®
RT7274/79/80/81
2A, 18V, 700kHz ACOTTM Synchronous Step-Down Converter
General Description
Features
The RT7274/79/80/81 is a synchronous step-down DC/
DC converter with Advanced Constant On-Time (ACOTTM)
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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 include
thermal shutdown for RT7274/79/80/81; output under
voltage protection and output over voltage protection for
RT7279/80 only.
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ACOTTM Mode Enables Fast Transient Response
4.5V to 18V Input Voltage Range
2A Output Current
High Efficient Internal N-MOSFET Optimized for
Lower Duty Cycle Applications
105mΩ
Ω Internal Low-Side N-MOSFET
Advanced Constant On-Time Control
Allows Ceramic Output Capacitor
700kHz Switching Frequency
Adjustable Output Voltage from 0.765V to 8V
Adjustable and Pre-biased Soft-Start
Cycle-by-Cycle Current Limit
Input Under Voltage Lockout
Thermal Shutdown
RoHS Compliant and Halogen Free
Applications
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Industrial and Commercial Low Power Systems
Computer Peripherals
LCD Monitors and TVs
Green Electronics/Appliances
Point of Load Regulation for High-Performance DSPs,
FPGAs, and ASICs
Simplified Application Circuit
VIN
RT7274/79/80/81
VIN
SW
VINR*
Input Signal
Power Good
EN
VOUT
BOOT
FB
GND*
PGOOD*
PVCC
SS
VOUT*
PGND*
* : VINR pin for TSSOP-14 (Exposed Pad) only.
VOUT pin for TSSOP-14 (Exposed Pad) only.
PGND pin for TSSOP-14 (Exposed Pad) and WDFN-10L 3x3 only.
PGOOD pin for TSSOP-14 (Exposed Pad) and WDFN-10L 3x3 only.
GND pin for TSSOP-14 (Exposed Pad) and SOP-8 (Exposed Pad) only.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS7274/79/80/81-02 April 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
RT7274/79/80/81
Ordering Information
Marking Information
Discontinuous Operating Mode
RT7274GSP
RT7274
RT7274GSP : Product Number
RT7274
GSPYMDNN
Package Type
SP : SOP-8 (Exposed Pad-Option 2)
Lead Plating System
G : Green (Halogen Free and Pb Free)
YMDNN : Date Code
RT7280GCP
RT7280GCP : Product Number
RT7280
GCPYMDNN
RT7280
Package Type
CP : TSSOP-14 (Exposed Pad)
QW : WDFN-10L 3x3 (W-Type)
YMDNN : Date Code
RT7280GQW
2Y= : Product Code
Lead Plating System
G : Green (Halogen Free and Pb Free)
2Y=YM
DNN
YMDNN : Date Code
RT7279GCP
Forced PWM Mode
RT7279GCP : Product Number
RT7279
RT7279
GCPYMDNN
Package Type
CP : TSSOP-14 (Exposed Pad)
QW : WDFN-10L 3x3 (W-Type)
YMDNN : Date Code
RT7279GQW
Lead Plating System
G : Green (Halogen Free and Pb Free)
2Z= : Product Code
2Z=YM
DNN
YMDNN : Date Code
RT7281
RT7281GSP
Package Type
SP : SOP-8 (Exposed Pad-Option 2)
RT7281GSP : Product Number
RT7281
GSPYMDNN
Lead Plating System
G : Green (Halogen Free and Pb Free)
YMDNN : Date Code
Pin Configurations
VOUT
FB
PVCC
SS
GND
PGOOD
EN
14
2
13
3
4
12
PGND
11
5
10
6
9
7
15
8
VINR
VIN
BOOT
SW
SW
PGND
PGND
EN
FB
PVCC
SS
PGOOD
TSSOP-14 (Exposed Pad)
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
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2
1
2
3
4
5
PGND
(TOP VIEW)
11
10
9
8
7
6
VIN
VIN
BOOT
SW
SW
WDFN-10L 3x3
EN
8
FB
2
PVCC
3
SS
4
GND
9
VIN
7
BOOT
6
SW
5
GND
SOP-8 (Exposed Pad)
is a registered trademark of Richtek Technology Corporation.
DS7274/79/80/81-02 April 2013
RT7274/79/80/81
Functional Pin Description
Pin No.
Pin Name
SOP-8
(Exposed Pad)
Pin Function
TSSOP-14
(Exposed Pad)
WDFN-10L
3x3
1
--
--
VOUT
Output Voltage Sense Input. This terminal is used
for On-Time Adjustment.
2
2
2
FB
Feedback Input Voltage. Connect with feedback
resistive divider to the output voltage.
3
3
3
PVCC
5.1V Power Supply Output. PVCC is the output of
the internal 5.1V linear regulator powered by VIN
(WDFN-10L 3x3) or VINR (TSSOP-14L (Exposed
Pad)). Connect a 1μF capacitor from this pin to
GND.
4
4
4
SS
Soft-Start Control. Connect an external capacitor
between this pin and GND to set the soft- start
time.
5
--
6
5
--
PGOOD
Open Drain Power Good Output.
7
1
1
EN
Enable Control Input.
--
PGND
Power Ground. The exposed pad must be
soldered to a large PCB and connected to PGND
for maximum power dissipation.
8, 9,
11
15 (Exposed Pad) (Exposed Pad)
5,
GND
9 (Exposed Pad)
Analog Ground. The exposed pad must be
soldered to a large PCB and connected to GND
for maximum power dissipation.
10, 11
6, 7
6
SW
Switch Node.
12
8
7
BOOT
Bootstrap Supply for High-Side Gate Driver.
Connect a 0.1μF capacitor between the BOOT
and SW pin.
13
9, 10
8
VIN
Power Input. It is connected to the drain of the
internal high-side MOSFET. Connect VIN to the
input capacitor. For the WDFN-10L 3x3 package,
VIN also supplies power to the internal linear
regulator.
14
--
--
VINR
Supply Input for Internal Linear Regulator to the
Control Circuitry.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS7274/79/80/81-02 April 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
3
RT7274/79/80/81
Function Block Diagram
For TSSOP-14 (Exposed Pad) and WDFN-10L 3x3 Package
BOOT
PVCC
VIN
(WDFN-10L 3x3)
Internal
Regulator
Over Current
Protection PVCC
PVCC
VIBIAS
GND
(TSSOP-14 (Exposed Pad))
VOUT
(TSSOP-14 (Exposed Pad))
VIN
VREF
UGATE
Under & Over
Voltage Protection
Discharge
PVCC
Switch
Controller
SW
Driver
LGATE
PGND
SW
Ripple
Gen.
2µA
0.9 VREF
+
FB
- -
SS
FB
On-Time
FB
Comparator
PGOOD
+
-
VINR
(TSSOP-14 (Exposed Pad))
PGOOD
Comparator
EN
EN
For SOP-8 (Exposed Pad) Package
BOOT
PVCC
Internal
Regulator
PVCC
VIBIAS
Over Current
Protection
VIN
PVCC
VREF
UGATE
GND
Switch
Controller
PVCC
2µA
SW
Driver
LGATE
Ripple
Gen.
SW
+
- -
SS
FB
On-Time
FB
Comparator
EN
EN
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
DS7274/79/80/81-02 April 2013
RT7274/79/80/81
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 oneshot. 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.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS7274/79/80/81-02 April 2013
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
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RT7274/79/80/81
changes, the switch voltage drops change causing a
switching frequency variation with load current. Also, at
light loads if the inductor current goes negative, the switch
dead-time between the synchronous rectifier turn-off and
the high-side switch turn-on allows the switching node to
rise to the input voltage. This increases the effective ontime and causes the switching frequency to drop
noticeably.
One way to reduce these effects is to measure the actual
switching frequency and compare it to the desired range.
This has the added benefit eliminating the need to sense
the actual output voltage, potentially saving one pin
connection. ACOTTM uses this method, measuring the
actual switching frequency and modifying the on-time with
a feedback loop to keep the average switching frequency
in the desired range.
To achieve good stability with low-ESR ceramic capacitors,
ACOTTM uses a virtual inductor current ramp generated
inside the IC. This internal ramp signal replaces the ESR
ramp normally provided by the output capacitor's ESR.
The ramp signal and other internal compensations are
optimized for low-ESR ceramic output capacitors.
ACOTTM One-shot Operation
The RT7274/79/80/81 control algorithm is simple to
understand. The feedback voltage, with the virtual inductor
current ramp added, is compared to the reference voltage.
When the combined signal is less than the reference the
on-time one-shot is triggered, as long as the minimum
off-time one-shot is clear and the measured inductor
current (through the synchronous rectifier) is below the
current limit. The on-time one-shot turns on the high-side
switch and the inductor current ramps up linearly. After
the on-time, the high-side switch is turned off and the
synchronous rectifier is turned on and the inductor current
ramps down linearly. At the same time, the minimum offtime one-shot is triggered to prevent another immediate
on-time during the noisy switching time and allow the
feedback voltage and current sense signals to settle. The
minimum off-time is kept short (230ns typical) so that
rapidly-repeated on-times can raise the inductor current
quickly when needed.
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Discontinuous Operating Mode (RT7274/80 Only)
After soft start, the RT7279/81 operates in fixed frequency
mode to minimize interference and noise problems. The
RT7274/80 uses variable-frequency discontinuous
switching at light loads to improve efficiency. During
discontinuous switching, the on-time is immediately
increased to add “hysteresis” to discourage the IC from
switching back to continuous switching unless the load
increases substantially.
The IC returns to continuous switching as soon as an ontime is generated before the inductor current reaches zero.
The on-time is reduced back to the length needed for
700kHz switching and encouraging the circuit to remain
in continuous conduction, preventing repetitive mode
transitions between continuous switching and
discontinuous switching.
Current Limit
The RT7274/79/80/81 current limit is a cycle-by-cycle
“valley” type, measuring the inductor current through the
synchronous rectifier during the off-time while the inductor
current ramps down. The current is determined by
measuring the voltage between source and drain of the
synchronous rectifier, adding temperature compensation
for greater accuracy. If the current exceeds the upper
current limit, the on-time one-shot is inhibited until the
inductor current ramps down below the upper current limit
plus a wide hysteresis band of about 1A and drops below
the lower current limit level. Thus, only when the inductor
current is well below the upper current limit is another ontime permitted. This arrangement prevents the average
output current from greatly exceeding the guaranteed
upper current limit value, as typically occurs with other
valley-type current limits. If the output current exceeds
the available inductor current (controlled by the current
limit mechanism), the output voltage will drop. If it drops
below the output under-voltage protection level (see next
section) the IC will stop switching to avoid excessive heat.
The RT7279/81 also includes a negative current limit to
protect the IC against sinking excessive current and
possibly damaging the IC. If the voltage across the
synchronous rectifier indicates the negative current is too
is a registered trademark of Richtek Technology Corporation.
DS7274/79/80/81-02 April 2013
RT7274/79/80/81
high, the synchronous rectifier turns off until after the next
high-side on-time. RT7274/80 does not sink current and
therefore does not need a negative current limit.
Output Over-voltage Protection and Under-voltage
Protection
The RT7279/80 include output over-voltage protection
(OVP). If the output voltage rises above the regulation
level, the high-side switch naturally remains off and the
synchronous rectifier turns on. If the output voltage remains
high the synchronous rectifier remains on until the inductor
current reaches the negative current limit (RT7279) or until
it reaches zero (RT7280). If the output voltage remains
high, the IC's switches remain off. If the output voltage
exceeds the OVP trip threshold for longer than 5μs
(typical), the IC's OVP is triggered.
The RT7279/80 include output under-voltage protection
(UVP). If the output voltage drops below the UVP trip
threshold for longer than 250μs (typical) the IC's UVP is
triggered.
There are two different behaviors for OVP and UVP events
for the TSSOP-14 (Exposed Pad) packages.
`
Latch-Off Mode (TSSOP-14 (Exposed Pad) Only)
`
The RT7280GCP/RT7279GCP, use latch-off mode OVP
and UVP. When the protection function is triggered the
IC will shut down. The IC stops switching, leaving both
switches open, and is latched off. To restart operation,
toggle EN or power the IC off and then on again.
`
Hiccup Mode (WDFN-10L 3x3 Only)
`
The RT7279GQW/RT7280GQW, use hiccup mode OVP
and UVP. When the protection function is triggered, the
IC will shut down for a period of time and then attempt
to recover automatically. Hiccup mode allows the circuit
to operate safely with low input current and power
dissipation, and then resume normal operation as soon
as the overload or short circuit is removed.
Between these 2 levels there are 2 thresholds (1.2V typical
and 1.4V typical). When VEN exceeds the lower threshold
the internal bias regulators begin to function and supply
current increases above the shutdown current level.
Switching operation begins when VEN exceeds the upper
threshold. Unlike many competing devices, EN is a high
voltage input that can be safely connected to VIN (up to
18V) for automatic start-up.
Input Under-voltage Lock-out
In addition to the enable function, the RT7274/79/80/81
feature an under-voltage lock-out (UVLO) function that
monitors the internal linear regulator output (PVCC). To
prevent operation without fully-enhanced internal MOSFET
switches, this function inhibits switching when PVCC
drops below the UVLO-falling threshold. The IC resumes
switching when PVCC exceeds the UVLO-rising threshold.
Soft-Start (SS)
The RT7274/79/80/81 soft-start uses an external pin (SS)
to clamp the output voltage and allow it to slowly rise.
After VEN is high and PVCC exceeds its UVLO threshold,
the IC begins to source 2μA from the SS pin. An external
capacitor at SS is used to adjust the soft-start timing.
The available capacitance range is from 2.7nF to 220nF.
Do not leave SS unconnected.
During start-up, while the SS capacitor charges, the
RT7274/79/80/81 operate in discontinuous switching mode
with very small pulses. This prevents negative inductor
currents and keeps the circuit from sinking current.
Therefore, the output voltage may be pre-biased to some
positive level before start-up. Once the VSS ramp charges
enough to raise the internal reference above the feedback
voltage, switching will begin and the output voltage will
smoothly rise from the pre-biased level to its regulated
level. After VSS rises above about 2.2V output over-and
under-voltage protections are enabled and the RT7279/81
begins continuous-switching operation.
Shut-down, Start-up and Enable (EN)
Internal Regulator (PVCC)
The enable input (EN) has a logic-low level of 0.4V. When
VEN is below this level the IC enters shutdown mode and
supply current drops to less than 10μA. When VEN exceeds
its logic-high level of 1.6V the IC is fully operational.
An internal linear regulator (PVCC) produces a 5.1V supply
from VIN that powers the internal gate drivers, PWM logic,
reference, analog circuitry, and other blocks. If VIN is 6V
or greater, PVCC is guaranteed to provide significant power
for external loads.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS7274/79/80/81-02 April 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
7
RT7274/79/80/81
PGOOD Comparator
PGOOD is an open drain output controlled by a comparator
connected to the feedback signal. If FB exceeds 90% of
the internal reference voltage, PGOOD will be high
impedance. Otherwise, the PGOOD output is connected
to PGND.
External Bootstrap Capacitor (C6)
Connect a 0.1μF low ESR ceramic capacitor between
BOOT and SW. This bootstrap capacitor provides the gate
driver supply voltage for the high-side N-channel MOSFET
switch.
Over Temperature Protection
The RT7274/79/80/81 includes an Over Temperature
Protection (OTP) circuitry to prevent overheating due to
excessive power dissipation. The OTP will shut down
switching operation when the junction temperature
exceeds 150°C. Once the junction temperature cools
down by approximately 25°C the IC will resume normal
operation with a complete soft-start. For continuous
operation, provide adequate cooling so that the junction
temperature does not exceed 150°C.
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is a registered trademark of Richtek Technology Corporation.
DS7274/79/80/81-02 April 2013
RT7274/79/80/81
Absolute Maximum Ratings
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(Note 1)
Supply Input Voltage, VIN, VINR ------------------------------------------------------------------------------Switch Node, SW -------------------------------------------------------------------------------------------------Switch Node, SW (