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) VDD-Vt 2VDD-Vt VDD 0 Clk=0, Clk_bar=VDD Finite diode voltage drops, Vt VDD-Vt VDD-Vt

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

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) To turn on transistor Ms2; Vgs = 2  V Conditions: 1, Clk=Vdd,Clk_bar=0: v2, v3+  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 Eliminate transistor threshold drop Complete turn-off of switch, MS1 Next stage MP1MN1 MP1 used to turn on MS1 MN1 used to turn off MS1

Modified Dickson’s Charge Pump #2 (NCP-2) To turn on transistor MP2 and MS2; Vgs = 2  V Conditions: 1, Clk=Vdd,Clk_bar=0: v2, v3+  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: V th, R ds(on), ESR, C p, etc Efficiency estimation Output impedance (slow switching) [Performance limits of switched-capacitor DC-DC Converter] M=ideal conversion ratio q=charge supplied to the source Vout Ts=switching period  i = parasitic time constant

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

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

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

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 R ds(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