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RT7272B
3A, 36V, 500kHz Synchronous Step-Down Converter
General Description
The RT7272B is a high efficiency, current mode
synchronous step-down DC/DC converter that can deliver
up to 3A output current over a wide input voltage range
from 4.5V to 36V. The device integrates a 150mΩ high
side and a 80mΩ low side MOSFET to achieve high
conversion efficiency up to 95%. The current mode control
architecture supports fast transient response and simple
compensation circuit.
A cycle-by-cycle current limit function provides protection
against shorted output and an internal soft-start eliminates
input current surge during start-up. The RT7272B provides
complete protection functions such as input under-voltage
lockout, output under-voltage protection, over-current
protection and thermal shutdown.
The RT7272B is available in the thermal enhanced SOP-8
(Exposed Pad) package.
Features
4.5V to 36V Input Voltage Range
3A Output Current
Internal N-MOSFETs
Current Mode Control
Fixed Frequency Operation : 500kHz
Adjustable Output Voltage from 0.8V to 30V
High Efficiency Up to 95%
Stable with Low ESR Ceramic Output Capacitors
Cycle-by-Cycle Current Limit
Input Under-Voltage Lockout
Output Under-Voltage Protection
Thermal Shutdown Protection
Adjustable Current Limit
Power Saving Mode for High Efficiency at Light
Load
RoHS Compliant and Halogen Free
Ordering Information
RT7272B
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.
Applications
Distributed Power Systems
Pre-Regulator for Linear Regulators
Notebook Computers
Point of Load Regulator in Distributed Power System
Digital Set-top Boxes
Personal Digital Recorders
Broadband Communications
Flat Panel TVs and Monitors
Vehicle Electronics
Simplified Application Circuit
V
IN
C
IN
Enable
EN
RLIM
R
L
GND
COMP
FB
C
C
R
C
R2
VIN
BOOT
RT7272B
SW
R1
C
OUT
C
B
L
V
OUT
Copyright
©
2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7272B-07
February 2018
www.richtek.com
1
RT7272B
Pin Configurations
(TOP VIEW)
SW
BOOT
EN
GND
2
3
4
8
7
Marking Information
RT7272BGSP : Product Number
VIN
RLIM
COMP
FB
RT7272B
GSPYMDNN
YMDNN : Date Code
GND
9
6
5
SOP-8 (Exposed Pad)
Functional Pin Description
Pin No.
1
2
3
4,
9 (Exposed Pad)
5
Pin Name
SW
BOOT
EN
GND
FB
Pin Function
Switch Node Connect to external L-C filter.
Bootstrap Supply for High Side Gate Drive. A 100nF or greater capacitor is
recommended to connect from BOOT pin to SW pin.
Enable Control Input. A logic-high enables the converter; a logic-low forces the
device into shutdown mode.
Ground. The exposed pad must be soldered to a large PCB and connected to
GND for maximum thermal dissipation.
Feedback Input. This pin is connected to the converter output. It is used to set
the output of the converter to regulate to the desired value via an resistive
divider.
Compensation Node. COMP is used to compensate the regulation control loop.
Connect a series RC network from COMP to GND. In some cases, an additional
capacitor from COMP to GND is required.
Current Limit Setting. Connect to a resistor to determine current limit.
Power Input. The input voltage range is from 4.5V to 36V. Must bypass with a
suitable large ceramic capacitor.
6
7
8
COMP
RLIM
VIN
Function Block Diagram
VIN
Internal
Regulator
Shutdown
V
A
V
CC
Comparator
1.2V
+
-
Lockout
Comparator
-
1.7V
+
V
CC
Oscillator
Foldback
Control
0.4V
+
UV
S
+
R
-
Current
Comparator
Q
Q
150mΩ
SW
80mΩ
GND
Slope Comp
Current Sense
Amplifier
+
-
R
SENSE
V
A
BOOT
5kΩ
EN
-
UV
Comparator
V
CC
0.8V
+
+EA
-
SS
RLIM
FB
COMP
is a registered trademark of Richtek Technology Corporation.
Copyright
©
2018 Richtek Technology Corporation. All rights reserved.
www.richtek.com
2
DS7272B-07
February 2018
RT7272B
Operation
The RT7272B is a constant frequency, current mode
synchronous step-down converter. In normal operation,
the high side N-MOSFET is turned on when the S-R latch
is set by the oscillator and is turned off when the current
comparator resets the S-R latch. While the high side
N-MOSFET is turned off, the low side N-MOSFET is turned
on to conduct the inductor current until next cycle begins.
Error Amplifier
The error amplifier adjusts its output voltage by comparing
the feedback signal (V
FB
) with the internal 0.8V reference.
When the load current increases, it causes a drop in the
feedback voltage relative to the reference. The error
amplifier's output voltage then rises to allow higher inductor
current to match the load current.
Oscillator
The internal oscillator runs at fixed frequency 500kHz. In
short circuit condition, the frequency is reduced to 75kHz
for low power consumption.
Internal Regulator
The regulator provides low voltage power to supply the
internal control circuits and the bootstrap power for high
side gate driver.
Enable
The converter is turned on when the EN pin is higher than
2V. When the EN pin is lower than 0.4V, the converter will
enter shutdown mode and reduce the supply current to
0.5µA.
Soft-Start (SS)
An internal current source charges an internal capacitor
to build a soft-start ramp voltage. The FB voltage will track
the internal ramp voltage during soft-start interval. The
typical soft-start time is 2ms.
Current Setting
The current limit of high side MOSFET is adjustable by
an external resistor connected to the RLIM pin. The current
limit range is from 1.9A to 7A. When the inductor current
reaches the current limit threshold, the COMP voltage
will be clamped to limit the inductor current.
UV Comparator
If the feedback voltage (V
FB
) is lower than 0.4V, the UV
Comparator will go high to turn off the high side MOSFET.
The output under voltage protection is designed to operate
in Hiccup mode. When the UV condition is removed, the
converter will resume switching.
Thermal shutdown
The over temperature protection function will shut down
the switching operation when the junction temperature
exceeds 150°C. Once the junction temperature cools
down by approximately 20°C, the converter will
automatically resume switching.
Copyright
©
2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7272B-07
February 2018
www.richtek.com
3
RT7272B
Absolute Maximum Ratings
(Note 1)
−0.3V
to 40V
−0.3V
to (V
IN
+ 0.3V)
−0.3V
to 6V
−0.3V
to 40V
Supply Input Voltage, VIN -----------------------------------------------------------------------------------------
Switch Voltage, SW ------------------------------------------------------------------------------------------------
V
BOOT
−
V
SW
----------------------------------------------------------------------------------------------------------
Other Pins Voltage --------------------------------------------------------------------------------------------------
Power Dissipation, P
D
@ T
A
= 25°C
SOP-8 (Exposed Pad) --------------------------------------------------------------------------------------------- 2.041W
Package Thermal Resistance (Note 2)
SOP-8 (Exposed Pad),
θ
JA
---------------------------------------------------------------------------------------- 49°C/W
SOP-8 (Exposed Pad),
θ
JC
---------------------------------------------------------------------------------------
Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------------
Junction Temperature -----------------------------------------------------------------------------------------------
Storage Temperature Range --------------------------------------------------------------------------------------
15°C/W
260°C
150°C
−65°C
to 150°C
ESD Susceptibility (Note 3)
HBM (Human Body Model) ---------------------------------------------------------------------------------------- 2kV
Recommended Operating Conditions
(Note 4)
Supply Input Voltage, VIN ----------------------------------------------------------------------------------------- 4.5V to 36V
Junction Temperature Range --------------------------------------------------------------------------------------
−40°C
to 125°C
Ambient Temperature Range --------------------------------------------------------------------------------------
−40°C
to 85°C
Electrical Characteristics
(V
IN
= 12V, C
IN
= 20µF, T
A
= 25°C, unless otherwise specified)
Parameter
Shutdown Supply Current
Quiescent Current
Feedback Reference
Voltage
High Side Switch
On-Resistance
Low Side Switch
On-Resistance
High Side Switch Leakage
Current
Upper Switch Current Limit
Range
Upper Switch Current Limit
Lower Switch Current Limit
Symbol
V
EN
= 0V
I
Q
V
REF
R
DS(ON)1
R
DS(ON)2
Test Conditions
Min
--
--
0.788
--
--
Typ
0.5
0.9
0.8
150
80
0
--
2.5
3.5
5.5
1.5
Max
3
1.2
0.812
--
--
10
7
3.1
4.2
6.5
--
Unit
µA
mA
V
mΩ
mΩ
µA
A
V
EN
= 3V, V
FB
= 0.9V
4.5V
≤
V
IN
≤
36V
V
EN
= 0V, V
SW
= 0V
U
OC
Min. Duty Cycle, R
LIM
= 57.6kΩ
U
OC
(Note 5)
Min. Duty Cycle, R
LIM
= 84.5kΩ
Min. Duty Cycle, R
LIM
= 137kΩ
From Drain to Source
--
1.9
1.9
2.7
4.5
--
A
A
Copyright
©
2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS7272B-07
February 2018
RT7272B
Parameter
Oscillation Frequency
Short Circuit Oscillation
Frequency
Maximum Duty Cycle
Minimum On-Time
EN Input Voltage
Logic-High
Logic-Low
Input Under Voltage Lockout
Threshold
Input Under Voltage Lockout
Hysteresis
Thermal Shutdown
Thermal Shutdown Hysteresis
COMP to Current Sense
Transconductance
Error Amplifier
Transconductance
Load Regulation
Line Regulation
G
EA
Symbol
f
OSC1
f
OSC2
D
MAX
t
ON
V
IH
V
IL
V
UVLO
∆V
UVLO
T
SD
∆T
SD
G
CS
∆I
COMP
= ±10µA
V
IN
Rising
V
FB
= 0V
V
FB
= 0.7V
Test Conditions
Min
450
--
--
--
2
--
3.9
--
--
--
--
--
--
V
IN
= 4.5V to 36V
--
Typ
500
75
90
100
--
--
4.1
250
150
20
4.7
1000
--
--
Max
550
--
--
--
--
0.4
4.3
--
--
--
--
--
0.05
0.1
Unit
kHz
kHz
%
ns
V
V
mV
°C
°C
A/V
µA/V
%/A
%
∆V
LOAD
∆V
LINE
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.
Note 3.
Devices are ESD sensitive. Handling precaution is recommended.
Note 4.
The device is not guaranteed to function outside its operating conditions.
Note 5.
R
LIM
(kΩ) = [U
OC
x 24.14 x (1 + 0.024 x (U
OC
−
3.5))
−
1.3], where U
OC
is desired upper switch peak current limit
value.
Copyright
©
2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7272B-07
February 2018
www.richtek.com
5
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