IC Design of Power Management Circuits (I)
-
Upload
claudia-sin -
Category
Education
-
view
663 -
download
39
Transcript of IC Design of Power Management Circuits (I)
IC Design ofPower Management Circuits (I)
Wing-Hung KiIntegrated Power Electronics Laboratory
ECE Dept., HKUSTClear Water Bay, Hong Kong
www.ee.ust.hk/~eeki
International Symposium on Integrated CircuitsSingapore, Dec. 14, 2009
Ki 2
1. Switching Converters: Fundamentals and Control
2. Switching Converters: IC Design
3. Switching Converters: Stability and Compensation
4. Fundamentals of Bandgap References
5. Development of Integrated Charge Pumps
6. Introduction to Low Dropout Regulators
Tutorial Content
Ki 3
Part I
Switching Converters:Fundamentals and Control
Ki 4
Steady State AnalysisLossless elementsBuck, boost, buck-boost power stagesVolt-second balanceContinuous conduction modeDiscontinuous conduction modeRinging suppressionPseudo-continuous conduction modeEfficiency
Performance Evaluation Parameters
Control TopologiesPWM voltage mode controlPWM current mode control
Single-Inductor Multi-Input Multi-Output Converters
Content
Ki 5
Linear Regulator has Low Efficiency
C LR
oV
1R
2R
obV
VREF
NM
EAddV
Q1I oIQ3IQ2I
ddI
Efficiency of linear regulator is not high:
η = = = < <+
o o o o o o
in dd dd dd o Q dd
P V I V I V1
P V I V I I V
Can one design a power converter with efficiency close to 1?
power converter
Ki 6
Switches as Lossless Components
A power converter with high efficiency needs lossless components.Reactive elements: capacitors, inductorsActive elements: switches
swI+
−swV
= Vsw ×Isw= Vsw ×0= 0
switch closed
Psw
swI+
−swV
store & relax
PC = 0
store & relax
PL = 0
switch open
Psw = Vsw ×Isw= 0×Isw= 0
CL
Ki 7
Switching Converter: Heuristic Development (1)
LR
oV
ddV
LR
oV
ddV
1SW
o ddV V=
t
No regulation
o ddV DV=
t
Load cannot accept a pulsating supply voltageduty ratio = D
ddV
Ki 8
C LR
oV
ddV
L1SW
C LR
oV
ddV
L1SW
2SW
xV
Add a lossless filter to achieve small ripple voltage, but …when switch is off, inductor current cannot change instantaneously and cause spark (volt-second balance).
Add a second switch that operates complementarily to arrive at a functional switching converter.
o ddV DV=
t
ddV
Switching Converter: Heuristic Development (2)
Ki 9
Buck, Boost and Buck-Boost Converters (1)
C LR
oV
ddVL
xV
C LR
oV
ddV
LxV
C LR
oV
ddV L
xV
One L and one C gives a second order switching converter.
C has to be in parallel with RL for filtering, leaving three ways to place L, SW1 and SW2 between Vdd and RL .
Three types of converters:Step-down: buckStep-up: boostStep-up/down: buck-boost
(Boost-buck, or Cuk, is a 4th order converter)
Buck
Boost
Buck-boost
1SW
2SW
1SW
2SW1SW
2SW
Ki 10
Buck, Boost and Buck-Boost Converters (2)
C LR
oV
ddV
LNM
1D
xV
C LR
oV
ddV
L xV
C LR
oV
ddVL
xV
SW1 is the controlling switch that determines the duty ratio D, while SW2 provides a path for the inductor current i to flow when SW1 is off.
SW1 can be a power NMOS (MN ). If power PMOS is used, the phase has to be reversed.
To prevent i from going negative, SW2
is usually implemented by a diode (D1), but the forward drop gives a low efficiency.
Note that Vo of buck-boost is negative.
Buck
Boost
Buck-Boost
i
state 1
state 2
NMstate 1
state 2state 1
NM
state 2
i
i
1D
1D
Ki 11
I-V Relations of C and L
The I-V characteristics of a capacitor and an inductor are described by
= cc
dvi C
dt=
div L
dt
Approximations are very useful in many calculations:
Δ=
Δc
cV
i Ct
Δ=
Δi
v Lt
ci +
−
cv
+
−
v
i
For sinusoidal steady state, the phasor relations are:
= =ω
cc
c
v 1zi j C
= = ωv
z j Li
CL
Ki 12
Volt-Second Balance
= ⇒ Δ = Δdi V
v L I tdt L
Switching actions cause ripples for both inductor current (i ) and capacitor voltage (vc). In the steady state, both quantities return to the same value after one cycle.
+ − v i
L
=11
V (S )m
L= −2
2V (S )
mL
1t(or DT)
2t(or D ' T)
Inductor current has to obey volt-second balance (VS balance):
V (S1)×t1 + V (S2)×t2 = 0
⇒
m1 t1 = m2 t2 or m1 D = m2 D’
It is used to compute the conversion ratio M = Vo /Vdd .
i
ΔII
0A
Ki 13
Inductor, Input, Switch, Diode and Tail Currents
Consider the buck converter:
C LR
oV
ddV
L
NM
1D
xVi
ddi
i
di
tisi
di
ti
si
ddici
Input current idd : current through Vdd
Switch current is : i in State 1
Diode current id : i in State 2; even if diode is implemented by NMOS switch
Tail current it : current through the combination of C and RL .
Capacitor current ic : ac part of tail current
Load current io : averaged tail current
oI
oI
Ki 14
Continuous Conduction Mode
The converter is operating in continuous conduction mode (CCM) if the inductor current is always larger than zero.
Buck converter(Step-down)
m1 D = m2 D’⇒
(Vdd -Vo )D = Vo D’
⇒ = =0
dd
V M D
V
Boost converter(Step-up)
m1 D = m2 D’⇒
Vdd D = (Vo -Vdd )D’
⇒ = =−
0
dd
V 1 MV 1 D
Buck-boost converter(Step-up/down)
m1 D = m2 D’⇒
Vdd D = -Vo D’
−⇒ = =
−0
dd
V D MV 1 D
oVddV oVddV oVddV
+
−V
1S 1S
1S2S2S 2S+ −V + −V
Ki 15
Discontinuous Conduction Mode
When the switching converter is operation in CCM, one switching cycle has two states S1 and S2 . When the load current becomes smaller and smaller, eventually the inductor current would fall to zero, and the converter then operates in discontinuous conduction mode (DCM) with a third state S3 . During D3 T, all switches are open.
=11
V (S )m
L= −2
2V (S )
mL
DT
i
ΔI
2D T 3D T
=3V (S )0
L
=i 0
VS balance becomes:
m1 D = m2 D2
Ki 16
C LR
oV
ddVL1SW
2SW
xV
xC
When both switches are open, L, C and the parasitic capacitor Cx at Vx form a resonance circuit that leads to serious ringing.
Ringing Suppression
We may add a small switch to short the inductor when SW1 and SW2 are both off [Jung 99].
xV
i
C LR
oV
ddVL1SW
2SW
xV
xC
3SW
xV
i
oVddV
Ki 17
Pseudo-Continuous Conduction Mode
C LR
oV
ddV
L
1SW
2SW
xV
FWSW
By increasing the size of the ringing suppression switch, a switching converter may work in pseudo-continuous mode (PCCM). It was first employed in a single-inductor dual-output (SI-DO) converter to increase the current handling capability [Ma 03b]. When both SW1 and SW2 are open, the freewheel switch SWFW is closed to allow free-wheeling of i at Ipccm.
i
pccmI
i
0
Ki 18
Efficiency of Buck Converter
C LR
oV
ddV
L1S
η = =o o o
dd dd dd
P V IP V I
For an ideal buck converter working in CCM, the conversion ratio M is Vo /Vdd = D, and Io :Idd = 1:D, giving η=1. If conduction loss is accounted for, then Io /Idd is still 1/D, but M is modified as M=ηD, with
η = =+ +
+
o
s ddd
L
P 1R DR D'RP 1
R
ddI
R
dR
sR
2S
oI
Ki 19
Efficiency of 2nd Order Converters
By accounting for conduction losses due to switch, diode and inductor series resistance (Rs , Rd and R , respectively), the efficiencies of buck, boost and buck-boost converters are computed as [Ki 98]
Buck:
Boost:
Buck-boost:
η =+ +
+buck
s d
L
1R DR D'R
1R
η =+ +
+boost
s d2
L
1R DR D'R11
RD'
−η =+ +
+buck boost
s d2
L
1R DR D'R11
RD'
Ki 20
Performance Evaluation Parameters
For a good voltage regulator, the output voltage should remain constant even the input voltage, load current or temperature changes.
Steady state parameters:Line regulationLoad regulationTemperature coefficient
Small signal parameters:Power supply rejectionOutput impedance
Transient parameters:Line transient (settling times)Load transient (settling times)Reference tracking time
Ki 21
Line Regulation
in mV / VΔ=
Δo
dd
Vline reg.
V
Line regulation is the change of Vo w.r.t. the change in Vdd :
Δ=
Δo o
dd
V / VV
in % / V
Switching converters are non-linear circuits for large signal changes, and hand analysis is impossible. It could be obtained by simulation. In datasheets, line regulation is usually measured.
Ki 22
Power Supply Rejection
Power supply rejection (PSR) is the small signal change of Vo w.r.t. the small signal change in Vdd .
In transfer function form:
In dB:
Usually |vo /vdd | < 1, but we customarily give a positive PSR in dB.
Note: Line reg. ≈
PSR × ΔVdd
= o
dd
vPSR
v
= × dd
o
vPSR 20 log
v
For a good switching converter (also for bandgap reference and linear regulator), the output voltage should be a weak function w.r.t. the supply voltage. Hence, a small signal parameter, the power supply rejection, gives good indication of line regulation.
Ki 23
Load Regulation and Output Impedance
in mV /mAΔ=
Δo
o
Vload reg.
I
Load regulation is the change of Vo w.r.t. the change in Io :
Δ=
Δo o
o
V / VI
in % /mA
In datasheets, load regulation is usually measured.
In the small signal limit, load regulation is the output impedance:
= oo
o
dVR
dIΩin
Ki 24
Temperature Coefficient
Temperature coefficient (TC) is the change of a parameter X w.r.t. the change in T, and is a large signal parameter:
−Δ= =
Δ −2 1
2 1
X(T ) X(T )XTC
T T T
TC could be positive or negative.
oin [X] / C
Δ=
ΔX / X
Toin ppm / C
Ki 25
PWM Voltage Mode Control (1)
S
RQ
Q
refVA(s)
EACMP
gV
oV
ckramp
L
CLR
1R
2R
obVav
av
PM
NM
A regulated switching converter consists of the power stage and the feedback circuit.
For a buck converter, if an on-chip charge pump is not available, then the NMOS power switch is replaced by a PMOS power switch.
avramp
ck
Q
Q
Ki 26
The output voltage Vo is scaled down by the resistor string R1 and R2 . The scale factor is b = R2 /(R1 +R2 ).
The scaled output voltage bVo is compared to the reference voltage Vref to generate a lowpass filtered voltage Va through the compensator A(s).
At the start of the clock, the SR latch is set and the switch MP is turned on, starting the duty cycle. A sawtooth waveform (ramp) synchronized with the clock ramps up.
When the ramp reaches the level of Va (trip point), the SR latch is reset, terminating the duty cycle.
When the SR latch is set, i ramps up. When the SR latch is reset, i ramps down. In the steady state, i returns to the same level at the start of every clock cycle.
PWM Voltage Mode Control (2)
Ki 27
PWM Feedback Action
For stability, the control loop has to have negative feedback.
Assume Vo drops suddenly due to change in load or disturbance⇒
error voltage Verr = (Vref –bVo ) becomes larger⇒
Va = A(f)(Vref –Vo ) also becomes larger⇒
with a higher Va , it takes the ramp longer to reach Va⇒
duty ratio D is temporarily increased⇒
more current is dumped into the load⇒
Vo rises accordingly and eventually settles to the original value
Note that A(s) is the frequency response of the compensator, not of the op amp Aop (s).
Ki 28
PWM Current Mode Control
S
RQ
Q
refVA(s)
EACMP
ddV
oV
ck
L
CLR
1R
2R
obVav
av
i
fi R
i /N
fNR
PM
NM
A current mode controlled switching converter is realized by replacing the fixed voltage ramp with the inductor current ramp.
ddVcurrentsensor
Ki 29
Sub-harmonic Oscillation and Slope Compensation
Output of EA Va cannot change in one cycle. If inductor current is perturbed by an amount of ΔI1 , oscillation occurs if
Δ −= > ⇔ >
Δ2 2
1 1
I m1 D 0.5
I m
=a a fI V /R
Δ 1IΔ 2I
=a a fI V /R
Δ 1I Δ 2I
1m − 2m1m − 2m
To prevent oscillation, employ slope compensation by adding a negative slope to Ia (i.e., Va ) to suppress the change in ΔI2 .
Δ 1I Δ 2I
1m − 2m
− cm
=a a fI V /R
<D 0.5 >D 0.5
−Δ= < ⇔ >
Δ +c 22 2
c1 c 1
m mI m1 m
I m m 2
Ki 30
Current Mode PWM with Compensation Ramp
S
RQ
Q
refVA(s)
EACMP
ddV
oV
ck
L
CLR
1R
2R
obVav
i
i /N
fNR
PM
NM
V2I
ramp from OSCav
DT
1 c f(m m )R+ 2 c f(m m )R− −
bvbv
In practice, the output of EA (Va ) should not be tempered, and a compensation ramp of +mc is added to m1 instead.
compensationramp
ddV
Ki 31
Synchronous Rectification
ddV
L
CLR
iPM
NM
To eliminate loss due to forward diode drop, the power diode is replaced by a power NMOS MN , and the scheme is known as synchronous rectification. To eliminate short-circuit loss of MP and MN , a break-before-make (BBM) buffer is used.
S
RQ
QBBM
Buffer
PV NV
PQ,V
NV
Q(ck) φ1
φ =2 NV
φ1
φ2
φ =1
PV
φ2Additional logic is needed for DCM operation. Non-overlapping φ1 and φ2
Ki 32
Multiple-Output Converters
L
1CL1R
o2V
1S
L
2CL2R
o1V
1i
2i 2i
1i
0S
2S
0S
ddV
Consider two boost converters that operate in deep DCM:
T 2T
T 2T
ddV
Ki 33
Single-Inductor Multiple-Output Converters
2C
o2V
L
L2R
i
0S
2S
ddV
1CL1R
1S o1V i
Time-multiplexing allows sharing one inductor and diverting the inductor current to two or more outputs [Ma 03a]:
T 2T
Ki 34
SIMO Converter in PCCM
2C
o2V
L
L2R
i
0S
2S
ddV
1CL1R
1S o1Vi
To handle large load currents, raise the inductor current floor to operate in PCCM. Add a free-wheeling switch (SFW ) to short the inductor when the inductor current reaches Ipccm [Ma 03b].
T 2T
i
T 2T
FWS
pccmI
Ki 35
SI-MIMO Converter
Energy-harvesting Boost 1 Rechargeable Boost 2 Loadsource battery
srcV
srcV
batV
batV
loadV
loadV
Energy-harvesting SI-DIDO boost Load Rechargeablesource battery
batV
Some applications need two converters in series with reduce efficiency.
Reorganize by using a SI-DIDO converter that needs only one inductor [Lam 04b], [Lam 07b], [Sze 08].
Ki 36
Development of SI-MO and SI-MIMO Converters
The recent years sees active R&D activities of SI-MO and SI-MIMO switching converters for low power applications. It is important to recognize the contribution of the first developers.
The idea of SI-MO converters was first conceived in [Goder 97], and only boost sub-converters were considered.
An SI-DO converter with buck-boost sub-converters was discussed in [Ma 97] to demonstrate the switching flow graph modeling method.
SI-DO converters became commercial products [MAX 98, UCC 99].
The concept of SI-MO was reinvented [Li 00, Ma 00, Ma 01, May 01]. [Ma 01] stressed the importance of DCM operation for reducing cross-regulation. A systematic classification is discussed in [Ki 01].
DCM operation is extended to PCCM operation in [Ma 02].
The concept of SI-MIMO was conceived [Lam 04, Lam 07].
Ki 37
References: Switching Converter Fundamentals
Books:[Brown 01] M. Brown, Power Supply Cookbook, EDN, 2001.
[Erickson 01] R. W. Erickson and D. Maksimovic, Fundamentals of Power Electronics, 2nd Edition, Springer Science, 2001.
[Kassakian 91] J. G. Kassakian, M. F. Schlecht and G. C. Verghese, Principle of Power Electronics, Addison Wesley, 1991.
[Krein 98] P. E. Krein, Elements of Power Electronics, Oxford, 1998.
Papers:[Jung 99] S. H. Jung et. al., "An integrated CMOS DC-DC converter for
battery-operated systems," IEEE Power Elec. Specialists Conf., pp. 43–47, 1999.
[Ki 98] W. H. Ki, "Signal flow graph in loop gain analysis of DC-DC PWM CCM switching converters," IEEE TCAS-1, pp.644-655, June 1998.
Ki 38
[Goder 97] D. Goder and H. Santo, “Multiple output regulator with time sequencing,” US Patent 5,617,015, April 1, 1997.
[Ma 97] Y. H. Ma and K. M. Smedley, "Switching flow-graph nonlinear modeling method for multistate-switching converters," IEEE Trans. on Power Elec., pp.854–861, Sept., 1997.
[MAX 98] "MAX685: Dual-output (positive and negative) DC-DC converter for CCD and LCD", Maxim Datasheet, 1998.
[UCC 99] "UCC3941: 1V synchronous boost converter," Datasheet, Unitrode Semiconductor Products, Jan. 1999.
[Li 00] T. Li, "Single inductor multiple output boost regulator," US Patent 6,075,295, June 13, 2000.
[Ma 00] D. Ma and W. H. Ki, "Single-inductor dual-output integrated boost converter for portable applications," 4th Hong Kong IEEE Workshop on SMPS, pp. 42- 51, Nov. 2000.
References: Early Development of SI-MIMO Converters (1)
Ki 39
[Ma 01a] D. Ma, W. H. Ki, C. Y. Tsui and P. Mok, "A single-inductor dual-output integrated DC/DC boost converter for variable voltage scheduling", IEEE/ACM Asia South Pacific Design Automation Conf., LSI University Design Contest, pp.19–20, Jan. 2001.
[May 01] M. W. May, M. R. May and J. E. Willis, "A synchronous dual-output switching dc-dc converter using multibit noise-shaped switch control," IEEE Int’l Solid- State Circ. Conf., pp.358–359, Jan 2001.
[Ma 01b] D. Ma, W. H. Ki, P. Mok and C. Y. Tsui, "Single-inductor multiple-output switching converters with bipolar outputs", IEEE Int'l. Symp. on Circ. and Syst., pp. III-301 - III-304, Sydney, May 2001.
[Ma 01c] D. Ma, W. H. Ki, C. Y. Tsui and P. Mok, "A 1.8V single-inductor dual-output switching converter for power reduction techniques," IEEE Symp. on VLSI Circ., Kyoto, Japan, pp. 137-140, June 2001.
[Ki 01] W. H. Ki and D. Ma, "Single-inductor multiple-output switching converters", IEEE Power Elec. Specialists Conf., Vancouver, Canada, pp.226–231, June 2001.
[Ma 02] D. Ma, W.H. Ki, and C.Y. Tsui, "A pseudo-CCM / DCM SIMO switching converter with freewheel switching", IEEE Int'l Solid–State Circ. Conf., San Francisco, pp.390–391+476. Feb. 2002.
References: Early Development of SI-MIMO Converters (2)
Ki 40
[Ma 03a] D. Ma, W. H. Ki, C. Y. Tsui and P. Mok, "Single-inductor multiple-output switching converters with time-multiplexing control in discontinuous conduction mode," IEEE J. of Solid-State Circ., pp. 89-100, Jan. 2003.
[Ma 03b] D. Ma, W. H. Ki and C. Y. Tsui, "A pseudo-CCM/DCM SIMO switching converter with freewheel switching," IEEE J. of Solid-State Circ., pp. 1007- 1014, June 2003.
[Lam 03] Y. H. Lam, W. H. Ki, C. Y. Tsui and P. Mok, "Single-inductor dual-input dual- output switching converter for integrated battery charging and power regulation," IEEE Int'l. Symp. on Circ. and Syst., Bangkok, Thailand, pp. III.447-III.450, May 2003.
[Lam 04] H. Lam, W. H. Ki, C. Y. Tsui and D. Ma, "Integrated 0.9V charge-control switching converter with self-biased current sensor," IEEE Int'l Midwest Symp. on Circ. & Sys., pp.II.305–II.308, July 2004.
[Koon 05] S. C. Koon, Y. H. Lam and W. H. Ki, "Integrated charge-control single- inductor dual-output step-up/step-down converter," IEEE Int'l. Symp. on Circ. and Syst., Kobe, Japan, pp. 3071-3074, May 2005.
[Lam 07] Y. H. Lam, W. H. Ki and C. Y. Tsui, "Single-inductor multiple-input multiple- output switching converter and method of use," US Patent 7,256,568, Aug 14, 2007.
[Ma 09] D. Ma, W. H. Ki, and C. Y. Tsui, "Single-inductor multiple-output switching converters in PCCM with freewheel switching," US Patent 7,432,614, Oct. 7, 2008.
References: Early Development of SI-MIMO Converters (3)