®
RT7277
3A, 18V, 700kHz ACOT
TM
Synchronous Step-Down Converter
General Description
The RT7277 is a synchronous step-down DC/DC converter
with Advanced Constant On-Time (ACOT
TM
) mode control.
It achieves high power density to deliver up to 3A output
current from a 4.5V to 18V input supply. The proprietary
ACOT
TM
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
capacitors for ensuring performance stabilization. In
addition, RT7277 keeps an excellent constant switching
frequency under line and load variation and the integrated
synchronous power switches with the ACOT
TM
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 also
include output under voltage protection, output over voltage
protection, and thermal shutdown.
Features
ACOT
TM
Mode Enables Fast Transient Response
4.5V to 18V Input Voltage Range
3A Output Current
60mΩ Internal Low Site N-MOSFET
Ω
Advanced Constant On-Time Control
Support All Ceramic Capacitors
Up to 95% Efficiency
700kHz Switching Frequency
Adjustable Output Voltage from 0.765V to 8V
Adjustable Soft-Start
Cycle-by-Cycle Current Limit
Input Under Voltage Lockout
Thermal Shutdown
RoHS Compliant and Halogen Free
Applications
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
Marking Information
RT7277GSP : Product Number
RT7277
GSPYMDNN
YMDNN : Date Code
Simplified Application Circuit
V
IN
C1
Chip Enable
C2
RT7277
VIN
SW
BOOT
EN
SS
C5
FB
PVCC
GND
L1
C6
C7
C3
R1
V
OUT
C4
V
PVCC
R2
Copyright
©
2013 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7277-01 March 2013
www.richtek.com
1
RT7277
Ordering Information
RT7277
Package Type
SP : SOP-8 (Exposed Pad-Option 2)
Lead Plating System
G : Green (Halogen Free and Pb Free)
Note :
Richtek products are :
RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
EN
FB
PVCC
SS
2
3
4
Pin Configurations
(TOP VIEW)
8
VIN
BOOT
SW
GND
GND
9
7
6
5
SOP-8 (Exposed Pad)
Functional Pin Description
Pin No.
1
2
3
4
5, 9
(Exposed Pad)
6
7
8
Pin Name
EN
FB
PVCC
SS
GND
SW
BOOT
VIN
Pin Function
Enable Input. A logic-high enables the converter; a logic-low forces the IC into
shutdown mode reducing the supply current to less than 10μA.
Feedback Input. It is used to regulate the output of the converter to a set value via
an external resistive voltage divider. The feedback threshold voltage is 0.765V
typically.
Internal Regulator Output. Connect a 1μF capacitor to GND to stabilize output
voltage.
Soft-Start Control Input. SS controls the soft-start period. Connect a capacitor from
SS to GND to set the soft-start period. A 3.9nF capacitor sets the soft-start period of
V
OUT
to 2.6ms.
Ground. The Exposed pad should be soldered to a large PCB and connected to
GND for maximum thermal dissipation.
Switch Node. Connect this pin to an external L-C filter.
Bootstrap Supply for High Side Gate Driver. Connect a 0.1μF or greater ceramic
capacitor from BOOT to SW pins.
Power Input. The input voltage range is from 4.5V to 18V. Must bypass with a
suitably large (
≥10μF
x 2) ceramic capacitor.
Copyright
©
2013 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
2
DS7277-01 March 2013
RT7277
Function Block Diagram
PVCC
BOOT
Internal
Regulator
PVCC
VIBIAS
V
REF
Over Current PVCC
Protection
UGATE
Switch
Controller
PVCC
2µA
SS
FB
On-Time
FB
Comparator
EN
- -
+
VIN
Driver
LGATE
SW
GND
Ripple
Gen.
SW
EN
Operation
In normal operation, the high side N-MOSFET is turned
on when the FB Comparator sets the Switch Controller,
and it is turned off when On-Time Controller resets the
Switch Controller. While the high side N-MOSFET is turned
off, the low side N-MOSFET is turned on and waits for the
FB Comparator to set the beginning of next cycle.
The FB Comparator sets the Switch Controller by
comparing the feedback signal (FB) from output voltage
with the internal 0.765V reference. When load transient
induces VOUT drop, the FB voltage will be less than its
threshold voltage. This means that the high side N-MOSFET
will turn on again immediately after minimum off-time
expired. The switching frequency will vary during the
transient period thus can provide a very fast transient
response. After the load transient finished, the RT7277
will be back to steady state with a constant switching
frequency.
Enable
Activate internal regulator once EN input level is higher
than the target level. Force IC to enter shutdown mode
when the EN input level is lower than 0.4V
Internal Regulator
Provide internal power for logic control and switch gate
drivers.
On-Time Controller
Control on-time according to VIN and SW to obtain
constant switching frequency.
Copyright
©
2013 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7277-01 March 2013
www.richtek.com
3
RT7277
Absolute Maximum Ratings
(Note 1)
−0.3V
to 21V
−0.8V
to (V
IN
+ 0.3V)
−5V
to 25V
−0.3V
to 6V
−0.3V
to 21V
2.041W
Supply Voltage, VIN -----------------------------------------------------------------------------------------------
Switch Voltage, SW -----------------------------------------------------------------------------------------------
<10ns -----------------------------------------------------------------------------------------------------------------
BOOT to SW, PVCC ----------------------------------------------------------------------------------------------
Other Pins Voltage -------------------------------------------------------------------------------------------------
Power Dissipation, P
D
@ T
A
= 25°C
SOP-8 (Exposed Pad) --------------------------------------------------------------------------------------------
Package Thermal Resistance (Note 2)
SOP-8 (Exposed Pad),
θ
JA
--------------------------------------------------------------------------------------- 49°C/W
SOP-8 (Exposed Pad),
θ
JC
-------------------------------------------------------------------------------------- 15°C/W
Junction Temperature Range ------------------------------------------------------------------------------------- 150°C
Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------ 260°C
Storage Temperature Range -------------------------------------------------------------------------------------
−65°C
to 150°C
ESD Susceptibility (Note 3)
HBM (Human Body Model) --------------------------------------------------------------------------------------- 2kV
Recommended Operating Conditions
(Note 4)
Supply Voltage, VIN ----------------------------------------------------------------------------------------------- 4.5V to 18V
Junction Temperature Range -------------------------------------------------------------------------------------
−40°C
to 125°C
Ambient Temperature Range -------------------------------------------------------------------------------------
−40°C
to 85°C
Electrical Characteristics
(V
IN
= 12V, T
A
= 25°C, unless otherwise specified)
Parameter
Supply Current
Shutdown Current
Quiescent Current
Logic Threshold
EN Voltage
Logic-High
Logic-Low
Symbol
I
SHDN
I
Q
Test Conditions
V
EN
= 0V
V
EN
= 3V, V
FB
= 1V
Min
--
--
2
--
Typ
1.5
0.7
--
--
0.765
0
5.1
--
--
110
Max
10
--
18
0.4
0.773
0.1
5.5
20
100
--
Unit
μA
mA
V
V
FB
Voltage and Discharge Resistance
Feedback Threshold Voltage
Feedback Input Current
V
PVCC
Output
V
PVCC
Output Voltage
Line Regulation
Load Regulation
Output Current
I
PVCC
V
PVCC
6V
≤
V
IN
≤
18V, 0 < I
PVCC
<
5mA
6V
≤
V
IN
≤
18V, I
PVCC
= 5mA
0
<
I
PVCC
<
5mA
V
IN
= 6V, V
PVCC
= 4V
4.7
--
--
--
V
mV
mV
mA
V
FB
I
FB
4.5V
≤
V
IN
≤
18V
V
FB
= 0.8V
0.757
−0.1
V
μA
Copyright
©
2013 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS7277-01 March 2013
RT7277
Parameter
R
DS(ON)
Switch On
Resistance
Current Limit
Current Limit
Thermal Shutdown
Thermal Shutdown Threshold
T
SD
--
--
V
IN
= 12V, V
OUT
= 1.05V
--
--
--
V
SS
= 0V
V
SS
= 0.5V
VIN Rising to Wake up V
PVCC
1.4
0.05
3.55
--
150
20
145
60
230
2
0.1
3.85
0.3
--
--
--
--
--
2.6
--
4.15
--
°C
°C
ns
ns
ns
μA
mA
Thermal Shutdown Hysteresis
ΔT
SD
On-Time Timer Control
On-Time
Minimum On-Time
Minimum Off-Time
Soft-Start
SS Charge Current
SS Discharge Current
UVLO
UVLO Threshold
Hysteresis
V
t
ON
t
ON(MIN)
t
OFF(MIN)
I
LIM
3.5
4.1
5.7
A
High Side
Low Side
R
DS(ON)_H
R
DS(ON)_L
--
--
90
60
--
--
mΩ
Symbol
Test Conditions
Min
Typ
Max
Unit
Note 1.
Stresses beyond those listed
“Absolute
Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in
the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may
affect device reliability.
Note 2.
θ
JA
is measured at T
A
= 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7.
θ
JC
is
measured at the exposed pad of the package. The PCB copper area of exposed pad is 70mm
2
.
Note 3.
Devices are ESD sensitive. Handling precaution is recommended.
Note 4.
The device is not guaranteed to function outside its operating conditions.
Copyright
©
2013 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7277-01 March 2013
www.richtek.com
5
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