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PDF RT7279 Data sheet ( Hoja de datos )
| Número de pieza | RT7279 | |
| Descripción | (RT7274 - RT7281) Synchronous Step-Down Converter | |
| Fabricantes | Richtek | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de RT7279 (archivo pdf) en la parte inferior de esta página. Total 22 Páginas | ||
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®
RT7274/79/80/81
2A, 18V, 700kHz ACOTTM Synchronous Step-Down Converter
General Description
The RT7274/79/80/81 is a synchronous step-down DC/
DC converter with Advanced Constant On-Time (ACOTTM)
mode control. It achieves high power density to deliver up
to 2A output current from a 4.5V to 18V input supply. The
proprietary ACOTTM mode offers an optimal transient
response over a wide range of loads and all kinds of ceramic
capacitors, which allows the device to adopt very low ESR
output capacitor for ensuring performance stabilization. In
addition, RT7274/79/80/81 keeps an excellent constant
switching frequency under line and load variation and the
integrated synchronous power switches with the ACOTTM
mode operation provides high efficiency in whole output
current load range. Cycle-by-cycle current limit provides
an accurate protection by a valley detection of low side
MOSFET and external soft-start setting eliminates input
current surge during startup. Protection functions indude
thermal shutdown for RT7274/79/80/81; output under
voltage protection and output over voltage protection for
RT7279/80 only.
Features
z ACOTTM Mode Enables Fast Transient Response
z 4.5V to 18V Input Voltage Range
z 2A Output Current
z High Efficient Internal N-MOSFET Optimized for
Lower Duty Cycle Applications
z 105mΩ Internal Low Side N-MOSFET
z Advanced Constant On-Time Control
z Allows Ceramic Output Capacitor
z 700kHz Switching Frequency
z Adjustable Output Voltage from 0.765V to 8V
z Adjustable and Pre-biased Soft-Start
z Cycle-by-Cycle Current Limit
z Input Under Voltage Lockout
z Thermal Shutdown
z RoHS Compliant and Halogen Free
Applications
z Industrial and Commercial Low Power Systems
z Computer Peripherals
z LCD Monitors and TVs
z Green Electronics/Appliances
z Point of Load Regulation for High-Performance DSPs,
FPGAs, and ASICs
Simplified Application Circuit
VIN
Input Signal
Power Good
RT7274/79/80/81
VIN SW
VINR*
EN
BOOT
FB
GND
PGOOD*
PVCC VOUT*
SS PGND*
VOUT
* : VINR pin for TSSOP-14 (Exposed Pad) only.
VOUT pin for TSSOP-14 (Exposed Pad) only.
PGND pin for TSSOP-14 (Exposed Pad) only.
PGOOD pin for TSSOP-14 (Exposed Pad) only.
Copyright ©2013 Richtek Technology Corporation. All rights reserved.
DS7274/79/80/81-01 February 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
Free Datasheet http://www.datasheet4u.com/
1 page  
Detailed Description
The RT7274/79/80/81 are high-performance 700kHz 2A
step-down regulators with internal power switches and
synchronous rectifiers. They feature an Advanced Constant
On-Time (ACOTTM) control architecture that provides
stable operation with ceramic output capacitors without
complicated external compensation, among other benefits.
The input voltage range is from 4.5V to 18V and the output
is adjustable from 0.765V to 8V.
The proprietary ACOTTM control scheme improves upon
other constant on-time architectures, achieving nearly
constant switching frequency over line, load, and output
voltage ranges. The RT7274/79/80/81 are optimized for
ceramic output capacitors. Since there is no internal clock,
response to transients is nearly instantaneous and inductor
current can ramp quickly to maintain output regulation
without large bulk output capacitance.
Constant On-Time (COT) Control
The heart of any COT architecture is the on-time one-
shot. Each on-time is a pre-determined “fixed” period
that is triggered by a feedback comparator. This robust
arrangement has high noise immunity and is ideal for low
duty cycle applications. After the on-time one-shot period,
there is a minimum off-time period before any further
regulation decisions can be considered. This arrangement
avoids the need to make any decisions during the noisy
time periods just after switching events, when the
switching node (SW) rises or falls. Because there is no
fixed clock, the high-side switch can turn on almost
immediately after load transients and further switching
pulses can ramp the inductor current higher to meet load
requirements with minimal delays.
Traditional current mode or voltage mode control schemes
typically must monitor the feedback voltage, current
signals (also for current limit), and internal ramps and
compensation signals, to determine when to turn off the
high-side switch and turn on the synchronous rectifier.
Weighing these small signals in a switching environment
is difficult to do just after switching large currents, making
those architectures problematic at low duty cycles and in
less than ideal board layouts.
RT7274/79/80/81
Because no switching decisions are made during noisy
time periods, COT architectures are preferable in low duty
cycle and noisy applications. However, traditional COT
control schemes suffer from some disadvantages that
preclude their use in many cases. Many applications require
a known switching frequency range to avoid interference
with other sensitive circuitry. True constant on-time control,
where the on-time is actually fixed, exhibits variable
switching frequency. In a step-down converter, the duty
factor is proportional to the output voltage and inversely
proportional to the input voltage. Therefore, if the on-time
is fixed, the off-time (and therefore the frequency) must
change in response to changes in input or output voltage.
Modern pseudo-fixed frequency COT architectures greatly
improve COT by making the one-shot on-time proportional
to VOUT and inversely proportional to VIN. In this way, an
on-time is chosen as approximately what it would be for
an ideal fixed-frequency PWM in similar input/output
voltage conditions. The result is a big improvement but
the switching frequency still varies considerably over line
and load due to losses in the switches and inductor and
other parasitic effects.
Another problem with many COT architectures is their
dependence on adequate ESR in the output capacitor,
making it difficult to use highly-desirable, small, low-cost,
but low-ESR ceramic capacitors. Most COT architectures
use AC current information from the output capacitor,
generated by the inductor current passing through the
ESR, to function in a way like a current mode control
system. With ceramic capacitors the inductor current
information is too small to keep the control loop stable,
like a current mode system with no current information.
ACOTTM Control Architecture
Making the on-time proportional to VOUT and inversely
proportional to VIN is not sufficient to achieve good
constant-frequency behavior for several reasons. First,
voltage drops across the MOSFET switches and inductor
cause the effective input voltage to be less than the
measured input voltage and the effective output voltage to
be greater than the measured output voltage. As the load
Copyright ©2013 Richtek Technology Corporation. All rights reserved.
DS7274/79/80/81-01 February 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
5
Free Datasheet http://www.datasheet4u.com/
5 Page 
RT7274/79/80/81
Typical Application Circuit
For TSSOP-14 (Exposed Pad) Package
VIN C1
C2
10µF x 2 0.1µF
Output Signal
PVCC
Input Signal
R3 100k
C4
1µF
C5
3.9nF
13
RT7279/80
VIN SW
10,
11
14 VINR
BOOT
FB
12
2
L1
2µH
C6
0.1µF
C3
6 PGOOD
7 EN
3 PVCC
4 SS
GND 5
VOUT 1
PGND 8, 9, 15 (Exposed Pad)
R1
8.25k
R2
22k
C7
22µF x 2
VOUT
1.05V/2A
For SOP-8 (Exposed Pad) Package
VIN
Enable
C1
10µF x 2
8
RT7274/81
VIN SW
6
C2
0.1µF
1 EN
BOOT 7
5, 9 (Exposed Pad) GND
FB 2
C5
3.9nF
4 SS
PVCC 3
L1
2µH
C6
0.1µF
C3
R1
8.25k
VOUT
1.05V/2A
C7
22µF x 2
C4 VPVCC
1µF
R2
22.1k
VOUT (V)
1
1.05
1.2
1.8
2.5
3.3
5
7
R1 (kΩ)
6.81
8.25
12.7
30.1
49.9
73.2
124
180
Table 1. Suggested Component Values
R2 (kΩ)
22.1
C3 (pF)
--
L1 (μH)
2
22.1 --
2
22.1 --
2
22.1 5 to 22
3.3
22.1 5 to 22
3.3
22.1 5 to 22
3.3
22.1 5 to 22
4.7
22.1 5 to 22
4.7
C7 (μF)
22 to 68
22 to 68
22 to 68
22 to 68
22 to 68
22 to 68
22 to 68
22 to 68
Copyright ©2013 Richtek Technology Corporation. All rights reserved.
DS7274/79/80/81-01 February 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
11
Free Datasheet http://www.datasheet4u.com/
11 Page | ||
| Páginas | Total 22 Páginas | |
| PDF Descargar | [ Datasheet RT7279.PDF ] | |
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