View
32
Download
0
Category
Tags:
Preview:
DESCRIPTION
Buck Regulator Architectures. 4.6 Voltage Mode Buck Regulators. Voltage-Mode Regulator. Output Filter. Modulator. Error Amplifier. Advantages and Disadvantages. Advantages Stable modulation/less sensitive to noise Single feedback path Can work over a wide range of duty cycles - PowerPoint PPT Presentation
Citation preview
Buck Regulator Architectures
4.6 Voltage Mode Buck Regulators
Voltage-Mode Regulator
2
VIN
COUT
RESR
RLOAD
L
RFB1
RFB2
VOUT
VREF
VFB
VP
ZC
+
-+
-T
VC
Output Filter
Error Amplifier
Modulator
Advantages and Disadvantages• Advantages
– Stable modulation/less sensitive to noise– Single feedback path– Can work over a wide range of duty cycles
• Disadvantages– Loop gain proportional to VIN
– LC double pole often drives Type III compensation– CCM and DCM differences - a compensation challenge– Slow response to input voltage changes– Current limiting must be done separately
3
Modulator Gain
4
VIN
VSW
+
-
T VRAMP
VP
VC
VC
P
INM V
VA
Output Filter
5
CO
RESR
RLOAD
LVOUTVSW
ZA
ZB
BA
B
SW
OUT
ZZZ
VV
* (Rx, Cy) indicate the components that drive the locations of the pole and the zero, detailed equations are in the notes
-40
-20
0
20
40
10 100 1,000 10,000 100,000 1,000,000
o
ESR
dB
Hz
(L, Co)
(Resr, Co)
Error Amplifier
The easiest place to compensate the entire loop is to adjust the compensation around the error amplifier. Several different approaches are possible.
6
RFB2VREF
VFB
ZF
+
-
VOUT
VC
ZI
I
F
OUT
C
ZZ
VV
Type II Compensation
7
RFB2
RFB1VREF
VFB
+
-
VOUT
VC
RCCC1
CC2ZF ZI
I
F
OUT
C
ZZ
VV
* (Rx, Cy) indicate the components that drive the locations of the pole and the zero (and k), detailed equations are in the notes
-20
0
20
40
60
10 100 1,000 10,000 100,000 1,000,000
pz k
ZF - ZI
dB
Hz
(Rc, Cc1) (Rc, Cc2)(Rc, RFB2)
Design Guidelines for Type II Compensation• Choose a large value for RFB2, between 2-200 kW• Set the mid-band gain k to give desired bandwidth• Set P equal to half the switching frequency:
P = 2p*Fsw/2
• Set z equal to the output filter double pole O
• Use the following equations to solve for the remaining variables
8
CPC2 Rω
1C Cz
C1 Rω1C FB2C kRR
Type III Compensation
9
RFB2
RFB1VREF
VFB
+
-
VOUT
VC
RC1CC1
CC2
CC3
RC2
ZFZI
I
F
OUT
C
ZZ
VV
* (Rx, Cy) indicate the components that drive the locations of the poles and zeros, detailed equations are in the notes
-20
0
20
40
60
10 100 1,000 10,000 100,000 1,000,000
1P1z
k
2P
2z
dB
Hz
(Rc1, Cc1)
(RFB2, Cc3)
(Rc1, Cc2) (Rc2, Cc3)
Design Guidelines for Type III Compensation• Choose a large value for RFB2, between 2-200 kW• Set the mid-band gain k to shift the open-loop gain up to give desired
bandwidth• Set P1 equal to half the switching frequency:
P1 = 2p * Fsw/2
• Set P2 equal to the output filter zero, ESR
• Set Z1 and Z2 equal to cancel out the output filter double pole
• Use the following equations to solve for the remaining variables
10
FB2Z2C3 Rω
1C C3P2
C2 Cω1R
FB2C1 kRR
3
C1Z1
FB2Z2C1 Rω
RωC cC
C1P1C2 Rω
1C
Internal Type III Voltage Mode Compensation (LM367x)
11
Typical Application CircuitInternal Block Diagram
Internal Type III Voltage Mode Compensation (LM285x)
12
PVINAVINENSSSGND PGND
SNS
SW
LM2852/3
+
Lo
Co
VIN = 3.3V
CIN VOUT = 2.5 V
Ramp and Clock
Generator
EN
PVIN
Gate Drive SW
PGND
Error Amp
PWM Comp
OscillatorReference
UVLO DAC
Zc2
400 kW
200
kW
200
kW
20 pF
Current Limit
+
-
+
-
Zc1
SNS
SS
Internally Compensated
Typical Application CircuitInternal Block Diagram
13
Thank you!
Recommended