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Buck Regulator Architectures
4.4 Constant On Time (COT) Buck Regulators
Constant ON-Time (COT) Hysteretic
Regulator
ON-time is constant, for a given VIN, as load current varies
• Advantages
+
+
VREF
• Disadvantages
– Requires ripple at
feedback comparator
– Sensitive to output noise,
because it translates to
feedback ripple
-
– Constant
frequency vs. VIN
– High Efficiency
at light load
– Fast transient
response
VIN
Modulator
Error
Comparator
VFB
One-Shot
Inversely
Proportional
to VIN
Power
Stage
L
VOUT
C
RL
RC
(ESR)
RF1
RF2
Ripple is needed to properly
switch the comparator!!
2
Frequency of Operation (Continuous)
TON is the on-time and FS is
the operating frequency. The
constant on-time controller
sets the on-time of the Buck
switch.
K is a constant and RON is a
programming resistor. VIN is
in the denominator as
expected, setting the on-time
inversely proportional to VIN.
Rearrange and substitute
TON into the first equation,
then solve for FS
3
Constant ON-Time Achieves Nearly
Constant Frequency
• Switching frequency is almost constant; the variations are due to effects
of RDS-ON, diode voltage and input impedance of the RON pin
• Note: A resistor from VIN to RON sets the ON-time
4
Constant On-Time Regulator
Waveforms (Discontinuous)
For a COT regulator, the constant frequency relationship holds true
provided the inductor current remains continuous. At light loading
conditions the current in the inductor will become discontinuous. Shown
here are the switching waveforms for a Buck regulator controlled with
constant on-time control in the discontinuous conduction mode, which
means the ramping inductor current returns to zero every cycle.
5
Initial Configuration Circuit
Input
Voltage
VCC
VIN
C1
C3
RON
BST
C4
L1
RON/SD
SW
VOUT
D1
LM2695
R1
RTN
SGND
R3
C2
FB
R2
Ripple here is greater than
that at FB by the ratio of
(R1+R2)/R2.
Ripple here must be
>25 mVp-p
• Ripple voltage at VOUT is the inductor’s ripple current x R3
• Since the inductor’s ripple current increases as VIN increases, the ripple
voltage at VOUT increases along with it
6
Initial Config. Transient Response
Load Transient Response
400 mA
100 mA
50 mV
Output Voltage
LM2695 Initial Circuit
VIN = 12V, VOUT = 10V
7
Reduce the Ripple With
One Capacitor!
Intermediate Ripple Configuration
Input
Voltage
VCC
VIN
C1
C3
RON
BST
C4
L1
RON/SD
SW
VOUT
D1
LM2695
C5
RTN
SGND
R1
R3
C2
FB
R2
Ripple here can
now be a minimum
of 25 mVp-p - same
as at FB.
Ripple here must be
>25 mVp-p
Adding C5 allows the ripple at FB to be same as at VOUT
without the attenuation of R1 & R2.
This reduces the ripple, but does not eliminate it
8
COT Transient Response With One
Capacitor Added
Load Transient Response
400 mA
100 mA
20 mV
Output Voltage
LM2695 Intermediate Ripple Configuration
VIN = 12V, VOUT = 10V
9
How to Achieve Minimum Ripple
Input Voltage
VCC
VIN
C1
C3
RON
BST
R3 has been
removed.
C4
L1
RON/SD
VOUT
SW
LM2695
D1
R4
C6
C2
C7
FB
RTN
R1
SGND
Ripple here must be
>25 mVp-p
Ripple here
R2 depends on C2's
ESR, and the
inductor ripple
current.
10
Minimum Ripple-Circuit Transient
Response
Load Transient Response
400 mA
100 mA
10 mV
Output Voltage
LM2695 Minimum Ripple Configuration
VIN = 12V, VOUT = 10V
11
Good To Know:
What Happens if R3 is Removed?
BST
C4
The circuit regulates poorly with a lot of
noise and jitter!!
L1
SW
VOUT
D1
R1
SGND
C2
VSW
FB
R2
tON
Ripple here must be
>25 mVp-p
VOUT
tOFF
SW Pin
Preferred waveform
VOUT
Ripple
Going down when it
should be going up!!
12
Good To Know:
Don’t Put Too Much Output Capacitance!
VIN
VIN
VCC
C3
C1
RON
BST
C4
L1
RON/SD
SW
LM2695
VOUT
D1
Load
R1 R3
RTN
SGND FB
C2
Distributed capacitance
around the PC board
R2
13
Other Items To Keep In Mind
• The flyback diode should be a Schottky, not an Ultra-fast!
• A 0.1 μF ceramic chip capacitor adjacent to the VIN pin is mandatory!
• PC board traces must be routed carefully!
Keep the loops physically small to minimize radiated EMI.
14
Thank you!
15