Fairchild SEMICONDUCTOR RC5050, Application Note

Page 1: ...nment such as with personal computers however a CPU voltage change requires a CPU board re design to accommo date the new voltage requirement The 5 bit DAC in the RC5050 and the RC5051 reads the voltage ID code that is programmed into modern processors and provides the appro priate CPU voltage In this manner the PC board does not have to be re designed each time the CPU voltage changes The CPU can...

Page 2: ...Processor Voltage Identification There are four voltage identification Pins VID3 VID0 on the Pentium Pro processor package which can be used to support automatic selection of the power supply voltage These pins are internally unconnected or are shorted to ground VSS The logic status of the VID pins defines the voltage required by the processor In order to address future low voltage microprocessors...

Page 3: ...are listed in Table 4 Table 4 RC5050 and RC5051 Differences Main Control Loop Refer to the RC5051 Block Diagram illustrated in Figure 2 The control loop of the regulator contains two main sections the analog control block and the digital control block The analog section consists of signal conditioning amplifiers feeding into a set of comparators which provide the inputs to the digital control bloc...

Page 4: ...ith the Pentium Pro DC DC converter speci fication to provide a constant voltage monitor on the VFB pin The circuit compares the VFB signal to the VREF volt age and outputs an active low interrupt signal to the CPU when the power supply voltage exceeds 12 of nominal The Power Good flag provides no other control function to the RC5050 or the RC5051 Output Enable OUTEN The DC DC converter accepts an...

Page 5: ...use of a smaller inductor value Unfortunately operation at lower frequencies increases the amount of energy storage that must be provided by the bulk output capacitors during load transients due to slower loop response of the controller In addition the efficiency losses due to switching of the MOSFETs increase as the operating frequency is increased Thus efficiency is optimized at lower operating ...

Page 6: ...2 13 14 15 16 17 18 19 20 5V PWRGD RC5051 VCC ENABLE C10 0 1µF C11 0 1µF 10K CEXT C7 0 1 µF C6 4 7 µF DS1 1N5817 C1 1000 µF C2 C3 1000 µF 1000 µF C5 0 1µF D1 1N4691 C12 1µF L1 1 3µ H M1 IRF7413 M2 IRF7413 C8 C9 0 1 µF 0 1µF RSENSE 6mΩ C13 1500 µ F L2 2 5 µH VREF C14 1500 µF C4 0 1 µF 1500 µF C16 1500 µF C15 12V VID4 47 R5 R6 100pF M3 IRF7413 M4 IRF7413 ...

Page 7: ...plied by the manufacturers and are approximations only Table 3 MOSFET Selection Table Manufacturer Model Conditions1 RDS ON mΩ Package Thermal Resistance Typ Max Fuji 2SK1388 VGS 4V ID 17 5A TJ 25 C 25 37 TO 220 ΦJA 75 TJ 125 C 37 Siliconix SI4410DY VGS 4 5V ID 5A TJ 25 C 16 5 20 SO 8 SMD ΦJA 50 TJ 125 C 28 34 National Semiconductor NDP706AL VGS 5V ID 40A TJ 25 C 13 15 TO 220 ΦJA 62 5 ΦJC 1 5 NDP7...

Page 8: ...t the voltage of the RC5050 output driver When the MOSFET switches off the source of the MOSFET is at 0 6V VCCQP is charged through the Schottky diode to 4 5V Thus the capacitor CP is charged to 5V When the MOS FET turns on the source of the MOSFET voltage is equal to 5V The capacitor voltage follows and hence provides a voltage at VCCQP equal to 10V The Schottky diode is required to provide the c...

Page 9: ...ere CRSS is the reverse transfer capacitance of the high side MOSFET Example Efficiency POUT pIN IOUT VOUT IOUT VOUT PLOSS PLOSS PDMOSFET PDCOIL PDSENSER PDGATE PDDIODE PDTRAN PDCAP PDIC PDMOSFET IOUT 2 RDS ON DutyCycle DutyCycle VOUT VD VIN VD VSW PDCOIL IOUT 2 RCOIL PDSENSER IOUT 2 RSENSE PDGATE qGATE f 5V PDDIODE Vf ID 1 Dutycycle PDTRAN VIN 2 CRSS ILOAD f IDRIVE PDCAP IRMS 2 ESR PDIC VCC ICC D...

Page 10: ...erminal of an internal comparator with hysterisis The other comparator terminal has the threshold voltage nominally of 120mV Table 6 states the limits for the comparator threshold of the Switching Regulator Table 6 RC5050 Short Circuit Comparator Threshold Voltage When designing the external current sense circuitry pay careful attention to the output limitations during normal operation and during ...

Page 11: ... 0 2 x 0 001 1 oz Cu trace 0 45 x 0 065 x 0 200 0 25 x 0 125 x 0 025 0 200 x 0 04 x 0 160 0 200 x 0 04 x 0 100 Power capability 50A in 1 watt 3W and 5W available 1 watt 1 watt 1 watt Temperature Coefficient 4 000 ppm 30 ppm 75 ppm 30 ppm 20 ppm Cost 10 000 piece Low included in motherboard 0 31 0 47 0 09 0 09 Based on the Tolerance Factor in the above table for an embedded PC trace resistor and fo...

Page 12: ...where ρ Resistivity µΩ mil L Length mils W Width mils and t Thickness mils For 1oz copper t 1 35 mils ρ 717 86 µΩ mil 1 L 1 W 1 Square For example you can layout a 5 30mΩ embedded sense resistor using the equations above L W 10 Therefore to model 5 30mΩ embedded sense resistor you need W 200 mils and L 2000 mils Refer to Figure 9 Figure 9 5 30mΩ Sense Resistor 10 You can also implement the sense r...

Page 13: ...t Condition The RC5050 and RC5051 controllers respond to an output short circuit by drastically changing the duty cycle of the gate drive signal to the power MOSFET In doing this the power MOSFET is protected from stress and from eventual failure Figure 13A shows the gate drive signal of a typical RC5050 operating in continuous mode with a load current of 10A The duty cycle is set by the ratio of ...

Page 14: ...n summary with proper heat sink the Schottky diode is not over stressed during a short circuit condition Schottky Diode Selection The application circuit diagram of Figure 3 shows a Schottky diode DS1 In non synchronous mode DS1 is used as a fly back diode to provide a constant current path for the inductor when M1 is turned off Table 10 shows the characteristics of several Schottky diodes Note th...

Page 15: ...d as follows Because the control loop response of the controller is not instantaneous the initial load transient must be supplied entirely by the output capacitors The initial voltage deviation is determined by the total ESR of the capacitors used and the parasitic resistance of the output traces For a detailed analysis of capacitor requirements in a high end microprocessor system please refer to ...

Page 16: ... mode a 1A schottky diode such as the 1N5817 should be substituted for the MBR2015CT 2 A target RDS ON value of 10mΩ should be used for each output driver switch Refer to Table 3 for alternative MOSFETs 1 L1 Pulse Engineering PE 53680 1 3µH inductor 1 L2 Pulse Engineering PE 53681 2 5µH inductor Optional helps reduce ripple on 5v line 2 4 note 2 M1 M4 International Rectifier IRF7413 N Channel Logi...

Page 17: ...the DC DC converter Place the output bulk capacitors as close to the CPU as possible to optimize their ability to supply instantaneous current to the load in the event of a current transient Additional space between the output capacitors and the CPU allows the parasitic resistance of the board traces to degrade the DC DC converter s performance under severe load transient conditions causing higher...

Page 18: ...r load regulation can be due to many causes Check the voltages and signals at the critical pins 6 The VREF pin should be at the voltage set by the VID pins If the power supply pins and the VID pins are correct the VREF should have the correct voltage 7 Next check the oscillator pin You should see a saw tooth wave at the frequency set by the external capacitor 8 When the VREF and CEXT pins are chec...

Page 19: ...10 0 5 2 505 1 0 2 504 2 0 2 501 3 0 2 496 4 0 2 493 5 0 2 493 6 0 2 492 7 0 2 492 8 0 2 491 9 0 2 490 10 0 2 989 11 0 2 488 12 0 2 486 13 0 2 485 13 9 2 484 Load Regulation 0 5A 13 9A 0 84 Low to High Current Step 0 5A 9 9A 76 0mV Refer to Attachment A for Scope Picture High to Low Current Step 9 9A 0 5A 70mV Refer to Attachment B for Scope Picture Low to High Current Step 0 5A 12 4A 97 6mV Refer...

Page 20: ...t G for Scope Picture Component Case Temperature Note The values for case temperatures are within guidelines That is case temperatures for all components should be below 105 C 25 C Ambient Device Description Case Temperature Iload 9 9A C Case Temperature Iload 12 4A C Case Temperature Iload 13 9A C Q3A MOSFET K1388 57 63 66 3 Q3B MOSFET K1388 58 64 66 6 L1 Inductor Unknown 53 56 61 2 Q2 Schottky D...

Page 21: ...APPLICATION NOTE AN50 21 Attachment C Attachment D Attachment E Attachment F Attachment G ...

Page 22: ...tions and compo nents selection layout guidelines and considerations guide lines for debugging and performance evaluations RC5050 Evaluation Board Raytheon Electronics provides an evaluation board to verify ing system level performance of the RC5050 The evaluation board serves as a guide to performance expectations when using the supplied external components and PCB layout Call Raytheon Electronic...

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Page 24: ...sonable expected to result in a significant injury of the user 2 A critical component in any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system or to affect its safety or effectiveness www fairchildsemi com Fairchild Semiconductor Corporation Americas Customer Response Center Tel 1 888 522 5372 ...