ACOT (Advanced Constant-On Time) Synchronous Step-Down Converters

Selection Guide
SG002–Oct 2015
ACOT™ (Advanced Constant-On Time) Synchronous Step-Down Converters
Introducing our new cost-effective, high performance ACOT Buck converters for Set Top Boxes, Industrial and commercial low power systems,
computer peripherals and LCD Monitors and TVs.
ACOT Key Features
The proprietary ACOT control scheme not only features ultra-fast transient response but also improves upon legacy constant on-time
architectures, achieving constant average frequency over line, load and output ranges to minimize interference and noise problems. The ACOT
Buck converters are stable with and optimized for ceramic output capacitors without external ripple injection scheme. The ACOT family includes
latch-off, hiccup and constant current protection modes.
Richtek ACOT™ (Advanced Constant On Time) Synchronous Step-Down Converter
Constant Frequency
MLCC Suitable
Frequency Locked Loop (FLL)
Pulse Shaping Regulators (PSR™)
provides constant average frequency
supports MLCC-only operation with
over line & load.
lowest component count.
Ultrafast Transient Response
T-on control with ‘’instant on’’ response
provides ultrafast transient response.
SG002-00 OCT 2015
© 2015 Richtek Technology Corporation
1
Selection Guide
Comparison Table of Richtek Buck Topologies
Topology
CMCOT
Current Mode
Constant-On-Time
Current Mode
ACOT
Advanced
Constant-On-Time
Response to Load Steps
moderate
fast
extremely fast
Current Sense
current sense limits min. ON time
low side current sense
not required
Min. ON Time
larger, limits the min. achievable
duty-cycle
small min. On time allows small
duty-cycles
small min. On time allows small
duty-cycles
Frequency
stable fixed f SW
constant average f SW
constant average fSW
Stable with MLCC
√
√
√
Slope Compensation
√
not required
not required
Synchronized to ext.
Clock
√
x
x
used for applications with steady
load conditions.
(some CM buck converters can be
synchronized to an external clock)
used in applications with moderate
load transients, or applications that
require small minimum ON times
(i.e. high switching frequency in
combination with larger step-down
ratios)
applications with severe fast load
transients , such as DDR, Core
SoC, FPGA and ASIC supplies
Applications
Which Buck topology is most suitable for your application?
Input voltage, output voltage and nature of the load will in most cases determine which topology is most suitable. Systems with steady load
conditions can use CM buck converter. To avoid sensitive frequency bands, some CM buck converters can be synchronized to an external
clock. Applications with severe fast load transients (such as seen in Core and DDR rails), should be driven by ACOT buck converters. Note
that switching frequency can deviate considerably during dynamic loads. CMCOT buck converter performance lies in between CM and ACOT
buck converters, so it can be used in applications with moderate load transients, or applications that require small minimum ON times (i.e.
high switching frequency in combination with larger step-down ratios). The dynamic frequency deviation during load transients is less than
ACOT.
For more information, please read the application notes ’’Comparing Buck converter topologies’’ and ’’Choosing the optimal Buck converter for
your application’’.
SG002-00 OCT 2015
© 2015 Richtek Technology Corporation
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Selection Guide
Example of ACOT™ Product Overview: RT7275 & RT7276
3A, 4.5V to 18V VIN, 700kHz ACOT™ Synchronous Step-Down Converter
RT7275/76 datasheet download (PDF, 356KB)
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
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Simplified Application Circuit
Ultra Fast Transient Response
RT7275: Steady 700kHz Switching Frequency at all loads
RT7276: Steady 700kHz Switching Frequency in continuous
conduction mode
Internal 90mΩ Switch and 60mΩ Synchronous Rectifier
0.765V to 8V Adjustable Output Voltage
Optional Output Discharge Function(TSSOP-14 only)
Power Good Indication
Externally-adjustable, Pre-biased compatible Soft-Start
Cycle-by-cycle current limit
Current Limit with Latch-off Mode Under-Voltage
Protection(TSSOP-14); Current Limit with Hiccup Mode
Under-Voltage Protection (DFN-10L 3x3)
Output Over-Voltage Protection
Pin Configurations
EN
FB
PVCC
SS
PGOOD
1
2
3
4
5
PGND
Features
11
10
9
8
7
9
Datasheet
VIN
VIN
BOOT
SW
SW
WDFN-10L 3x3
SG002-00 OCT 2015
© 2015 Richtek Technology Corporation
VOUT
FB
PVCC
SS
GND
PGOOD
EN
14
2
13
3
4
12
PGND
5
6
7
11
10
15
9
8
VINR
VIN
BOOT
SW
SW
PGND
PGND
TSSOP-14 (Exposed Pad)
3
Selection Guide
ACOT™ Product Family and Sample Ordering Information


VIN range up to 6.0V, IOUT up to 2A
Key features: Power Good, low quiescent current, and ±2% VREF
VIN
Current rating
Package
f SW
Over current
behaviour
IQ
P/N
PSM/PWM
Force-PWM
For enhanced light load
efficiency
2.5V-6.0V
2A
EN
FB
AGND
VOUT
1
8
2
7
3
6
4
5
1.5MHz
PGOOD
VIN
LX
PGND
Cycle-by-cycle
Current limit
22µA
*Coming soon
*Coming soon
RT5784A
RT5784B
RT5785A*Coming soon
RT5785B*Coming soon
RT5795A
-
AGND
7
6
5
2
3
4
PGND
VIN
LX
VOUT
FB
8
PGOOD
EN
WDFN-8JL 2x1.5 (FC)
2.5V-5.5V
EN
PGND
AGND
FB
PGND
TSOT-23-8 (FC)
1
2
3
4
VIN
LX
PGOOD
VOS
8
7
6
5
9
2.7MHz
30µA
WDFN-8SL 2x2
VIN rangeup to 18.0V, IOUT up to 3A in SOT-23 and WDFN 2x3 small packages
Key features: simple and easy low BOM cost design with excellent FCOL thermal performance, 500kHz f SW and ±1.5% VREF
EN
PGOOD
8
7
6
5
2
3
4
Over current behaviour
P/N
Hiccup
PSM/PWM
For enhanced light load
efficiency
Constant current
Force-PWM
RT7255A
✓
✓
RT7255B
RT7255C
✓
RT7255D
✓
FB
SS
1.5A
VIN
Package
BOOT
4.3V-18V
Current rating
GND
VIN
SW


SOT-23-8
2A
2.5A
(3Apeak)
3A
4.3V-18V
1.5A
BOOT
GND
LX
VIN
1
2
3
4
GND
4.5V-18V
9
8
7
6
5
SS
PGOOD
EN
FB
✓
RT6232A
RT6232B
✓
RT6233A
RT6233B
✓
RT6234A*Coming soon
RT6234B*Coming soon
✓
RT7285/A
WDFN-8L 2x3
SW VIN EN
6
5
4
2
3
BOOT GND FB
✓
4.5V-18V
1.5A
6
EN BOOT
5
4
2
3
RT7285C
✓
T/SOT-23-6
FB
RT7285B
RT7285D
✓
RT6211A
✓
RT6211B
✓
VIN GND LX
SOT-23-6
4.3V-18V
2.5A
(3A peak)
SW VIN EN
6
5
4
2
3
BOOT GND FB
✓
2A
BOOT EN
6
2.5A
(3A peak)
4.3V-18V
3A
(3.5A peak)
FB
✓
2
3
✓
SW VIN EN
6
5
4
2
3
BOOT GND FB
3A
BOOT EN
6
FB
5
4
2
3
RT6212B
RT6213A
RT6213B
✓
RT7295A
✓
✓
RT7295B
RT7295C
✓
TSOT-23-6 (FC)
4.5V-18V
RT7294D
RT6212A
✓
4
GND LX VIN
RT7294C
✓
5
TSOT-23-6 (FC)
RT7294B
✓
TSOT-23-6 (FC)
4.5V-18V
RT7294A
✓
RT7295D
✓
✓
✓
RT6214A
RT6214B
GND LX VIN
TSOT-23-6 (FC)
SG002-00 OCT 2015
© 2015 Richtek Technology Corporation
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Selection Guide


VIN range up to 18.0V, IOUT up to 5.5A in TSSOP-14, WDFN and SOP-8 packages
Key features: high performance, programmable Soft-Start, Power Good in TSSOP-14 & WDFN-10L 3x3, 650kHz f SW and ±1% VREF
VIN
Current
Package
Features
rating
Over current behaviour
P/N
VOUT
Discharge
function
Latch-off
Constant
PSM/PWM
current
For enhanced light
✓
✓
✓
RT7280GCP
RT7279GCP
✓
✓
✓
RT7276GCP
RT7275GCP
✓
✓
✓
RT7232GCP
RT7231GCP
✓
✓
✓
RT7236GCP
RT7235GCP
Power
Good
4.5V-18V 2A
3A
4A
5A
VOUT
FB
PVCC
SS
GND
PGOOD
EN
VINR
VIN
BOOT
SW
SW
PGND
PGND
14
2
13
3
12
PGND
4
11
5
10
6
9
15
7
8
TSSOP-14 (Exposed Pad)
2A
4A
5A
1
2
3
10
9
PGND
3A
EN
FB
PVCC
SS
PGOOD
4
5
8
7
6
11
VIN
VIN
BOOT
SW
SW
WDFN-10L 3x3
5.5A
Force-PWM
load efficiency
✓
✓
RT7280GQW
RT7279GQW
✓
✓
RT7276GQW
RT7275GQW
✓
✓
RT7232GQW
RT7231GQW
✓
✓
RT7236GQW
RT7235GQW
✓
✓
RT6206BHGQW
RT6206AHGQW
2A
✓
RT7274GSP
RT7281GSP
3A
✓
RT7278GSP
RT7277GSP
✓
RT7234GSP
RT7233GSP
RT6201AHGSP
RT6201BHGSP
RT7240GSP
RT7239GSP
4A
EN
FB
2
4A
PVCC
3
SS
4
5A


Hiccup
8
GND
VIN
7
BOOT
6
SW
5
GND
9
✓
✓
SOP-8 (Exposed Pad)
5A
✓
RT6202AHGSP
RT6202BHGSP
5.5A
✓
RT6206BHGSP
RT6206AHGSP
VIN range up to 18.0V, IOUT up to 12A in UQFN packages
Key features: high current ACOT parts, programmable Soft-Start, Power Good, FCOL technology for low parasitic and good thermal
performance and ±1.0% VREF
VIN
Current
rating
Package
f SW
Over current
behaviour
RON
P/N
PSM/PWM
Force-PWM
For enhanced light load
BOOT
6A
SW
4.5V-18V 5A
GND
efficiency
13
12
11
650kHz
GND
1
10
VIN
EN
2
9
VIN
FB
3
8
PGOOD
NC
4
7
NC
60m/22m
RT6207A
RT6207B
51m/18m
RT6236A
RT6236B
40m/16m
RT6237A
RT6237B
35m/14m
RT6238A
RT6238B
30m/12m
RT6239A
RT6239B
12m/5.4m
RT6242A
RT6242B
6
PVCC
SS
5
Cycle-by-cycle
Current limit
GND
13
12
500kHz
1
11
GND
FB
2
10
GND
PVCC
3
9
GND
PGOOD
4
8
VIN
5
6
7
VIN
AGND
SW
9A
14
BOOT
8A
EN
7A
SS
UQFN-13JL 2x3 (FC)
EN
SW
SW
12A
RLIM
UQFN-14L 2x3 (FC)
16
15
14
13
SW
PVCC
3
10
SW
RT
4
9
BOOT
5
6
7
8
PGOOD
SW
11
GND
12
2
SS
1
FB
VIN
AGND
Adj. f SW
from
300kHz
to
700kHz
Adj. current limit
from
10A to 15A
UQFN-16JL 3x3 (FC)
SG002-00 OCT 2015
© 2015 Richtek Technology Corporation
5
Selection Guide


VIN range up to 23.0V, IOUT up to 8A in WDFN, UQFN packages
Key features: 500kHz, Power Goodand±1% VREF accuracy.
Vin
Current
rating
Package
Over current
behaviour
Latch-off
RDSON
P/N
Hiccup
PSM/PWM
Force-PWM
For enhanced light load
efficiency
4A
PGOOD
VBYP
VIN
EN
FB
1
2
3
10
9
PGND
5V-23V
4
5
8
7
6
11
SW
SW
SW
BOOT
VCC
✓
✓
67mΩ/41mΩ
RT6219A
RT6219BL
67mΩ/41mΩ
RT6219AH
RT6219BH
31mΩ/20mΩ
RT6220A
RT6220BL
31mΩ/20mΩ
RT6220AH
RT6220BH
RDSON
P/N
WDFN-10L 3x3
✓
AGND
EN
FB
VCC
BOOT
6A
14 13 12 11 10
VIN
1
PGND
2
15
9
SW
8
SW
SW
16
SW
4
5
6
✓
7
VBYP
PGOOD
AGND
AGND
VOUT
3
UQFN-16L 3x3 (FC)
VIN
Current
rating
Package
Over current
behaviour
With LDO
Buck
VOUT
PSM/PWM
For enhanced light load
EN2
efficiency
5V-23V
8A
7
6
5
8
VIN
9
GND
70mA,
3.35VLDO
Fixed
3.3V
27mΩ/10mΩ
RT7238B
70mA,
5.1VLDO
Fixed
5.0V
RT7238C
70mA,
Adj. VLDO,
Adj. VOUT
0.6V– 5.0V
RT7238D
4
3
2
10
1
LX
BOOT
LDO
VOUT
NC
PGOOD
EN1
Cycle-by-cycle
Current limit
LDO
RT7238B
UQFN-10L 3x3 (FC)
7
6
5
8
VIN
9
GND
4
3
2
10
1
LX
BOOT
NC
VOUT
NC
PGOOD
EN
BYP
RT7238C/E
UQFN-10L 3x3 (FC)
7
6
5
8
VIN
9
GND
4
3
2
1
10
LX
BOOT
LDO
FB
ILMT
PGOOD
EN
RT7238D
UQFN-10L 3x3 (FC)
Technical Support
ACOT™ Video
ACOT™ video Youtube
Richtek Designer
Application Note




SOT-23 FCOL Thermal considerations
ACOT™ Stability Testing
Comparing Buck converter topologies

Choosing the optimal Buck converter for
your application
ACOT Stability Design Tool
ACOT™ video Tudou: ENGLISH,CHINESE
ACOT™ video YouKu: ENGLISH,CHINESE
SG002-00 OCT 2015
© 2015 Richtek Technology Corporation
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