparator. Drives low if Phase current is unable to match the level of the SHARE bus due to an external fault. Connect externa pull-up. Reference voltage input from the Control IC. Current sensing and PWM operation referenced to this pin. System reference voltage for internal circuitry 9/30/04 IR3088 SYSTEM THEORY OF OPERATION XPhaseTM Architecture The XPhaseTM architecture is designed for multiphase interleaved buck converters which are used in applications requiring small size, design flexibility, low voltage, high current and fast transient response. The architecture can control converters of any phase number flexibility facilitates the design trade-off of multiphase converters. The scalable architecture can be applied to other applications which require high current or multiple output voltages. As shown in Figure 1, the XPhaseTM architecture consists of a Control IC and a scalable array of phase converters each using a single Phase IC. The Control IC communicates with the Phase ICs through a 5-wire analog bus, i.e. bias voltage, phase timing, average current, error amplifier output, and VID voltage. The Control IC incorporates all the system functions, i.e. VID, PWM ramp oscillator, error amplifier, bias voltage, and fault protections etc. The Phase IC implements the functions required by the converter of each phase, i.e. the gate drivers, PWM comparator and latch, over-voltage protection, and current sensing and sharing. There is no unused or redundant silicon with the XPhaseTM architecture compared to others such as a 4 phase controller that can be configured for 2, 3, or 4 phase operation. PCB Layout is easier since the 5 wire bus eliminates the need for point-to-point wiring between the Control IC and each Phase. The critical gate drive and current sense connections are short and local to the Phase ICs. This improves the PCB layout by lowering the parasitic inductance of the gate drive circuits and reducing the noise of the current sense signal. POWER GOOD PHASE FAULT VR HOT 12V ENABLE VID5 VID0 VID1 IR3081 CONTROL IC PHASE FAULT CIN >> BIAS VOLTAGE VOUT SENSE+ >> PHASE TIMING VID2 << CURRENT SENSE VID3 >> PWM CONTROL VID4 >> VID VOLTAGE CURRENT SHARE IR3088 PHASE IC VOUT+ 0.1uF COUT VOUT- PHASE HOT CCS RCS VOUT SENSE- PHASE FAULT CURRENT SHARE IR3088 PHASE IC 0.1uF PHASE HOT CCS CONTROL BUS RCS ADDITIONAL PHASES INPUT/OUTPUT Figure 1. System Block Diagram Page 6 of 34 9/30/04 IR3088 PWM Control Method The PWM block diagram of the XPhaseTM architecture is shown in Figure 2. Feed-forward voltage mode control with trailing edge modulation is used. A high-gain wide-bandwidth voltage type error amplifier in the Control IC is used for the voltage control loop. An external RC circuit connected to the input voltage and ground is used to program the slope of the PWM ramp and to provide the feed-forward control at each phase. The PWM ramp slope will change with the input voltage and automatically compensate for changes in the input voltage. The input voltage can change due to variations in the silver box output voltage or due to the wire and PCB-trace voltage drop related to changes in load current. VIN CONTROL IC VPEAK RAMPIN+ RMPOUT RPHS1 VVALLEY PWM LATCH CLOCK PULSE GENERATOR RAMPIN- - EAIN VBIAS + - + + - RVFB X 0.91 + ISHARE FB 10K CURRENT SENSE AMPLIFIER 20mV IROSC X34 RDRP VDRP AMP CSIN+ + IFB VOSNS- CSCOMP - SHARE ADJUST ERROR AMPLIFIER BODY BRAKING COMPARATOR + RAMP DISCHARGE CLAMP SCOMP + ERROR AMP GND - CPWMRMP EAOUT GATEL ENABLE PWMRMP VOSNSVDAC VOUT COUT R + - RPWMRMP VOSNS+ RESET DOMINANT + RPHS2 VDAC VBIAS REGULATOR GATEH S PWM COMPARATOR - RAMP GENERATOR + 50% DUTY CYCLE PHASE IC SYSTEM REFERENCE VOLTAGE BIASIN CCS RCS CCS RCS CSIN- DACIN VDRP + - IIN RAMPIN+ PWM LATCH CLOCK PULSE GENERATOR + RPHS1 PHASE IC SYSTEM REFERENCE VOLTAGE BIASIN - RAMPIN- GATEH S PWM COMPARATOR - EAIN RESET DOMINANT R GATEL + RPHS2 ENABLE PWMRMP + RPWMRMP - X 0.91 - + SHARE ADJUST ERROR AMPLIFIER + ISHARE 10K 20mV CURRENT SENSE AMPLIFIER - CSIN+ + X34 - CSCOMP - CPWMRMP BODY BRAKING COMPARATOR + RAMP DISCHARGE CLAMP SCOMP CSIN- DACIN Figure 2. PWM Block Diagram Frequency and Phase Timing Control An oscillator with programmable frequency is located in the Control IC. The output of the oscillator is a 50% duty cycle triangle waveform with peak and valley voltages of approximately 5V and 1V respectively. This signal is used to program both the switching frequency and phase timing of the Phase ICs. The Phase IC is programmed by resistor divider RPHS1 and RPHS2 connected between the VBIAS reference voltage and the Phase IC LGND pin. A comparator in the Phase ICs detects the crossing of the oscillator waveform over the voltage generated by the resistor divider and triggers a clock pulse that starts the PWM cycle. The peak and valley voltages track the VBIAS voltage reducing potential Phase IC timing errors. Figure 3 shows the Phase timing for an 8 phase converter. Note that both slopes of the triangle waveform can be used for phase timing by swapping the RMPIN+ and RMPIN– pins, as shown in Figure 2. Page 7 of 34 9/30/04 IR3088 50% RAMP DUTY CYCLE RAMP (FROM CONTROL IC) SLOPE = 80mV / % DC VPEAK (5.0V) VPHASE4&5 (4.5V) SLOPE = 1.6mV / ns @ 200kHz SLOPE = 8.0mV / ns @ 1MHz VPHASE3&6 (3.5V) VPHASE2&7 (2.5V) VPHASE1&8 (1.5V) VVALLEY (1.00V) CLK1 PHASE IC CLOCK PULSES CLK2 CLK3 CLK4 CLK5 CLK6 CLK7 CLK8 Figure 3. 8 Phase Oscillator Waveforms PWM Operation The PWM comparator is located in the Phase IC. Upon receiving a clock pulse, the PWM latch is set; the PWMRMP voltage begins to increase; the low side driver is turned off, and the high side driver is then turned on after the nonoverlap time. When the PWMRMP voltage exceeds the Error Amplifier’s output voltage, the PWM latch is reset. This turns off the high side driver and then turns on the low side driver after the non-overlap time; it activates the Ramp Discharge Clamp, which quickly discharges the PWMRMP capacitor to the VDAC voltage of the Control IC until the next clock pulse. The PWM latch is reset dominant allowing all phases to go to zero duty cycle within a few tens of nanoseconds in response to a load step decrease. Phases can overlap and go to 100% duty cycle in response to a load step increase with turn-on gated by the clock pulses. An Error Amplifier output voltage greater than the common mode input range of the PWM comparator results in 100% duty cycle regardless of the voltage of the PWM ramp. This arrangement guarantees the Error Amplifier is always in control and can demand 0 to 100% duty cycle as required. It also favors response to a load step decrease which is appropriate given the low output to input voltage ratio of most systems. The inductor current will increase much more rapidly than decrease in response to load transients. This control method is designed to provide “single cycle transient response” where the inductor current changes in response to load transients within a single switching cycle maximizing the effectiveness of the power train and minimizing the output capacitor requirements. An additional advantage of the architecture is that differences in ground or input voltage at the phases have no effect on operation since the PWM ramps are referenced to VDAC. Figure 4 depicts PWM operating waveforms under various conditions. Page 8 of 34 9/30/04 IR3088 PHASE IC CLOCK PULSE EAIN PWMRMP VDAC 91% VDAC GATEH GATEL STEADY-STATE OPERATION DUTY CYCLE INCREASE DUE TO LOAD INCREASE DUTY CYCLE DECREASE DUE TO VIN INCREASE (FEED-FORWARD) DUTY CYCLE DECREASE DUE TO LOAD DECREASE (BODY BRAKING) OR FAULT (VCC UV, VCCVID UV, OCP, VID=11111X) STEADY-STATE OPERATION Figure 4. PWM Operating Waveforms Body BrakingTM In a conventional synchronous buck converter, the minimum time required to reduce the current in the inductor in response to a load step decrease is; TSLEW = L * ( I MAX − I MIN ) VO The slew rate of the inductor current can be significantly increased by turning off the synchronous rectifier in response to a load step decrease. The switch node voltage is then forced to decrease until conduction of the synchronous rectifier’s body diode occurs. This increases the voltage across the inductor from Vout to Vout + VBODYDIODE. The minimum time required to reduce the current in the inductor in response to a load transient decrease is now; TSLEW = L * ( I MAX − I MIN ) VO + VBODYDIODE Since the voltage drop in the body diode is often higher than output voltage, the inductor current slew rate can be increased by 2X or more. This patent pending technique is referred to as “body braking” and is accomplished through the “0% Duty Cycle Comparator” located in the Phase IC. If the Error Amplifier’s output voltage drops below 91% of the VDAC voltage this comparator turns off the low side gate driver. Lossless Average Inductor Current Sensing Inductor current can be sensed by connecting a resistor and a capacitor in parallel with the inductor and measuring the voltage across the capacitor, as shown in Figure 5. The equation of the sensing network is, vC ( s) = vL ( s) 1 RL + sL = iL ( s) 1 + sRCS CCS 1 + sRCS CCS Usually the resistor Rcs and capacitor Ccs are chosen so that the time constant of Rcs and Ccs equals the time constant of the inductor which is the inductance L over the inductor DCR (RL). If the two time constants match, the voltage across Ccs is proportional to the current through L, and the sense circuit can be treated as if only a sense resistor with the value of RL was used. The mismatch of the time constants does not affect the measurement of inductor DC current, but affects the AC component of the inductor current. Page 9 of 34 9/30/04 IR3088 vL iL Current Sense Amp L RL RCS CCS VO CO vCc S CSOUT Figure 5. Inductor Current Sensing and Current Sense Amplifier The advantage of sensing the inductor current versus high side or low side sensing is that actual output current being delivered to the load is obtained rather than peak or sampled information about the switch currents. The output voltage can be positioned to meet a load line based on real time information. Except for a sense resistor in series with inductor, this is the only sense method that can support a single cycle transient response. Other methods provide no information during either load increase (low side sensing) or load decrease (high side sensing). An additional problem associated with peak or valley current mode control for voltage positioning is that they suffer from peak-to-average errors. These errors will show in many ways but one example is the effect of frequency variation. If the frequency of a particular unit is 10% low, the peak to peak inductor current will be 10% larger and the output impedance of the converter will drop by about 10%. Variations in inductance, current sense amplifier bandwidth, PWM prop delay, any added slope compensation, input voltage, and output voltage are all additional sources of peak-to-average errors. Current Sense Amplifier This is a high speed differential current sense amplifier, as shown in Figure 5. Its gain decreases with increasing temperature and is nominally 34 at 25ºC and 29 at 125ºC (-1470 ppm/ºC). This reduction of gain tends to compensate the 3850 ppm/ºC increase in inductor DCR. Since in most designs the Phase IC junction is hotter than the inductor these two effects tend to cancel such that no additional temperature compensation of the load line is required. The current sense amplifier can accept positive differential input up to 100mV and negative up to -20mV before clipping. The output of the current sense amplifier is summed with the DAC voltage and sent to the Control IC and other Phases through an on-chip 10KΩ resistor connected to the ISHARE pin. The ISHARE pins of all the phases are tied together and the voltage on the share bus represents the average current being delivered to the load and is used by the Control IC for voltage positioning and current limit protection. Average Current Share Loop Current sharing between phases of the converter is achieved by the average current share loop in each Phase IC. The output of the current sense amplifier is compared with the share bus less a 20mV offset. If current in a phase is smaller than the average current, the share adjust error amplifier of the phase will activate a current source that reduces the slope of its PWM ramp thereby increasing its duty cycle and output current. The crossover frequency of the current share loop can be programmed with a capacitor at the SCOMP pin so that the share loop does not interact with the output voltage loop. Page 10 of 34 9/30/04 IR3088 IR3088 THEORY OF OPERATION Block Diagram The Block diagram of the IR3088 is shown in Figure 6, and specific features are discussed in the following sections. PWM LATCH RAMP COMPARATOR S PWM COMPARATOR - EAIN - - R + SYSTEM REFERENCE VOLTAGE BIASIN GATEH RESET DOMINANT + GATE NON-OVERLAP COMPARATORS ENABLE + + PWMRMP - RAMP DISCHARGE CLAMP - RAMP SLOPE ADJUST 2V - + RMPIN- VCCH + RMPIN+ CLOCK PULSE GENERATOR VCCL GATEL + SCOMP - X 0.91 - + SHARE ADJUST ERROR AMP ISHARE PGND INTERNAL CIRCUIT BIAS VCC + + X34 VRHOT COMPARATOR VOLTAGE PROPORTIONAL TO ABSOLUTE TEMPERATURE - - CSINPHSFLT + + - HOTSET + CSIN+ - - LGND DACIN CURRENT SENSE AMP + + 20mV 10K VRHOT 0% DUTY CYCLE COMPARATOR FAULT COMPARATOR Figure 6. IR3088 Block Diagram Tri-State Gate Drivers The gate drivers can deliver up to 3A peak current. An adaptive non-overlap circuit monitors the voltage on the GATEH and GATEL pins to prevent MOSFET shoot-through current while minimizing body diode conduction. An Enable signal is provided by the Control IC to the Phase IC without the addition of a dedicated signal line. The Error Amplifier output of the Control IC drives low in response to any fault condition such as input under voltage or output overload. The IR3088 0% duty cycle comparator detects this and drives both gate outputs low. This tri-state operation prevents negative inductor current and negative output voltage during power-down. The Gate Drivers revert to a high impedance “off” state if VCCL and VCCH supply voltages are below the normal operating range. An 80kΩ resistor is connected across the GATEH/GATEL and PGND pins to prevent the GATEH/GATEL voltage from rising due to leakage or other causes under these conditions. Thermal Monitoring (VRHOT) The IR3088 senses its own die temperature and produces a voltage at the input of the VRHOT comparator that is proportional to temperature. An external resistor divider connected from VBIAS to the HOTSET pin and ground can be used to program the thermal trip point of the VRHOT comparator. The VRHOT pin is an open-collector output and should be pulled up to a voltage source through a resistor. If the thermal trip point is reached the VRHOT output drives low. Page 11 of 34 9/30/04 IR3088 Phase Fault It is possible for multiphase converters to appear to be working correctly with one or more phases not functioning. The output voltage can still be regulated and the full load current may still be delivered. However, the remaining phase(s) will be stressed far beyond their intended design limits and are likely to fail. Loss of a phase can occur due to poor solder connections or mounting during the manufacturing process, or can occur in the field. The most common failure mode of a buck converter is failure of the high side MOSFET. The IR3088 has the ability to detect if a phase stops switching and can provide this information to the system through the PHSFLT output pin. If a phase stops switching its output current will drop to zero and the output of its IR3088 current sense amplifier will be the DACIN voltage. The Share Adjust Amplifier reacts to this by increasing the Ramp Slope Adjust current until it exceeds the externally programmable PWM Ramp bias current. This will cause the voltage at the PWMRMP pin to drop below its normal operating range. The Fault Comparator trips and drives the PHSFLT output to ground when the voltage on the PWMRMP pin falls below 91% of the DACIN voltage. PHSFLT is an open-collector output and should be pulled up to a voltage source through a resistor. APPLICATIONS INFORMATION POWERGOOD VRHOT PHASE FAULT 12V RCSRVCC 10 ohm RCP CSIN+ 18 19 16 20 17 CSIN- DACIN BIASIN DBST PGND GATEL VCCL CIN VOUT SENSE+ 15 14 13 VOUT+ 12 DISTRIBUTION IMPEDANCE 11 COUT VCC VOUT10 LGND SCOMP 6 CSCOMP GATEH CVCCL RVCC VOUT SENSE- RPWMRMP CVCC CCP RCS- 17 CCP1 CSIN+ 16 18 19 17 CSIN- DACIN 20 DBST ISHARE SCOMP 6 CSCOMP PGND GATEL VCCL CIN 15 14 13 12 11 VCC VRHOT GATEH 10 HOTSET RPHASE22 RPHASE23 PHSFLT BIASIN 4 LGND RSHARE 5 CVDAC RCS+ CBST VCCH IR3088 PHASE IC RMPIN- PWMRMP 3 RVDAC RMPIN+ EAIN 2 9 1 CCS+ CCS- RBIASIN 7 VDAC 15 CPWMRMP 8 16 ROCSET ROSC ISHARE RPHASE21 RPHASE31 19 14 ROSC OCSET TRM4 VID4 21 20 18 VRHOT RPHASE21 25 23 LGND RMPOUT 24 26 27 N/C SS/DEL PWRGD 22 FB VDRP IIN 8 RFB VCC EAOUT VID3 13 7 VID2 12 VID4 ENABLE 28 6 VID1 TRM3 VID3 IR3081 CONTROL IC VID0 VOSNS- 5 TRM2 4 VID2 BBFB TRM1 3 VID1 VBIAS VID5 9 VID0 OSCDS 11 2 10 1 VID5 RFB1 HOTSET PWMRMP 5 CFB 0.1uF RCS+ CBST VCCH IR3088 PHASE IC RMPIN- EAIN 4 1nF ENABLE RMPIN+ 9 CSS/DEL 3 7 2 CPWMRMP 8 1 PHSFLT RPHASE11 RSS/DEL RFB22 CCS+ CCS- RBIASIN RFB21 CVCC 0.1uF CVCCL RVCC RPWMRMP CVCC RCS- RPHASE33 CSIN+ 16 19 18 20 17 CSIN- DACIN BIASIN DBST GATEH PGND GATEL CIN 15 14 13 12 11 VCC VCCL 10 SCOMP 6 CSCOMP EAIN ISHARE LGND VRHOT PWMRMP HOTSET RPHASE32 5 9 4 RCS+ CBST VCCH IR3088 PHASE IC RMPIN- 7 3 RMPIN+ CPWMRMP 8 2 PHSFLT RPHASE31 1 CCS+ CCS- RBIASIN CVCCL RVCC RPWMRMP CVCC Figure 7. IR3081/3088 12V to 2.5V 3 Phase Converter Page 12 of 34 9/30/04 IR3088 POWERGOOD VRHOT PHASE FAULT 12V RCS- VGATE RVCC 10 ohm QGATE CCS+ CCS- 19 RCP 18 16 CSIN+ 19 17 CSIN- 20 18 DACIN PHSFLT CIN PGND GATEL VOUT SENSE+ 15 14 L 13 VOUT+ 12 DISTRIBUTION IMPEDANCE 11 COUT VCC VOUT10 LGND VCCL 9 SCOMP CSCOMP GATEH CVCCL RVCC VOUT SENSE- RPWMRMP CVCC CCP RCS- 17 CCS+ CCP1 16 CSIN+ 17 CSIN- 19 CIN GATEH PGND GATEL 15 14 L 13 12 11 VCC 10 SCOMP CSCOMP DBST CBST VCCL LGND ISHARE 9 VRHOT 6 RPHASE23 CVDAC HOTSET RCS+ RCS+ VCCH IR3088 PHASE IC RMPIN- RPHASE22 5 18 20 4 DACIN BIASIN 3 RSHARE RMPIN+ PWMRMP 2 EAIN 1 PHSFLT RDRP RVDAC CCS- 20k 7 RBIASIN VDAC 15 CPWMRMP 8 RDRP1 CDRP 16 ROCSET ROSC ISHARE 6 RPHASE13 RBBDRP 14 ROSC 13 8 OCSET 21 20 VRHOT RPHASE21 23 LGND 25 26 27 24 RMPOUT SS/DEL N/C PWRGD 22 FB VDRP IIN VID4 RFB VCC EAOUT VID3 TRM4 VID4 7 VID2 12 6 BBFB VID1 TRM3 VID3 VBIAS IR3081 CONTROL IC VID0 RBBFB RFB1 VID5 VOSNS- 5 TRM2 4 VID2 TRM1 3 VID1 9 VID0 OSCDS 11 2 10 1 ENABLE 28 0.1uF HOTSET RPHASE12 5 ENABLE VID5 BIASIN 4 CFB CBST VCCH IR3088 PHASE IC RMPIN- PWMRMP 3 RMPIN+ EAIN CSS/DEL 2 1nF DBST 7 1 RCS+ 20k CPWMRMP 8 RBIASIN DGATE RPHASE11 RGATE RSS/DEL CVCC 0.1uF CVCCL RVCC RPWMRMP CVCC RCSCCS+ CCSRBIASIN RCS+ 20k 16 CSIN+ 17 CSIN- 19 18 PGND GATEL CIN 15 14 L 13 12 11 VCC 10 SCOMP CSCOMP GATEH VCCL LGND ISHARE 9 VRHOT 6 RPHASE33 DACIN BIASIN HOTSET RPHASE32 5 PWMRMP 4 VCCH IR3088 PHASE IC RMPIN- EAIN 3 RMPIN+ 7 2 CPWMRMP 8 1 PHSFLT RPHASE31 20 DBST CBST CVCCL RVCC RPWMRMP CVCC RCSCCS+ CCS- 16 CSIN+ 17 18 19 PGND GATEL CIN 15 14 L 13 12 11 VCC 10 SCOMP CSCOMP GATEH VCCL LGND ISHARE 9 VRHOT 6 RPHASE43 CSIN- BIASIN HOTSET RPHASE42 5 CBST VCCH IR3088 PHASE IC RMPIN- PWMRMP 4 RMPIN+ EAIN 3 7 2 DACIN 20 DBST PHSFLT RPHASE41 1 RCS+ 20k CPWMRMP 8 RBIASIN CVCCL RVCC RPWMRMP CVCC RCSCCS+ CCS- 16 CSIN+ 17 18 19 CIN PGND GATEL 15 14 L 13 12 11 VCC 10 SCOMP CSCOMP GATEH VCCL LGND ISHARE 9 VRHOT 6 RPHASE53 CSIN- BIASIN HOTSET RPHASE52 5 PWMRMP 4 CBST VCCH IR3088 PHASE IC RMPIN- EAIN 3 RMPIN+ 7 2 DACIN 20 DBST PHSFLT RPHASE51 1 RCS+ 20k CPWMRMP 8 RBIASIN CVCCL RVCC RPWMRMP CVCC RCSCCS+ RBIASIN CCS- 20k RCS+ RPHASE63 16 CSIN+ 17 CSIN- 19 18 DACIN BIASIN PGND GATEL VCCL 15 14 L 13 12 11 VCC SCOMP 6 CSCOMP GATEH 10 ISHARE LGND VRHOT 9 HOTSET RPHASE62 5 PWMRMP 4 CIN VCCH IR3088 PHASE IC RMPIN- EAIN 3 RMPIN+ 7 2 CPWMRMP 8 1 PHSFLT RPHASE61 20 DBST CBST CVCCL RVCC RPWMRMP CVCC Figure 8. IR3081/3088 Six Phase VRM/EVRD 10 Converter Page 13 of 34 9/30/04 IR3088 DESIGN PROCEDURES - IR3081 AND IR3088 CHIPSET IR3081 EXTERNAL COMPONENTS Oscillator Resistor Rosc The oscillator of IR3081 generates a triangle waveform to synchronize the phase ICs, and the switching frequency of the each phase converter equals the oscillator frequency, which is set by the external resistor ROSC according to the curve in Figure 13 of IR3081 Data Sheet. Soft Start Capacitor CSS/DEL and Resistor RSS/DEL Because the capacitor CSS/DEL programs four different time parameters, i.e. soft start delay time, soft start time, over-current latch delay time, and power good delay time, they should be considered together while choosing CSS/DEL. The SS/DEL pin voltage controls the slew rate of the converter output voltage, as shown in Figure 10 of IR3081 Data Sheet. After the ENABLE pin voltage rises above 0.6V, there is a soft-start delay time tSSDEL, after which the error amplifier output is released to allow the soft start. The soft start time tSS represents the time during which converter voltage rises from zero to VO. tSS can be programmed by an external capacitor, which is determined by Equation (1). I *t 70 * 10 −6 * t SS (1) C SS / DEL = CHG SS = VO VO Once CSS/DEL is chosen, the soft start delay time tSSDEL, the over-current fault latch delay time tOCDEL, and the delay time tVccPG from output voltage (VO) in regulation to Power Good are fixed and shown in Equations (2), (3) and (4) respectively. *1.3 CSS / DEL *1.3 C (2) = tSSDEL = SS / DEL I CHG 70 *10− 6 t OCDEL = tVccPG = C SS / DEL * 0.09 C SS / DEL * 0.09 = I DISCHG 6 *10 −6 CSS / DEL * (3.91 − VO − 1.3) CSS / DEL * (3.91 − VO − 1.3) = I CHG 70 *10−6 (3) (4) If faster over-current protection is required, a resistor in series with the soft start capacitor CSS/DEL can be used to reduce the over-current fault latch delay time tOCDEL, and the resistor RSS/DEL is determined by Equation (5). Equation (1) for soft start capacitor CSS/DEL and Equation (4) for power good delay time tVccPG are unchanged, while the equation for soft start delay time tSS/DEL (Equation 2) is changed to Equation (6). Considering the worst case values of charge and discharge current, RSS/DEL should be no grater than 10kΩ. 0.09 − RSS / DEL = tSSDEL = tOCDEL * I DISCHG t ∗ 6 *10−6 0.09 − OCDEL CSS / DEL CSS / DEL = I DISCHG 6 *10−6 (5) CSS / DEL * (1.3 − RSS / DEL ∗ I CHG ) CSS / DEL * (1.3 − RSS / DEL * 70 *10−6 ) (6) = I CHG 70 *10−6 VDAC Slew Rate Programming Capacitor CVDAC and Resistor RVDAC The slew rate of VDAC down-slope SRDOWN can be programmed by the external capacitor CVDAC as defined in Equation (7), where ISINK is the sink current of VDAC pin as shown in Figure 15 of IR3081 Data Sheet. The resistor RVDAC is used to compensate VDAC circuit and is determined by Equation (8). The slew rate of VDAC up-slope Page 14 of 34 9/30/04 IR3088 SRUP is proportional to that of VDAC down-slope and is given by Equation (9), where ISOURCE is the source current of VDAC pin as shown in Figure15 of IR3081 Data Sheet. CVDAC = I SINK SR DOWN RVDAC = 0.5 + SRUP = 3.2 ∗ 10 −15 CVDAC 2 I SOURCE CVDAC (7) (8) (9) Over Current Setting Resistor ROCSET The inductor DC resistance is utilized to sense the inductor current. The copper wire of inductor has a constant temperature coefficient of 3850 ppm/°C, and therefore the maximum inductor DCR can be calculated from Equation (10), where RL_MAX and RL_ROOM are the inductor DCR at maximum temperature TL_MAX and room temperature T_ROOM respectively. R L _ MAX = R L _ ROOM ∗ [1 + 3850 * 10 −6 ∗ (T L _ MAX − TROOM )] (10) The current sense amplifier gain of IR3088 decreases with temperature at the rate of 1470 ppm/°C, which compensates part of the inductor DCR increase. The phase IC die temperature is only a couple of degrees Celsius higher than the PCB temperature due to the low thermal impedance of MLPQ package. The minimum current sense amplifier gain at the maximum phase IC temperature TIC_MAX is calculated from Equation (11). GCS _ MIN = GCS _ ROOM ∗ [1 − 1470 * 10 −6 ∗ (TIC _ MAX − TROOM )] (11) The total input offset voltage (VCS_TOFST) of current sense amplifier in phase ICs is the sum of input offset (VCS_OFST) of the amplifier itself and that created by the amplifier input bias currents flowing through the current sense resistors RCS+ and RCS-. VCS _ TOFST = VCS _ OFST + I CSIN + ∗ RCS + − I CSIN − ∗ RCS − (12) The over current limit is set by the external resistor ROCSET as defined in Equation (13), where ILIMIT is the required over current limit. IOCSET, the bias current of OCSET pin, changes with switching frequency setting resistor ROSC and is determined by the curve in Figure 14 of IR3081 Data Sheet. KP is the ratio of inductor peak current over average current in each phase and is calculated from Equation (14). ROCSET = [ KP = I LIMIT ∗ RL _ MAX ∗ (1 + K P ) + VCS _ TOFST ] ∗ GCS _ MIN / I OCSET n (VI − VO ) ∗ VO /( L ∗ VI ∗ f SW ∗ 2) IO / n (13) (14) No Load Output Voltage Setting Resistor RFB and Adaptive Voltage Positioning Resistor RDRP A resistor between FB pin and the converter output is used to create output voltage offset VO_NLOFST, which is the difference between VDAC voltage and output voltage at no load condition. Adaptive voltage positioning further lowers the converter voltage by RO*IO, where RO is the required output impedance of the converter. RFB is not only determined by IFB, the current flowing out of FB pin as shown in Figure 14 of IR3081 Data Sheet, but also affected by the adaptive voltage positioning resistor RDRP and total input offset voltage of current sense amplifiers. RFB and RDRP are determined by (15) and (16) respectively. Page 15 of 34 9/30/04 IR3088 R FB = R L _ MAX ∗ VO _ NLOFST − VCS _ TOFST ∗ n ∗ RO R DRP = I FB ∗ R L _ MAX R FB ∗ R L _ MAX ∗ GCS _ MIN (15) (16) n ∗ RO Body BrakingTM Related Resistors RBBFB and RBBDRP The body brakingTM during Dynamic VID can be disabled by connecting BBFB pin to ground. If the feature is enabled, Resistors RBBFB and RBBDRP are needed to restore the feedback voltage of the error amplifier after Dynamic VID step down. Usually RBBFB and RBBDRP are chosen to match RFB and RDRP respectively. IR3088 EXTERNAL COMPONENTS PWM Ramp Resistor RPWMRMP and Capacitor CPWMRMP PWM ramp is generated by connecting the resistor RPWMRMP between a voltage source and PWMRMP pin as well as the capacitor CPWMRMP between PWMRMP and LGND. Choose the desired PWM ramp magnitude VRAMP and the capacitor CPWMRMP in the range of 100pF and 470pF, and then calculate the resistor RPWMRMP from Equation (17). To achieve feed-forward voltage mode control, the resistor RRAMP should be connected to the input of the converter. RPWMRMP = VIN * f SW * CPWMRMP * [ln(VIN VO − VDAC ) − ln(VIN − VDAC − VPWMRMP )] (17) Inductor Current Sensing Capacitor CCS+ and Resistors RCS+ and RCSThe DC resistance of the inductor is utilized to sense the inductor current. Usually the resistor RCS+ and capacitor CCS+ in parallel with the inductor are chosen to match the time constant of the inductor, and therefore the voltage across the capacitor CCS+ represents the inductor current. If the two time constants are not the same, the AC component of the capacitor voltage is different from that of the real inductor current. The time constant mismatch does not affect the average current sharing among the multiple phases, but affect the current signal ISHARE as well as the output voltage during the load current transient if adaptive voltage positioning is adopted. Measure the inductance L and the inductor DC resistance RL. Pre-select the capacitor CCS+ and calculate RCS+ as follows. RCS + = L RL C CS + (18) The bias current flowing out of the non-inverting input of the current sense amplifier creates a voltage drop across RCS+, which is equivalent to an input offset voltage of the current sense amplifier. The offset affects the accuracy of converter current signal ISHARE as well as the accuracy of the converter output voltage if adaptive voltage positioning is adopted. To reduce the offset voltage, a resistor RCS- should be added between the amplifier inverting input and the converter output. RCS − = RCS + (19) If RCS- is not used, RCS+ should be chosen so that the offset voltage is small enough. Usually RCS+ should be less than 2 kΩ and therefore a larger CCS+ value is needed. Page 16 of 34 9/30/04 IR3088 Over Temperature Setting Resistors RHOTSET1 and RHOTSET2 The threshold voltage of VRHOT comparator is proportional to the die temperature TJ (ºC) of phase IC. Determine the relationship between the die temperature of phase IC and the temperature of the power converter according to the power loss, PCB layout and airflow etc, and then calculate HOTSET threshold voltage corresponding to the allowed maximum temperature from Equation (20). V HOTSET = 4.73 * 10 −3 * T J + 1.241 (20) There are two ways to set the over temperature threshold, central setting and local setting. In the central setting, only one resistor divider is used, and the setting voltage is connected to HOTSET pins of all the phase ICs. To reduce the influence of noise on the accuracy of over temperature setting, a 0.1uF capacitor should be placed next to HOTSET pin of each phase IC. In the local setting, a resistor divider per phase is needed, and the setting voltage is connected to HOTSET pin of each phase. The 0.1uF decoupling capacitor is not necessary. Use VBIAS as the reference voltage. If RHOTSET1 is pre-selected, RHOTSET2 can be calculated as follows. RHOTSET 2 = RHOTSET 1 ∗ VHOTSET VBIAS − VHOTSET (21) Phase Delay Timing Resistors RPHASE1 and RPHASE2 The phase delay of the interleaved multiphase converter is programmed by the resistor divider connected at RMPIN+ or RMPIN- depending on which slope of the oscillator ramp is used for the phase delay programming of phase IC, as shown in Figure 3. If the upslope is used, RMPIN+ pin of the phase IC should be connected to RMPOUT pin of the control IC and RMPIN- pin should be connected to the resistor divider. When RMPOUT voltage is above the trip voltage at RMPIN- pin, the PWM latch is set. GATEL becomes low, and GATEH becomes high after the non-overlap time. If down slope is used, RMPIN- pin of the phase IC should be connected to RMPOUT pin of the control IC and RMPIN+ pin should be connected to the resistor divider. When RMPOUT voltage is below the trip voltage at RMPIN- pin, the PWM latch is set. GATEL becomes low, and GATEH becomes high after the non-overlap time. Use VBIAS voltage as the reference for the resistor divider since the oscillator ramp magnitude from control IC tracks VBIAS voltage. Try to avoid both edges of the oscillator ramp for better noise immunity. Determine the ratio of the programming resistors corresponding to the desired switching frequencies and phase numbers. If the resistor RPHASEx1 is pre-selected, the resistor RPHASEx2 is determined as: R PHASEx 2 = RAPHASEx ∗ R PHASEx1 1 − RAPHASEx (22) Combined Over Temperature and Phase Delay Setting Resistors RPHASE1, RPHASE2 and RPHASE3 The over temperature setting resistor divider can be combined with the phase delay resistor divider to save one resistor per phase. Calculate the HOTSET threshold voltage VHOTSET corresponding to the allowed maximum temperature from Equation (20). If the over temperature setting voltage is lower than the phase delay setting voltage, VBIAS*RAPHASEx, connect RMPIN+ or RMPIN- pin between RPHASEx1 and RPHASEx2, and connect HOTSET pin between RPHASEx2 and RPHASEx3. Pre-select RPHASEx1, Page 17 of 34 RPHASEx 2 = ( RAPHASEx ∗ VBIAS − VHOTSET ) * RPHASEx1 VBIAS ∗ (1 − RAPHASEx ) (23) RPHASEx3 = VHOTSET ∗ RPHASEx1 VBIAS * (1 − RAPHASEx ) (24) 9/30/04 IR3088 If the over temperature setting voltage is higher than the phase delay setting voltage, VBIAS*RAPHASEx, connect HOTSET pin between RPHASEx1 and RPHASEx2, and connect RMPIN+ or RMPIN- between RPHASEx2 and RPHASEx3. Pre-select RPHASEx1, R PHASEx 2 = (V HOTSET − RAPHASEx ∗ V BIAS ) ∗ R PHASEx1 V BIAS − V HOTSET (25) RPHASEx3 = RAPHASEx ∗ VBIAS * RPHASEx1 VBIAS − VHOTSET (26) Bootstrap Capacitor CBST Depending on the duty cycle and gate drive current of the phase IC, a 0.1uF to 1uF capacitor is needed for the bootstrap circuit. Decoupling Capacitors for Phase IC 0.1uF-1uF decoupling capacitors are required at VCC and VCCL pins of phase ICs. VOLTAGE LOOP COMPENSATION The adaptive voltage positioning (AVP) is usually adopted in the computer applications to improve the transient response and reduce the power loss at heavy load. Like current mode control, the adaptive voltage positioning loop introduces extra zero to the voltage loop and splits the double poles of the power stage, which make the voltage loop compensation much easier. Resistors RFB and RDRP are chosen according to Equations (15) and (16), and the selection of compensation types depends on the output capacitors used in the converter. For the applications using Electrolytic, Polymer or ALPolymer capacitors and running at lower frequency, type II compensation shown in Figure 9(a) is usually enough. While for the applications using only ceramic capacitors and running at higher frequency, type III compensation shown in Figure 9(b) is preferred. For applications where AVP is not required, the compensation is the same as for the regular voltage mode control. For converter using Polymer, AL-Polymer, and ceramic capacitors, which have much higher ESR zero frequency, type III compensation is required as shown in Figure 9(b) with RDRP and CDRP removed. CCP1 CCP1 RFB VO+ RCP VO+ RFB FB CCP RDRP VDAC CFB FB - EAOUT EAOUT VDRP RFB1 CCP RCP + (a) Type II compensation EAOUT VDRP RDRP VDAC EAOUT + CDRP (b) Type III compensation Figure 9. Voltage loop compensation network Type II Compensation for AVP Applications Determine the compensation at no load, the worst case condition. Choose the crossover frequency fc between 1/10 and 1/5 of the switching frequency per phase. Assume the time constant of the resistor and capacitor across the output inductors matches that of the inductor, and determine RCP and CCP from Equations (27) and (28), where LE and CE are the equivalent inductance of output inductors and the equivalent capacitance of output capacitors respectively. Page 18 of 34 9/30/04 IR3088 (2π ∗ fC ) 2 ∗ LE ∗ CE ∗ RFB ∗ VPWMRMP RCP = VO * 1 + (2π * fC * C * RC ) 2 10 ∗ L E ∗ C E C CP = (27) (28) RCP CCP1 is optional and may be needed in some applications to reduce the jitter caused by the high frequency noise. A ceramic capacitor between 10pF and 220pF is usually enough. Type III Compensation for AVP Applications Determine the compensation at no load, the worst case condition. Assume the time constant of the resistor and capacitor across the output inductors matches that of the inductor, the crossover frequency and phase margin of the voltage loop can be estimated by Equations (29) and (30), where RLE is the equivalent resistance of inductor DCR. f C1 = RDRP 2π * CE ∗ GCS * RFB ∗ RLE θ C1 = 90 − A tan(0.5) ∗ (29) 180 (30) π Choose the desired crossover frequency fc around fc1 estimated by Equation (29) or choose fc between 1/10 and 1/5 of the switching frequency per phase, and select the components to ensure the slope of close loop gain is -20dB /Dec around the crossover frequency. Choose resistor RFB1 according to Equation (31), and determine CFB and RDRP from Equations (32) and (33). 1 R FB 2 R FB1 = CFB = to R FB1 = 2 R FB 3 1 4π ∗ fC ∗ RFB1 C DRP = ( R FB + R FB1 ) ∗ C FB R DRP (31) (32) (33) RCP and CCP have limited effect on the crossover frequency, and are used only to fine tune the crossover frequency and transient load response. Determine RCP and CCP from Equations (34) and (35). RCP = C CP = (2π ∗ fC ) 2 ∗ LE ∗ CE ∗ RFB ∗ VPWMRMP VO 10 ∗ L E ∗ C E RCP (34) (35) CCP1 is optional and may be needed in some applications to reduce the jitter caused by the high frequency noise. A ceramic capacitor between 10pF and 220pF is usually enough. Type III Compensation for Non-AVP Applications Resistor RFB is chosen according to Equations (15), and resistor RDRP and capacitor CDRP are not needed. Choose the crossover frequency fc between 1/10 and 1/5 of the switching frequency per phase and select the desired phase margin θc. Calculate K factor from Equation (36), and determine the component values based on Equations (37) to (41), π θ (36) K = tan[ ∗ ( C + 1.5)] 4 180 Page 19 of 34 9/30/04 IR3088 RCP = RFB ∗ ( 2π ∗ LE ∗ CE ∗ fC ) 2 ∗ VPWMRMP VO ∗ K (37) CCP = K 2π ∗ fC ∗ RCP (38) CCP1 = 1 2π ∗ fC ∗ K ∗ RCP (39) CFB = K 2π ∗ fC ∗ RFB (40) R FB1 = 1 2π ∗ f C ∗ K ∗ C FB (41) CURRENT SHARE LOOP COMPENSATION The crossover frequency of the current share loop should be at least one decade lower than that of the voltage loop in order to eliminate the interaction between the two loops. A capacitor from SCOMP to ground is usually enough for the share loop compensation. Choose the crossover frequency of current share loop (fCI) based on the crossover frequency of voltage loop (fC), and determine the CSCOMP, CSCOMP = 0.65 * RPWMRMP *VI * I O * GCS _ ROOM * RLE * [1 + 2π * fCI * CE * (VO I O )] * FMI VO ∗ 2π ∗ fCI *1.05 *106 (42) Where FMI is the PWM gain in the current share loop, FMI = Page 20 of 34 RPWMRMP * CPWMRMP * f SW *V PWMRMP (VI − VPWMRMP − VDAC ) * (VI − VDAC ) (43) 9/30/04 IR3088 DESIGN EXAMPLE 1 - VRM 10 2U CONVERTER SPECIFICATIONS Input Voltage: VI=12 V DAC Voltage: VDAC=1.35 V No Load Output Voltage Offset: VO_NLOFST=20 mV Output Current: IO=105 ADC Maximum Output Current: IOMAX=120 ADC Output Impedance: RO=0.91 mΩ VCC Ready to VCC Power Good Delay: tVccPG=0-10mS Soft Start Time: tSS=2 mS Over Current Delay: tOCDEL=0.5mS Dynamic VID Down-Slope Slew Rate: SRDOWN=2.5mV/uS Over Temperature Threshold: TPCB=115 ºC POWER STAGE Phase Number: n=6 Switching Frequency: fSW=400 kHz Output Inductors: L=220 nH, RL=0.47 mΩ Output Capacitors: AL-Polymer, C=560uF, RC= 7mΩ, Number Cn=10 IR3081 EXTERNAL COMPONENTS Oscillator Resistor Rosc Once the switching frequency is chosen, ROSC can be determined from the curve in Figure 13 of IR3081 Data Sheet. For switching frequency of 400kHz per phase, choose ROSC=30.1kΩ Soft Start Capacitor CSS/DEL and Resistor RSS/DEL Because faster over-current protection is required, the soft start capacitor CSS/DEL in series with the resistor RSS/DEL is used. Calculate the soft start capacitor from the required soft start time. C SS / DEL = I CHG ∗ t SS 70 * 10 −6 ∗ 2 * 10 −3 = = 0.1uF VO 1.35 − 20 * 10 −3 Calculate the soft start resistor from the required over current delay time tOCDEL, 0.09 − RSS / DEL = tOCDEL ∗ I DISCHG 0.5 *10−3 ∗ 6 *10−6 0.09 − CSS / DEL 0.1 *10− 6 = 10kΩ = I DISCHG 6 *10− 6 The soft start delay time is tSSDEL = CSS / DEL ∗ (1.3 − RSS / DEL ∗ ICHG ) 0.1 *10−6 ∗ (1.3 − 10 *103 * 70 *10−6 ) = = 0.86mS I CHG 70 *10− 6 The power good delay time is tVccPG = CSS / DEL * (3.91 − VO − 1.3) 0.1*10−6 * (3.91 − 1.33 − 1.3) = = 1.8ms I CHG 70 *10− 6 Page 21 of 34 9/30/04 IR3088 VDAC Slew Rate Programming Capacitor CVDAC and Resistor RVDAC From Figure 15 of IR3081 Data Sheet, the sink current of VDAC pin corresponding to 400kHz (ROSC=30.1kΩ) is 76uA. Calculate the VDAC down-slope slew-rate programming capacitor from the required down-slope slew rate. CVDAC = I SINK 76 * 10 −6 = = 30.4nF , Choose CVDAC=33nF SR DOWN 2.5 * 10 −3 / 10 −6 Calculate the programming resistor. RVDAC = 0.5 + 3.2 * 10 −15 CVDAC 2 = 0.5 + 3.2 * 10 −15 (33 * 10 −9 ) 2 = 3.5Ω From Figure 15 of IR3081 Data Sheet, the source current of VDAC pin is 110uA. The VDAC up-slope slew rate is SRUP = I SOURCE 110 * 10 −6 = = 3.3mV / uS CVDAC 33 * 10 −9 Over Current Setting Resistor ROCSET The room temperature is 25ºC and the target PCB temperature is 100 ºC. The phase IC die temperature is about 1 ºC higher than that of phase IC, and the inductor temperature is close to PCB temperature. Calculate Inductor DC resistance at 100 ºC, RL _ MAX = RL _ ROOM ∗ [1 + 3850*10−6 ∗ (TL _ MAX − TROOM )] = 0.47 *10−3 ∗ [1 + 3850*10−6 ∗ (100 − 25)] = 0.61mΩ The current sense amplifier gain is 34 at 25ºC, and its gain at 101ºC is calculated as, G CS _ MIN = G CS _ ROOM ∗ [1 − 1470 *10 −6 ∗ (T IC _ MAX − T ROOM )] = 34 ∗ [1 − 1470 *10 −6 ∗ (101 − 25)] = 30.2 Set the over current limit at 135A. From Figure 14 of IR3081 Data Sheet, the bias current of OCSET pin (IOCSET) is 41uA with ROSC=30.1kΩ. The total current sense amplifier input offset voltage is 0.55mV, which includes the offset created by the current sense amplifier input resistor mismatch. Calculate constant KP, the ratio of inductor peak current over average current in each phase, KP = (V I − VO ) ∗ VO /( L ∗ V I ∗ f SW ∗ 2) (12 − 1.33) ∗ 1.33 /( 220 *10 −9 ∗ 12 ∗ 400 * 10 3 ∗ 2) = = 0.3 135 / 6 I LIMIT / n ROCSET = [ =( RLIMIT ∗ RL _ MAX ∗ (1 + K P ) + VCS _ TOFST ] ∗ GCS _ MIN / I OCSET n 135 ∗ 0.61 *10 −3 ∗ 1.3 + 0.55 *10 − 3 ) ∗ 30.2 /( 41 *10 − 6 ) = 13.3kΩ 6 No Load Output Voltage Setting Resistor RFB and Adaptive Voltage Positioning Resistor RDRP From Figure 14 of IR3081 Data, the bias current of FB pin is 41uA with ROSC=30.1kΩ. R FB = RDRP = R L _ MAX ∗ V O _ NLOFST − V CS _ TOFST ∗ n ∗ R O I FB ∗ R L _ MAX RFB ∗ RL _ MAX ∗ GCS _ MIN n ∗ RO Page 22 of 34 = = 0 .61 * 10 −3 ∗ 20 * 10 −3 − 0 .55 * 10 −3 ∗ 6 ∗ 0 .91 * 10 −3 41 * 10 − 6 ∗ 0 .61 * 10 − 3 = 365 Ω 365 ∗ 0.61*10−3 ∗ 30.2 = 1.21kΩ 6 ∗ 0.91*10−3 9/30/04 IR3088 Body Braking Related Resistors RBBFB and RBBDRP N/A. The body braking during Dynamic VID is disabled. IR3088 EXTERNAL COMPONENTS PWM Ramp Resistor RPWMRMP and Capacitor CPWMRMP Set PWM ramp magnitude VPWMRMP=0.8V. Choose 220pF for PWM ramp capacitor CPWMRMP, and calculate the resistor RPWMRMP, VO RPWMRMP = VIN ∗ f SW ∗ CPWMRMP ∗ [ln(VIN − VDAC ) − ln(VIN − VDAC − VPWMRMP )] = 1.33 12 ∗ 400 *10 3 ∗ 220 *10 −12 ∗ [ln(12 − 1.35) − ln(12 − 1.35 − 0.8)] = 16.1kΩ , choose RPWMRMP=16.2kΩ Inductor Current Sensing Capacitor CCS+ and Resistors RCS+ and RCSChoose CCS+=47nF and calculate RCS+, RCS + = L RL 220 *10−9 /(0.47 *10−3 ) = = 10.0kΩ CCS + 47 *10−9 The bias currents of CSIN+ and CSIN- are 0.25uA and 0.4uA respectively. Calculate resistor RCS-, RCS − = RCS + = 10.0kΩ Over Temperature Setting Resistors RHOTSET1 and RHOTSET2 Use central over-temperature setting and set the temperature threshold at 115 ºC, which corresponds to the IC die temperature of 116 ºC. Calculate the HOTSET threshold voltage corresponding to the temperature thresholds. V HOTSET = 4.73 * 10 −3 * TJ + 1.241 = 4.73 * 10 −3 ∗ 116 + 1.241 = 1.79V Pre-select RHOTSET1=10.0kΩ, R HOTSET 2 = R HOTSET 1 ∗ V HOTSET 10 *10 3 ∗1.79 = 3.57 kΩ = 6.8 − 1.79 V BIAS − V HOTSET Phase Delay Timing Resistors RPHASE1 and RPHASE2 Use central over-temperature setting and set the temperature threshold at 115 ºC, which corresponds to the IC die temperature of 116 ºC. Calculate the HOTSET threshold voltage corresponding to the temperature thresholds. The phase delay resistor ratios for phases 1 to 6 at 400kHz of switching frequencies are RAPHASE1=0.628, RAPHASE2=0.415, RAPHASE3=0.202, RAPHASE4=0.246, RAPHASE5=0.441 and RAPHASE6=0.637 starting from downslope. Pre-select RPHASE11=RPHASE21=RPHASE31=RPHASE41=RPHASE51= RPHASE61=10kΩ, RPHASE12 = RAPHASE1 0.628 ∗ RPHASE11 = ∗ 10 *103 = 16.9kΩ 1 − RAPHASE1 1 − 0.628 RPHASE22=7.15kΩ, RPHASE32=2.55kΩ, RPHASE42=3.24kΩ, PPHASE52=7.87kΩ, RPHASE62=17.4kΩ Page 23 of 34 9/30/04 IR3088 Bootstrap Capacitor CBST Choose CBST=0.1uF Decoupling Capacitors for Phase IC and Power Stage Choose CVCC=0.1uF, CVCCL=0.1uF VOLTAGE LOOP COMPENSATION Type II compensation is used for the converter with AL-Polymer output capacitors. Choose the crossover frequency fc=40kHz, which is 1/10 of the switching frequency per phase, and determine Rcp and CCP. RCP = CCP = (2π ∗ fC )2 ∗ LE ∗ CE ∗ RFB ∗ VRAMP VO * 1 + (2π * fC * C * RC )2 10 ∗ LE ∗ CE RCP = = (2π ∗ 40 ∗103 )2 ∗ (220 ∗10−9 / 6) ∗ (560 ∗10−6 ∗10) ∗ 365 ∗ 0.8 (1.35 − 20 ∗10−3 ) * 1 + (2π * 40 *103 * 560 *10−6 * 7 *10−3 )2 10 ∗ (220 ∗ 10−9 / 6) ∗ (560 ∗ 10−6 *10) 2.0 ∗103 = 2.0kΩ = 71nF , Choose CCP=68nF Choose CCP1=47pF to reduce high frequency noise. CURRENT SHARE LOOP COMPENSATION The crossover frequency of the current share loop fCI should be at least one decade lower than that of the voltage loop fC. Choose the crossover frequency of current share loop fCI=4kHz, and calculate CSCOMP, FMI = RPWMRMP * CPWMRMP * f SW *V PWMRMP 16.2 *103 * 220 *10−12 * 400 *103 * 0.8 = = 0.011 (VI − VPWMRMP − VDAC ) * (VI − VDAC ) (12 − 0.8 − 1.35) * (12 − 1.35) CSCOMP = = 0.65 * RPWMRMP *VI * I O * GCS _ ROOM * RLE * [1 + 2π * fCI * CE * (VO I O )] * FMI VO ∗ 2π ∗ fCI *1.05 *106 0.65 *16.2 *10 3 *12 *105 * 34 * (0.47 *10 −3 6) * [1 + 2π * 4 *10 3 * 560 *10 −6 *10 * (1.33 − 105 * 9.1*10 −4 ) 105] * 0.011 (1.33 − 105 * 9.1*10 − 4 ) ∗ 2π ∗ 4 *10 3 *1.05 *10 6 = 31.4nF Choose CSCOMP=33nF. Page 24 of 34 9/30/04 IR3088 DESIGN EXAMPLE 2 - EVRD 10 HIGH FREQUENCY ALL-CERAMIC CONVERTER SPECIFICATIONS Input Voltage: VI=12 V DAC Voltage: VDAC=1.3 V No Load Output Voltage Offset: VO_NLOFST=20 mV Output Current: IO=105 ADC Maximum Output Current: IOMAX=120 ADC Output Impedance: RO=0.91 mΩ VCC Ready to VCC Power Good Delay: tVccPG=0-10mS Soft Start Time: tSS=2.9mS Over Current Delay: tOCDEL=2.1mS Dynamic VID Down-Slope Slew Rate: SRDOWN=2.5mV/uS Over Temperature Threshold: TPCB=115 ºC POWER STAGE Phase Number: n=6 Switching Frequency: fSW=800 kHz Output Inductors: L=100 nH, RL=0.5 mΩ Output Capacitors: Ceramic, C=22uF, RC= 2mΩ, Number Cn=62 IR3081 EXTERNAL COMPONENTS Oscillator Resistor Rosc Once the switching frequency is chosen, ROSC can be determined from the curve in Figure 13 of IR3081 Data Sheet. For switching frequency of 800kHz per phase, choose ROSC=13.3kΩ Soft Start Capacitor CSS/DEL and Resistor RSS/DEL Because faster over-current protection is required, the soft start capacitor CSS/DEL in series with the resistor RSS/DEL is used. Calculate the soft start capacitor from the required soft start time. CSS / DEL = I CHG ∗ tSS 70 *10−6 ∗ 2.9 *10−3 = = 0.16uF , choose CSS/DEL=0.15uF VO 1.3 − 20 *10−3 Calculate the soft start resistor from the required over current delay time tOCDEL, 0.09 − RSS / DEL = tOCDEL ∗ I DISCHG 2.1 *10−3 ∗ 6 *10−6 0.09 − CSS / DEL 0.15 *10− 6 = = 1kΩ I DISCHG 6 *10− 6 The soft start delay time is t SSDEL = C SS / DEL ∗ (1.3 − R SS / DEL ∗ I CHG ) 0.15 * 10 −6 ∗ (1.3 − 1 * 10 3 * 70 * 10 −6 ) = = 2.6mS I CHG 70 * 10 −6 The power good delay time is tVccPG = C SS / DEL ∗ (3.91 − VO − 1.3) 0.15 * 10 −6 * (3.91 − 1.28 − 1.3) = = 2.85ms I CHG 70 * 10 −6 Page 25 of 34 9/30/04 IR3088 VDAC Slew Rate Programming Capacitor CVDAC and Resistor RVDAC From Figure 15 of IR3081 Data Sheet, the sink current of VDAC pin corresponding to 800kHz (ROSC=13.3kΩ) is 170uA. Calculate the VDAC down-slope slew-rate programming capacitor from the required down-slope slew rate. CVDAC = I SINK 170 * 10 −6 = = 68nF SR DOWN 2.5 * 10 −3 / 10 −6 Calculate the programming resistor. RVDAC = 0.5 + 3.2 * 10 −15 CVDAC 2 = 0.5 + 3.2 * 10 −15 (68 * 10 −9 ) 2 = 1.2Ω From Figure 15 of IR3081 Data Sheet, the source current of VDAC pin is 250uA. The VDAC up-slope slew rate is SRUP = I SOURCE 250 *10 −6 = = 3.7 mV / uS CVDAC 68 *10 −9 Over Current Setting Resistor ROCSET The room temperature is 25ºC and the target PCB temperature is 100 ºC. The phase IC die temperature is about 1 ºC higher than that of phase IC, and the inductor temperature is close to PCB temperature. Calculate Inductor DC resistance at 100 ºC, RL _ MAX = RL _ ROOM ∗ [1 + 3850*10−6 ∗ (TL _ MAX − TROOM )] = 0.5 *10−3 ∗ [1 + 3850*10−6 ∗ (100 − 25)] = 0.64mΩ The current sense amplifier gain is 34 at 25ºC, and its gain at 101ºC is calculated as, G CS _ MIN = G CS _ ROOM ∗ [1 − 1470 *10 −6 ∗ (T IC _ MAX − T ROOM )] = 34 ∗ [1 − 1470 *10 −6 ∗ (101 − 25)] = 30.2 Set the over current limit at 135A. From Figure 14 of IR3081 Data Sheet, the bias current of OCSET pin (IOCSET) is 90uA with ROSC=13.3kΩ. The total current sense amplifier input offset voltage is 0.55mV, which includes the offset created by the current sense amplifier input resistor mismatch. Calculate constant KP, the ratio of inductor peak current over average current in each phase, KP = (VI − VO ) ∗ VO /( L ∗ VI ∗ f SW ∗ 2) (12 − 1.28) ∗ 1.28 /(100 *10−9 ∗ 12 ∗ 800 *103 ∗ 2) = = 0.32 I LIMIT / n 135 / 6 ROCSET = [ =( RLIMIT ∗ RL _ MAX ∗ (1 + K P ) + VCS _ TOFST ] ∗ GCS _ MIN / I OCSET n 135 ∗ 0.64 *10 − 3 ∗ 1.32 + 0.55 *10 −3 ) * 30.2 /(90 *10 − 6 ) = 6.34 kΩ 6 No Load Output Voltage Setting Resistor RFB and Adaptive Voltage Positioning Resistor RDRP From Figure 14 of IR3081 Data Sheet, the bias current of FB pin is 90uA with ROSC=13.3kΩ. RFB = RL _ MAX ∗ VO _ NLOFST − VCS _ TOFST ∗ n ∗ RO RDRP = I FB ∗ RL _ MAX RFB ∗ RL _ MAX ∗ GCS _ MIN n ∗ RO Page 26 of 34 = = 0.64 *10−3 ∗ 20 *10−3 − 0.55 *10−3 ∗ 6 ∗ 0.91 *10−3 = 162Ω 90 *10−6 * 0.64 *10−3 162 ∗ 0.64 *10−3 * 30.2 = 576Ω 6 ∗ 0.91 *10−3 9/30/04 IR3088 Body Braking Related Resistors RBBFB and RBBDRP N/A. The body braking during Dynamic VID is disabled. IR3088 EXTERNAL COMPONENTS PWM Ramp Resistor RPWMRMP and Capacitor CPWMRMP Set PWM ramp magnitude VPWMRMP=0.75V. Choose 100pF for PWM ramp capacitor CPWMRMP, and calculate the resistor RPWMRMP, VO RPWMRMP = VIN * f SW * CPWMRMP * [ln(VIN − VDAC ) − ln(VIN − VDAC − VPWMRMP )] = 1.28 12 ∗ 800 *103 ∗100 *10− 12 ∗ [ln(12 − 1.3) − ln(12 − 1.3 − 0.75)] = 18.2kΩ Inductor Current Sensing Capacitor CCS+ and Resistors RCS+ and RCSChoose 47nF for capacitor CCS+, and calculate RCS+, RCS + = L RL 100 *10−9 /(0.5 *10−3 ) = = 4.22kΩ CCS + 47 *10−9 The bias currents of CSIN+ and CSIN- are 0.25uA and 0.4uA respectively. Calculate resistor RCS-, RCS − = RCS + = 4.22kΩ Combined Over Temperature and Phase Delay Setting Resistors RPHASEx1, RPHASEx2 and RPHASEx3 The over temperature setting resistor divider is combined with the phase delay resistor divider. Set the temperature threshold at 115 ºC, which corresponds to the IC die temperature of 116 ºC, and calculate the HOTSET threshold voltage corresponding to the temperature thresholds. V HOTSET = 4.73 * 10 −3 ∗ TJ + 1.241 = 4.73 * 10 −3 ∗ 116 + 1.241 = 1.79V The phase delay resistor ratios for phases 1 to 6 at 800kHz of switching frequencies are RAPHASE1=0.665, RAPHASE2=0.432, RAPHASE3=0.198, RAPHASE4=0.206, RAPHASE5=0.401 and RAPHASE6=0.597 starting from downslope. The over temperature setting voltage of phases 1, 2, 5, and 6 is lower than the phase delay setting voltage, VBIAS*RAPHASEx. Pre-select RPHASE11=10kΩ, RPHASEx 2 = ( RAPHASEx ∗ VBIAS − VHOTSET ) * RPHASEx1 (0.665 ∗ 6.8 − 1.79) ∗10 *103 = 12.1kΩ = 6.8 ∗ (1 − 0.665) VBIAS ∗ (1 − RAPHASEx ) RPHASEx3 = VHOTSET ∗ RPHASEx1 1.79 ∗ 12.1 *103 = = 7.87 kΩ VBIAS * (1 − RAPHASEx ) 6.8 * (1 − 0.665) RPHASE21=10kΩ, RPHASE22=2.94kΩ, RPHASE23=4.64kΩ RPHASE51=10kΩ, RPHASE52=2.32kΩ, RPHASE53=4.42kΩ RPHASE61=10kΩ, RPHASE62=8.25kΩ, RPHASE63=6.49kΩ Page 27 of 34 9/30/04 IR3088 The over temperature setting voltage of Phases 3 and 4 is higher than the phase delay setting voltage, VBIAS*RAPHASEx. Pre-select RPHASEX1=10kΩ, R PHASE 32 = (V HOTSET − RAPHASE 3 ∗ V BIAS ) ∗ R PHASE 31 (1.79 − 0.198 ∗ 6.8) ∗10 *10 3 = 887Ω = V BIAS − V HOTSET 6.8 − 1.79 RPHASE 33 = RAPHASE 3 ∗ VBIAS * RPHASE 31 0.198 ∗ 6.8 ∗ 10 *103 = 2.67 kΩ = 6.8 − 1.79 VBIAS − VHOTSET RPHASE41=10kΩ, RPHASE42=768Ω, RPHASE43=2.80kΩ Bootstrap Capacitor CBST Choose CBST=0.1uF Decoupling Capacitors for Phase IC and Power Stage Choose CVCC=0.1uF, CVCCL=0.1uF VOLTAGE LOOP COMPENSATION Type III compensation is used for the converter with only ceramic output capacitors. The crossover frequency and phase margin of the voltage loop can be estimated as follows. f C1 = R DRP 576 = = 146 kHz −6 2π ∗ C E ∗ G CS ∗ R FB ∗ R LE 2π ∗ (62 ∗ 22 * 10 ) ∗ 34 ∗ 162 ∗ (0.5 * 10 − 3 / 6) θC1 = 90 − A tan(0.5) ∗ Choose RFB1 = 180 π = 63° 2 2 ∗ RFB = ∗ 162 = 110Ω 3 3 Choose the desired crossover frequency fc (=140kHz) around fc1 estimated above, and calculate CFB = 1 4π ∗ fC ∗ RFB1 CDRP = RCP = CCP = = 1 = 5.2nF , choose CFB=5.6nF 4π ∗ 140 *103 ∗ 110 ( RFB + RFB1 ) ∗ CFB (162 + 110) ∗ 5.6 *10−9 = = 2.7 nF RDRP 576 ( 2π ∗ fC ) 2 ∗ LE ∗ CE ∗ RFB ∗ VRAMP (2π ∗140 *103 ) 2 ∗ (100 *10−9 / 6) ∗ (22 *10−6 ∗ 62) ∗162 * 0.75 = = 1.65kΩ VO 1.3 − 20 *10−3 10 ∗ LE ∗ CE RCP = 10 ∗ (100 *10−9 / 6) ∗ ( 22 *10−6 * 62) 1.65 ∗ 103 = 27nF Choose CCP1=47pF to reduce high frequency noise. CURRENT SHARE LOOP COMPENSATION The crossover frequency of the current share loop fCI should be at least one decade lower than that of the voltage loop fC. Choose the crossover frequency of current share loop fCI=3.5kHz, and calculate CSCOMP, Page 28 of 34 9/30/04 IR3088 FMI = RPWMRMP * CPWMRMP * f SW *V PWMRMP 18.2 *103 *100 *10−12 * 800 *103 * 0.75 = = 0.011 (VI − VPWMRMP − VDAC ) * (VI − VDAC ) (12 − 0.75 − 1.3) * (12 − 1.3) CSCOMP = = 0.65 * RPWMRMP *VI * I O * GCS _ ROOM * RLE * [1 + 2π * fCI * CE * (VO I O )] * FMI VO ∗ 2π ∗ fCI *1.05 *106 0.65 *18.2 *10 3 *12 *105 * 34 * (0.5 *10 −3 6) * [1 + 2π * 3500 * 22 *10 −6 * 62 * (1.33 − 105 * 9.1*10 −4 ) 105] * 0.011 (1.33 − 105 * 9.1*10 − 4 ) ∗ 2π ∗ 3500 *1.05 *10 6 = 20.6nF Choose CSCOMP=22nF Page 29 of 34 9/30/04 I IR3088 LAYOUT GUIDELINES The following layout guidelines are recommended to reduce the parasitic inductance and resistance of the PCB layout, therefore minimizing the noise coupled to the IC. • Dedicate at least one middle layer for a ground plane, which is then split into signal ground plane (LGND) and power ground plane (PGND). • Connect PGND to LGND pins of each phase IC to the ground tab, which is tied to LGND and PGND planes respectively through vias. • In order to reduce the noise coupled to SCOMP pin of phase IC, use a dedicated wire to connect the capacitor CSCOMP directly to LGND pin. However, connect PWM ramp capacitor CPWMRMP, phase delay programming resistor RPHASE2 or RPHASE3, decoupling capacitor CVCC to LGND plane through vias. • Place current sense resistors and capacitors (RCS+, RCS-, CCS+, and CCS-) close to phase IC. Use Kelvin connection for the inductor current sense wires, but separate the two wires by ground polygon. The wire from the inductor terminal to RCS- should not cross over the fast transition nodes, i.e. switching nodes, gate drive outputs and bootstrap nodes. • Place the decoupling capacitors CVCC and CVCCL as close as possible to VCC and VCCL pins of the phase IC respectively. • Place the phase IC as close as possible to the MOSFETs to reduce the parasitic resistance and inductance of the gate drive paths. • Place the input ceramic capacitors close to the drain of top MOSFET and the source of bottom MOSFET. Use combination of different packages of ceramic capacitors. • There are two switching power loops. One loop includes the input capacitors, top MOSFET, inductor, output capacitors and the load; another loop consists of bottom MOSFET, inductor, output capacitors and the load. Route the switching power paths using wide and short traces or polygons; use multiple vias for connections between layers. LGND PLANE To Signal Bus To LGND Plane SCOMP PHSFLT LGND PGND PLANE Page 30 of 34 To Switching Node Ground Polygon CCS- RCS- CCS+ RCS+ VCCH To Bottom To Top MOSFET MOSFET CBST GATEH CSIN+ PGND VCC GATEL CVCCL CSIN- VCCL To PGND Plane RBIASIN DACIN DBST CSCOMP EAIN CVCC To Gate Drive Voltage BIASIN PWMRMP To LGND Plane RPHASE1 RMPIN+ RMPIN- VRHOT HOTSET ISHARE EAIN RPWMRMP RPHASE2 To LGND Plane CPWMRMP To VIN Ground Polygon To Inductor 9/30/04 I IR3088 PCB Metal and Component Placement • Lead land width should be equal to nominal part lead width. The minimum lead to lead spacing should be ≥ 0.2mm to minimize shorting. • Lead land length should be equal to maximum part lead length + 0.2 mm outboard extension + 0.05mm inboard extension. The outboard extension ensures a large and inspectable toe fillet, and the inboard extension will accommodate any part misalignment and ensure a fillet. • Center pad land length and width should be equal to maximum part pad length and width. However, the minimum metal to metal spacing should be ≥ 0.17mm for 2 oz. Copper (≥ 0.1mm for 1 oz. Copper and ≥ 0.23mm for 3 oz. Copper) • Four 0.3mm diameter vias shall be placed in the pad land spaced at 1.2mm, and connected to ground to minimize the noise effect on the IC, and to transfer heat to the PCB. Page 31 of 34 9/30/04 I IR3088 Solder Resist • The solder resist should be pulled away from the metal lead lands by a minimum of 0.06mm. The solder resist mis-alignment is a maximum of 0.05mm and it is recommended that the lead lands are all Non Solder Mask Defined (NSMD). Therefore pulling the S/R 0.06mm will always ensure NSMD pads. • The minimum solder resist width is 0.13mm, therefore it is recommended that the solder resist is completely removed from between the lead lands forming a single opening for each “group” of lead lands. • At the inside corner of the solder resist where the lead land groups meet, it is recommended to provide a fillet so a solder resist width of ≥ 0.17mm remains. • The land pad should be Solder Mask Defined (SMD), with a minimum overlap of the solder resist onto the copper of 0.06mm to accommodate solder resist mis-alignment. In 0.5mm pitch cases it is allowable to have the solder resist opening for the land pad to be smaller than the part pad. • Ensure that the solder resist in-between the lead lands and the pad land is ≥ 0.15mm due to the high aspect ratio of the solder resist strip separating the lead lands from the pad land. • The 4 vias in the land pad should be tented with solder resist 0.4mm diameter, or 0.1mm larger than the diameter of the via. Page 32 of 34 9/30/04 I IR3088 Stencil Design • The stencil apertures for the lead lands should be approximately 80% of the area of the lead lands. Reducing the amount of solder deposited will minimize the occurrence of lead shorts. Since for 0.5mm pitch devices the leads are only 0.25mm wide, the stencil apertures should not be made narrower; openings in stencils < 0.25mm wide are difficult to maintain repeatable solder release. • The stencil lead land apertures should therefore be shortened in length by 80% and centered on the lead land. • The land pad aperture should be striped with 0.25mm wide openings and spaces to deposit approximately 50% area of solder on the center pad. If too much solder is deposited on the center pad the part will float and the lead lands will be open. • The maximum length and width of the land pad stencil aperture should be equal to the solder resist opening minus an annular 0.2mm pull back to decrease the incidence of shorting the center land to the lead lands when the part is pushed into the solder paste. Metal and Solder Resist Page 33 of 34 9/30/04 IR3088 PACKAGE INFORMATION 20L MLPQ (4 x 4 mm Body) – θJA = 32oC/W, θJC = 3oC/W Data and specifications subject to change without notice. This product has been designed and qualified for the Consumer market. Qualification Standards can be found on IR’s Web site. IR WORLD HEADQUARTERS: 233 Kansas St., El Segundo, California 90245, USA Tel: (310) 252-7105 TAC Fax: (310) 252-7903 Visit us at www.irf.com for sales contact information. www.irf.com Page 34 of 34 9/30/04