Switched Capacitor DC DC Converters Topologies and Applications

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					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
  Easy to implement (open-loop system)
  Fast transient but large ripple
  High efficiency but limited output power
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
Ideal Dickson’s Charge
Pump(Phase 2)
                                                  3VDD-2Vt
VDD                             2VDD-Vt
                                                  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
Dickson’s Charge Pump




                                                 C1=C2=C3=C




      (Body effect can be significant at later stages)
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]
Modified Switch


                   CTS




 •Static Charge Transfer Switches (CTS)
 •Eliminate transistor threshold drop
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
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
    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
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
Complete Circuit(NCP-2)




  •Careful PMOS well connection to prevent latch-up
  •Diode-connected output stage used
Modified Dickson’s Charge
Pump #3 (NCP-3)
NCP-3 uses boosted clock at output stage
Converters Output Voltage
Results
Optimum Capacitance
Selection




          [A Low-Ripple Switched-Capacitor DC-DC Up converter for Low-voltage applications]
Efficiency and Output Impedance
  Power loss due to: Vth, Rds(on), ESR, Cp,
  etc                         [Performance limits of switched-capacitor DC-DC Converter]




  Efficiency estimation
                 M=ideal conversion ratio




  Output impedance (slow switching)
                               [Performance limits of switched-capacitor DC-DC Converter]


                 Ts=switching period
                 i= parasitic time constant


               q=charge supplied to the source Vout
 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]
H-bridge Topology
  Commercial
  products (Linear
  Technology,
  Fairchild, Maxim …)
  Buck or Boost
  functions
  Negative voltage
  generation
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
Application (1): Flash Memory
  Floating gate programming
  Control gate voltage >> Vdd




                                [ee141 lecture]
Application (1): Flash Memory
Nominal VDD= 5V
Application (2): Sample Switches

  S/H circuit– constant
  vgs sampling with all
  input level
  Reduces distortion
  Reduces Rds(on)

          Voltage
          doubler
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
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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