Switched Capacitor DC-DC Converters: Topologies and Applications

1
Switched Capacitor DC-DC Converters Topologies
and Applications

• Bill Tsang and Eddie Ng

2
Outline

• Motivations
• Dicksons 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 Dicksons Charge Pump(Phase 1)
2VDD-Vt
VDD
VDD-Vt
VDD-Vt
VDD-Vt
0
VDD

• Clk0, Clk_barVDD
• Finite diode voltage drops, Vt

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Ideal Dicksons Charge Pump(Phase 2)
3VDD-2Vt
2VDD-Vt
VDD
2VDD-2Vt
VDD-Vt
VDD-Vt
VDD
0

• ClkVDD, Clk_bar0
• Maximum voltage stress on diodes 2VDD-Vt gt
  reliability issue
• Maximum voltage stress on capacitors VCn
  n(VDD-Vt) gt reliability issue

6
Dicksons Charge Pump
C1C2C3C
(Body effect can be significant at later stages)
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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 Dicksons Charge Pump 1 (NCP-1)
Conditions
1, ClkVdd,Clk_bar0 v2, v3?V
To turn on transistor Ms2 Vgs 2?V
2, Clk0,Clk_barVDD v1, v2?V,v3
To turn off transistor Ms2 Vgs 2?V
impossible
10
Modified Dicksons 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

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Modified Switch 2
MN1 used to turn off MS1
MP1 used to turn on MS1
MP1
MN1
Next stage

• Eliminate transistor threshold drop
• Complete turn-off of switch, MS1

12
Modified Dicksons Charge Pump 2 (NCP-2)
Conditions
1, ClkVdd,Clk_bar0 v2, v3?V
To turn on transistor MP2 and MS2 Vgs 2?V
2, Clk0,Clk_barVDD v1, v2?V,v3
To turn on transistor MN2 and turn off MS2 Vgs
2?V
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Complete Circuit(NCP-2)

• Careful PMOS well connection to prevent latch-up
• Diode-connected output stage used

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Modified Dicksons Charge Pump 3 (NCP-3)

• NCP-3 uses boosted clock at output stage

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Converters Output Voltage Results
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Optimum Capacitance Selection

• A Low-Ripple Switched-Capacitor DC-DC Up
  converter for Low-voltage applications

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Efficiency and Output Impedance

• Power loss due to Vth, Rds(on), ESR, Cp, etc
• Efficiency estimation
• Output impedance (slow switching)

Performance limits of switched-capacitor DC-DC
Converter
Mideal conversion ratio
Performance limits of switched-capacitor DC-DC
Converter
Tsswitching period ?i parasitic time constant
qcharge supplied to the source Vout
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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
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H-bridge Topology

• Commercial products (Linear Technology,
  Fairchild, Maxim )
• Buck or Boost functions
• Negative voltage generation

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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
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Application (1) Flash Memory

• Floating gate programming
• Control gate voltage gtgt Vdd

ee141 lecture
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Application (1) Flash Memory
Nominal VDD 5V
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Application (2) Sample Switches

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

Voltage doubler
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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

gt2VGS
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Conclusion

• Different Dicksons SC converters discussed
• Optimal Capacitor size selection
• Discussion of cross-coupled doublers
• Commercial product Full H-bridge
• Applications Flash, ADC, Amplifier, LCD driver