Download 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
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
Vo
VDD-Vt
0
VDD
C1
clk
clk_bar
• Clk=0, Clk_bar=VDD
• Finite diode voltage drops, Vt
C2
C3
Ideal Dickson’s Charge
Pump(Phase 2)
3VDD-2Vt
2VDD-Vt
VDD
2VDD-2Vt
VDD-Vt
VDD
Vo
VDD-Vt
VDD
clk
0
clk_bar
C1
C2
C3
• 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
V2+dV2
V1+dV1
V2
V
V1
v1
Vth
v2
VDD
Vo
Cp
C1
Cp
C2
Cp
C3
C1=C2=C3=C
clk
clk_bar
V  V
V  Vth
Io
C

C  C p f (C  C p )
(Body effect can be significant at later stages)
Vout  VDD  N * (V  Vt )  Vt
Non-idealities
Threshold voltage drop
[Mos charge pumps for low-voltage operation]

Vth  Vtho  γ VBS  2φF  2φF

Parasitic capacitor divider voltage drop
Low conversion efficiency and pumping
gain G  V  V  V  V (V )
V2
2
1
tn
2
Limited maximum number of stages
2
 VDD  Vtho

Vout,max  
 2φF   2φF
γ


[An on-chip High-voltage generator circuit for
EEPROMs with a power supply voltage below 2V]
Modified Switch
MD1
VDD
MD1
VDD
MS1
CTS
clk
2VDD
clk
•Static Charge Transfer Switches (CTS)
•Eliminate transistor threshold drop
Modified Dickson’s Charge Pump #1 (NCP-1)
dV
v3
dV
V2
dV
v1
MD1
MD3
MD2
v2
v1
MD4
v3
Vo
VDD
MS1
MS3
MS2
MS4
Cp
Cp
Cp
C1
Cp
C2
Cp
C3
C4
C5
clk
clk_bar
Conditions:
1, Clk=Vdd,Clk_bar=0: v2, v3+V
To turn on transistor Ms2; Vgs = 2V
2 * V  Vtn (V2 )
2, Clk=0,Clk_bar=VDD: v1, v2+V,v3
To turn off transistor Ms2; Vgs = 2V
2 * V  Vtn (V1 )
impossible
Modified Dickson’s Charge Pump #1
(NCP-1)
Static Charge Transfer Switches (CTS)
Better voltage pumping gain than diodes
GV 2  V2  V1  V
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
MD1
MS1
MN1 used to turn off MS1
MP1 used to turn on MS1
MN1
MP1
Next
stage
clk
• Eliminate transistor threshold drop
• Complete turn-off of switch, MS1
Modified Dickson’s Charge Pump #2 (NCP-2)
dV
dV
dV
MD1
MS1
v2
C1
v3
MS3
MS2
MN2
Cp
MD3
MD2
v1
MP2
Cp
C2
Cp
C3
clk
clk_bar
Conditions:
1, Clk=Vdd,Clk_bar=0: v2, v3+V
To turn on transistor MP2 and MS2; Vgs = 2V
2 * V  Vtp
2 * V  Vtn (V2 )
2, Clk=0,Clk_bar=VDD: v1, v2+V,v3
To turn on transistor MN2 and turn off MS2; Vgs = 2V
2 * V  Vtn (V1 )
Complete Circuit(NCP-2)
dV
dV
dV
MD1
v2
v1
MS1
MD3
MD2
MD4
v3
Vo
MS3
MS2
MS4
q
MN2
MP2
Cp
Cp
Cp
C1
Cp
C2
Cp
C3
clk
clk_bar
•Careful PMOS well connection to prevent latch-up
•Diode-connected output stage used
C4
C5
Modified Dickson’s Charge
Pump #3 (NCP-3)
NCP-3 uses boosted clock at output stage
dV
dV
dV
MD1
MD3
MD2
MD4
Vi
Vo
MS1
MS3
MS2
MS4
q
C5
HV
Clock
Generator
clk
Cp
Cp
clk
clk_bar
C1
Cp
C2
Cp
C3
C4
Converters Output Voltage
Results
Optimum Capacitance
Selection
V  VDD
Ci
I out

Ci  C p f (Ci  C p )
Ctot  N * Ci
Vout  VDD  N * V

C  C V

i
p
out
 VDD 
CiVDD  I out / f
Ci
Ctot 2Ci  C p CiVDD  I out / f   Ci Ci  C p VDD 

(Vout  VDD )
2
Ci
CiVDD  I out / f 
Ci ,min
 I out
I out

 
VDD f
 VDD f
2
 C p I out
 
 VDD f
[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
Efficiency estimation
[Performance limits of switched-capacitor DC-DC Converter]
Vout

M *Vin
M=ideal conversion ratio
Output impedance (slow switching)
[Performance limits of switched-capacitor DC-DC Converter]
 i  Ts
 Vout
1
Ro 

q / Ts
fC
Ts=switching period
i= parasitic time constant
q=charge supplied to the source Vout
Cross-Coupled Charge Pump
VDD
2Vdd RL
Vo 
RL  Rds( on)
I
Vripple  L
2f
M10
M9
 1
1 



 CL C1  CL 
Vo 
1 
  sC1  sC L  
I L 
RL 
Vo
1
C1
RL
CL1
C2
phi2
phi1
• 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
1
Commercial
products (Linear
Technology,
Fairchild, Maxim …)
Buck or Boost
3
functions
Negative voltage
generation
2
4
Oscillator and Control
H-bridge Topologies
Vin
1
2
Phase 1: transistors in red are on
Phase 2: transistors in blue are on
Vin
3
4
Vout
1
doubler
phi1
2
phi2
phi1
phi2
Vout = 2Vin
Vout
Vin
1
3
4
Vin
Vout
phi1
inverter
phi2
2
phi1
phi2
3
4
Vin
Vout = -Vin
phi1
Splitter
phi2
Vout = 0.5 Vin
phi1
phi2
Application (1): Flash Memory
Floating gate programming
Control gate voltage >> Vdd
[ee141 lecture]
Application (1): Flash Memory
Nominal VDD= 5V
Application (2): Sample Switches
vicm
vicm
phi1
phi2d
Ci
Vi+
S/H circuit– constant
vgs sampling with all
input level
Reduces distortion
Reduces Rds(on)
M10
CL
phi1d
phi2
Cs
-
+
Vo+
OTA
+
-
Cs
phi1d
Vo-
phi2
Vi-
CL
phi2d
phi1
vicm
Ci
vicm
VDD
M9
Phi_bar
A
M4
M5
M8
M3
M7
Voltage
doubler
C1
C2
C3
M2
Phi
M1
M9
M6
M11
Vo
VSS
Phi_bar
Vin
Application (3): Low voltage
Amplifier
Positive zero in
Miller compensation
1/gm pole-zero
cancellation
VDD
[charge-pump assisted
low-power/low-voltage CMOS Opamp Design]
V-
Charge pump
V+
>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