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
5.Ideal Dickson’s Charge Pump(Phase 2)
VDD-Vt
2VDD-2Vt
2VDD-Vt
3VDD-2Vt
VDD
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
VDD-Vt
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)
To turn on transistor Ms2; Vgs = 2V
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 = 2V
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
Eliminate transistor threshold drop
Complete turn-off of switch, MS1
Next stage
MP1
MN1
MP1 used to turn on MS1
MN1 used to turn off MS1
12.Modified Dickson’s Charge Pump #2 (NCP-2)
To turn on transistor MP2 and MS2; Vgs = 2V
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 = 2V
13.Complete Circuit(NCP-2)
Careful PMOS well connection to prevent latch-up
Diode-connected output stage used
16.Optimum Capacitance Selection
[A Low-Ripple Switched-Capacitor DC-DC Up converter for Low-voltage applications]
17.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]
[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
18.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
19.H-bridge Topology
Commercial products (Linear Technology, Fairchild, Maxim …)
Buck or Boost functions
Negative voltage generation
20.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
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