®
RT7277
3A, 18V, 700kHz ACOTTM 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
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
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 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 also
include output under voltage protection, output over voltage
protection, and thermal shutdown.
Marking Information
RT7277GSP : Product Number
RT7277
GSPYMDNN
YMDNN : Date Code
Features
zz
ACOTTM Mode Enable s Fa st T ra n sient Respon se
z
zz
zz
z 4.5V to 18V Input Voltage Range
zz
zz
z 3A Output Current
zz
ΩΩ
zz
z 60m
Ω Internal Low Site N-MOSFET
zz
ΩΩ
zz
z Advanced Constant On-Time Control
zz
zz
z Support All Ceramic Capa citors
zz
zz
z Up to 95% Efficiency
zz
zz
z 700kHz Switching Frequency
zz
zz
z Adjustable Output Voltage from 0.765V to 8V
zz
zz
z Adjustable Soft-Start
zz
zz
z Cycle-by-Cycle Current Limit
zz
zz
z Input Under Voltage Lockout
zz
zz
z Thermal Shutdown
zz
zz
z RoHS Compliant and Halogen Free
zz
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
RT7277
V
IN
VIN
SW
C1 C2
Chip Enable
EN
SS
BOOT
FB
PVCC
C5
GND
Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
©
DS7277-01 March 2013 www.richtek.com
L1
C6
C4
V
C3
PVCC
R1
R2
C7
V
OUT
1
RT7277
Ordering Information
Pin Configurations
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.
Functional Pin Description
Pin No. Pin Name Pin Function
1 EN
2 FB
3 PVCC
4 SS
5, 9
(Exposed Pad)
GND
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
to 2.6ms.
V
OUT
Ground. The Exposed pad should be soldered to a large PCB and connected to
GND for maximum thermal dissipation.
(TOP VIEW)
EN
FB
PVCC
SS
2
GND
3
4
SOP-8 (Exposed Pad)
8
VIN
7
BOOT
6
9
SW
5
GND
6 SW Switch Node. Connect this pin to an external L-C filter.
7 BOOT
8 VIN
Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
2
©
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.
DS7277-01 March 2013www.richtek.com
Function Block Diagram
RT7277
Operation
SS
FB
PVCC
Internal
Regulator
VIBIAS
PVCC
On-Time
2µA
PVCC
V
REF
Ripple
Gen.
Over Current
Protection
Switch
Controller
+
--
FB
Comparator
BOOT
PVCC
Driver
SW
EN
EN
VIN
UGATE
SW
LGATE
GND
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)
z Supply Voltage, VIN ----------------------------------------------------------------------------------------------- −0.3V to 21V
z Switch Voltage, SW ----------------------------------------------------------------------------------------------- −0.8V to (V
<10ns ----------------------------------------------------------------------------------------------------------------- −5V to 25V
z BOOT to SW, PVCC ---------------------------------------------------------------------------------------------- −0.3V to 6V
z Other Pins Voltage ------------------------------------------------------------------------------------------------- −0.3V to 21V
z Power Dissipation, P
@ TA = 25°C
D
SOP-8 (Exposed Pad) -------------------------------------------------------------------------------------------- 2.041W
z Package Thermal Resistance (Note 2)
SOP-8 (Exposed Pad), θJA--------------------------------------------------------------------------------------- 49°C/W
SOP-8 (Exposed Pad), θJC-------------------------------------------------------------------------------------- 15°C/W
z Junction Temperature Range------------------------------------------------------------------------------------- 150°C
z Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------ 260°C
z Storage Temperature Range ------------------------------------------------------------------------------------- −65°C to 150°C
z ESD Susceptibility (Note 3)
HBM (Human Body Model)--------------------------------------------------------------------------------------- 2kV
+ 0.3V)
IN
Recommended Operating Conditions
z Supply Voltage, VIN ----------------------------------------------------------------------------------------------- 4.5V to 18V
z Junction Temperature Range------------------------------------------------------------------------------------- −40°C to 125°C
z Ambient Temperature Range------------------------------------------------------------------------------------- −40°C to 85°C
(Note 4)
Electrical Characteristics
(VIN = 12V, T
Supply Current
Shutdown Current I
Quiescent Current IQ V
Logic Threshold
EN Voltage
VFB Voltage and Discharge Resistance
Feedback Threshold Voltage VFB 4.5V ≤ V
Feedback Input Current IFB V
V
Output
PVCC
V
Output Voltage V
PVCC
Line Regulation 6V ≤ V
Load Regulation 0 < I
Output Current I
= 25°C, unless otherwise specified)
A
Parameter Symbol Test Conditions Min Typ Max Unit
V
SHDN
= 0V -- 1.5 10 μA
EN
= 3V, VFB = 1V -- 0.7 -- mA
EN
Logic-High 2 -- 18
Logic-Low -- -- 0.4
≤ 18V 0.757 0.765 0.773 V
IN
= 0.8V −0.1 0 0.1 μA
FB
6V ≤ V
PVCC
V
PVCC
IN
≤ 18V, 0 < I
IN
≤ 18V, I
IN
< 5mA -- -- 100 mV
PVCC
= 6V, V
PVCC
= 4V -- 110 -- mA
PVCC
< 5mA 4.7 5.1 5.5 V
PVCC
= 5mA -- -- 20 mV
V
Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
©
DS7277-01 March 2013www.richtek.com
4
RT7277
Parameter Symbol Test Conditions Min Typ Max Unit
R
Switch On
Resistance
DS(ON)
High Side
R
DS(ON)_H
-- 90 --
mΩ
Low Side
R
DS(ON)_L
-- 60 --
Current Limit
Current Limit I
3.5 4.1 5.7 A
LIM
Thermal Shutdown
Thermal Shutdown Threshold TSD -- 150 -- °C
Thermal Shutdown Hysteresis ΔTSD -- 20 -- °C
On-Time Timer Control
On-Time tON V
Minimum On-Time t
Minimum Off-Time t
ON(MIN)
OFF(MIN)
-- 60 -- ns
= 12V, V
IN
= 1.05V -- 145 -- ns
OUT
-- 230 -- ns
Soft-Start
SS Charge Current VSS = 0V 1.4 2 2.6 μA
SS Discharge Current VSS = 0.5V 0.05 0.1 -- mA
UVLO
UVLO Threshold VIN Rising to Wake up V
3.55 3.85 4.15
PVCC
V
Hysteresis -- 0.3 --
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
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
is measured at T
JA
measured at the exposed pad of the package. The PCB copper area of exposed pad is 70mm
= 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC is
A
2
.
Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
DS7277-01 March 2013 www.richtek.com
©
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