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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
+
+
VREF
– Requires ripple at
feedback comparator
– Sensitive to output noise,
because it translates to
feedback ripple
-
– Constant
frequency vs. VIN
– High Efficiency
– 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
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
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
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
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
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
```
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