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Switched Capacitor DC-DC Converters: Topologies and Applications

Switched Capacitor DC-DC Converters: Topologies and Applications. Bill Tsang and Eddie Ng. Outline. Motivations Dickson’s Charge Pump Other Various Charge Pumps Applications Conclusion. Motivations. Inductorless On-chip integration Low cost High switching frequency

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Switched Capacitor DC-DC Converters: Topologies and Applications

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  1. Switched Capacitor DC-DC Converters: Topologies and Applications Bill Tsang and Eddie Ng

  2. Outline • Motivations • Dickson’s Charge Pump • Other Various Charge Pumps • Applications • Conclusion

  3. Motivations • Inductorless • On-chip integration • Low cost • High switching frequency • Easy to implement (open-loop system) • Fast transient but large ripple • High efficiency but limited output power

  4. Ideal Dickson’s Charge Pump(Phase 1) 2VDD-Vt VDD VDD-Vt VDD-Vt VDD-Vt 0 VDD • Clk=0, Clk_bar=VDD • Finite diode voltage drops, Vt

  5. Ideal Dickson’s Charge Pump(Phase 2) 3VDD-2Vt 2VDD-Vt VDD 2VDD-2Vt VDD-Vt VDD-Vt VDD 0 • Clk=VDD, Clk_bar=0 • Maximum voltage stress on diodes 2VDD-Vt => reliability issue • Maximum voltage stress on capacitors VCn =n(VDD-Vt) => reliability issue

  6. Dickson’s Charge Pump C1=C2=C3=C (Body effect can be significant at later stages)

  7. Non-idealities • Threshold voltage drop [Mos charge pumps for low-voltage operation] • Parasitic capacitor divider voltage drop • Low conversion efficiency and pumping gain • Limited maximum number of stages [An on-chip High-voltage generator circuit for EEPROMs with a power supply voltage below 2V]

  8. Modified Switch CTS • Static Charge Transfer Switches (CTS) • Eliminate transistor threshold drop

  9. Modified Dickson’s Charge Pump #1 (NCP-1) Conditions: 1, Clk=Vdd,Clk_bar=0: v2, v3+V To turn on transistor Ms2; Vgs = 2V 2, Clk=0,Clk_bar=VDD: v1, v2+V,v3 To turn off transistor Ms2; Vgs = 2V impossible

  10. Modified Dickson’s Charge Pump #1 (NCP-1) • Static Charge Transfer Switches (CTS) • Better voltage pumping gain than diodes • Lower voltage equals upper voltage of pervious stage • Utilizing higher voltage from following stage to drive CTS • Reverse charge sharing since CTS cannot turn off completely

  11. Modified Switch #2 MN1 used to turn off MS1 MP1 used to turn on MS1 MN1 MP1 Next stage • Eliminate transistor threshold drop • Complete turn-off of switch, MS1

  12. Modified Dickson’s Charge Pump #2 (NCP-2) Conditions: 1, Clk=Vdd,Clk_bar=0: v2, v3+V To turn on transistor MP2 and MS2; Vgs = 2V 2, Clk=0,Clk_bar=VDD: v1, v2+V,v3 To turn on transistor MN2 and turn off MS2; Vgs = 2V

  13. Complete Circuit(NCP-2) • Careful PMOS well connection to prevent latch-up • Diode-connected output stage used

  14. Modified Dickson’s Charge Pump #3 (NCP-3) NCP-3 uses boosted clock at output stage

  15. Converters Output Voltage Results

  16. Optimum Capacitance Selection • [A Low-Ripple Switched-Capacitor DC-DC Up converter for Low-voltage applications]

  17. Efficiency and Output Impedance [Performance limits of switched-capacitor DC-DC Converter] • Power loss due to: Vth, Rds(on), ESR, Cp, etc • Efficiency estimation • Output impedance (slow switching) M=ideal conversion ratio [Performance limits of switched-capacitor DC-DC Converter] Ts=switching period i= parasitic time constant q=charge supplied to the source Vout

  18. Cross-Coupled Charge Pump • PMOS to transmit 2VDD to output • Bodies tied to source(highest voltage) to avoid forward biasing junction diodes [Area-efficient CMOS Charge Pumps for LCD Drivers]

  19. H-bridge Topology • Commercial products (Linear Technology, Fairchild, Maxim …) • Buck or Boost functions • Negative voltage generation

  20. H-bridge Topologies Phase 1: transistors in red are on Phase 2: transistors in blue are on Vout = 2Vin Vout = -Vin Vout = 0.5 Vin

  21. Application (1): Flash Memory • Floating gate programming • Control gate voltage >> Vdd [ee141 lecture]

  22. Application (1): Flash Memory Nominal VDD= 5V

  23. Application (2): Sample Switches • S/H circuit– constant vgs sampling with all input level • Reduces distortion • Reduces Rds(on) Voltage doubler

  24. Application (3): Low voltage Amplifier • Positive zero in Miller compensation • 1/gm pole-zero cancellation [charge-pump assisted low-power/low-voltage CMOS Opamp Design] >2VGS

  25. Conclusion • Different Dickson’s SC converters discussed • Optimal Capacitor size selection • Discussion of cross-coupled doublers • Commercial product: Full H-bridge • Applications: Flash, ADC, Amplifier, LCD driver

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