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RT7257G
3A, 18V, 800kHz Synchronous Step-Down Converter
General Description
The RT7257G is a high efficiency, monolithic synchronous
step-down DC/DC converter that can deliver up to 3A
output current from a 4.5V to 18V input supply. The
RT7257G's current mode architecture and external
compensation allow the transient response to be
optimized over a wide input voltage range and loads.
Cycle-by-cycle current limit provides protection against
shorted outputs, and soft-start eliminates input current
surge during start-up. The RT7257G also provides under
voltage protection and thermal shutdown protection. The
low current (<3μA) shutdown mode provides output
disconnection, enabling easy power management in
battery-powered systems. The RT7257G is available in
an SOP-8 (Exposed Pad) package.
Features
±
1.5% High Accuracy Reference Voltage
4.5V to 18V Input Voltage Range
3A Output Current
Integrated N-MOSFET Switches
Current Mode Control
Fixed Frequency Operation : 800kHz
Output Adjustable from 0.8V to 12V
Up to 95% Efficiency
Programmable Soft-Start
Stable with Low ESR Ceramic Output Capacitors
Cycle-by-Cycle Current Limit
Input Under Voltage Lockout
Output Under Voltage Protection
Thermal Shutdown Protection
RoHS Compliant and Halogen Free
Marking Information
RT7257GxZSP : Product Number
Applications
RT7257Gx
ZSPYMDNN
x : H or L
YMDNN : Date Code
Wireless AP/Router
Set-Top-Box
Industrial and Commercial Low Power Systems
LCD Monitors and TVs
Green Electronics/Appliances
Point of Load Regulation of High-Performance DSPs
Simplified Application Circuit
V
IN
C
IN
R
EN
VIN
BOOT
RT7257G
SW
EN
SS
C
SS
GND
COMP
FB
C
C
R
C
R2
R1
C
OUT
C
BOOT
L
V
OUT
Copyright
©
2016 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7257G-02
October 2016
www.richtek.com
1
RT7257G
Ordering Information
RT7257G
Package Type
SP : SOP-8 (Exposed Pad-Option 2)
Lead Plating System
Z : ECO (Ecological Element with
Halogen Free and Pb free)
H : UVP Hiccup
L : UVP Latch-Off
Note :
Richtek products are :
½
Pin Configurations
(TOP VIEW)
BOOT
VIN
SW
GND
2
3
4
8
7
SS
EN
COMP
FB
GND
9
6
5
SOP-8 (Exposed Pad)
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.
½
Functional Pin Description
Pin No.
1
2
3
4,
9 (Exposed Pad)
5
Pin Name
BOOT
VIN
SW
GND
FB
Pin Function
Bootstrap for High Side Gate Driver. Connect a 0.1F 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
suitable large ceramic capacitor.
Switch Node. Connect this pin to an external L-C filter.
Ground. The exposed pad must be soldered to a large PCB and connected to
GND for maximum power dissipation.
Feedback Input. It is used to regulate the output of the converter to a set value
via an external resistive voltage 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.
Enable Input. A logic high enables the converter; a logic low forces the
converter into shutdown mode reducing the supply current to less than 3A.
Attach this pin to VIN with a 100k pull up resistor for automatic startup.
Soft-Start Control Input. SS controls the soft-start period. Connect a capacitor
from SS to GND to set the soft-start period. A 0.1F capacitor sets the
soft-start period to 13.5ms.
6
COMP
7
EN
8
SS
Copyright
©
2016 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
2
DS7257G-02
October 2016
RT7257G
Function Block Diagram
VIN
Internal
Regulator
Shutdown
V
A
V
CC
Comparator
1.2V
+
-
Oscillator
Foldback
Control
0.4V
+
-
UV
Comparator
Current Sense
Amplifier
Slope Comp
+
-
R
SENSE
V
A
BOOT
S
+
R
-
Current
Comparator
Q
Q
110m
EN
5k
Lockout
Comparator
-
2.5V +
V
CC
6µA
SW
90m
GND
SS
0.8V
+
+EA
-
FB
COMP
Operation
Shutdown Comparator
Activate internal regulator once EN input level is larger
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.
Lockout Comparator
Activate the current comparator, release lock-out logic,
and enable the switches as EN input level is larger than
lockout voltage. Otherwise, the switches still locks out.
Oscillator
The oscillator provides internal clock and controls the
converter's switching frequency.
Foldback Control
Dynamically adjust the internal clock. It provides a slower
frequency as a lower FB voltage.
UV Comparator
As FB voltage is lower than the UV voltage, it will activate
a UV protect scheme.
Error Amplifier
The output voltage COMP of the error amplifier is adjusted
comparing FB signal with the internal reference voltage
and SS signal.
Current Sense Amplifier
R
SENSE
detects the peak current of the high side switch.
This signal is amplified by the current sense amplifier and
added with a slope compensation signal. Then, It controls
the switches by comparing this signal with the COMP
voltage.
Copyright
©
2016 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7257G-02
October 2016
www.richtek.com
3
RT7257G
Absolute Maximum Ratings
(Note 1)
−0.3V
to 20V
−0.3V
to (V
IN
+ 0.3V)
−0.3V
to 6V
−0.3V
to 20V
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) --------------------------------------------------------------------------------------------- 1.333W
Package Thermal Resistance (Note 2)
SOP-8 (Exposed Pad),
θ
JA
---------------------------------------------------------------------------------------- 75°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
(Note 4)
Recommended Operating Conditions
Supply Input 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
Shutdown Supply Current
Supply Current
Reference Voltage
Error Amplifier
Transconductance
High Side Switch
On-Resistance
Low Side Switch
On-Resistance
High Side Switch Leakage
Current
Upper Switch Current Limit
COMP to Current Sense
Transconductance
Oscillation Frequency
Short Circuit Oscillation
Frequency
Maximum Duty Cycle
Minimum On-Time
Symbol
V
EN
= 0V
Test Conditions
Min
--
--
0.788
--
--
--
Typ
0.5
0.8
0.8
940
110
90
0
5.1
5.1
800
270
84
100
Max
3
1.2
0.812
--
--
--
10
--
--
--
--
--
--
Unit
A
mA
V
A/V
m
m
A
A
A/V
kHz
kHz
%
ns
V
EN
= 3V, V
FB
= 0.9V
V
REF
G
EA
R
DS(ON)1
R
DS(ON)2
V
EN
= 0V, V
SW
= 0V
Min. Duty Cycle, V
BOOT
V
SW
= 4.8V
G
CS
f
OSC1
f
OSC2
D
MAX
t
ON
V
FB
= 0V
V
FB
= 0.7V
4.5V V
IN
18V
I
C
= ±10A
--
--
--
--
--
--
--
Copyright
©
2016 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS7257G-02
October 2016
RT7257G
Parameter
EN Input Threshold
Voltage
Logic-High
Logic-Low
Symbol
V
IH
V
IL
V
UVLO
V
UVLO
I
SS
t
SS
T
SD
V
SS
= 0V
C
SS
= 0.1F
V
IN
Rising
Test Conditions
Min
2.7
--
3.8
--
--
--
--
Typ
--
--
4.2
320
6
13.5
150
Max
18
0.4
4.5
--
--
--
--
Unit
V
V
mV
A
ms
C
Input Under Voltage Lockout
Threshold
Input Under Voltage Lockout
Hysteresis
Soft-Start Current
Soft-Start Period
Thermal Shutdown
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.
Copyright
©
2016 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7257G-02
October 2016
www.richtek.com
5
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