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RT7257B
3A, 18V, 1.2MHz Synchronous Step-Down Converter
General Description
The RT7257B 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
RT7257B'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 RT7257B 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 RT7257B 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 : 1.2MHz
Output Adjustable from 0.8V to 12V
Up to 95% Efficiency
Programmable Soft-Start
Stable with Low ESR Ceramic Output Capacitors
Cycle-by-Cycle Over Current Protection
Input Under Voltage Lockout
Output Under Voltage Protection
Thermal Shutdown Protection
RoHS Compliant and Halogen Free
Ordering Information
RT7257B
Package Type
SP : SOP-8 (Exposed Pad-Option 1)
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
Applications
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
Pin Configurations
(TOP VIEW)
BOOT
VIN
SW
GND
2
3
4
8
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.
SS
EN
COMP
FB
½
GND
9
7
6
5
Marking Information
RT7257BxZSP : Product Number
SOP-8 (Exposed Pad)
RT7257Bx
ZSPYMDNN
x : H or L
YMDNN : Date Code
Copyright
©
2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7257B-05
February 2018
www.richtek.com
1
RT7257B
Typical Application Circuit
V
IN
4.5V to 18V
2
C
IN
10µF x 2
R
EN
100k
VIN
BOOT
RT7257B
SW 3
7 EN
8 SS
GND
1
C
BOOT
L
0.1µF 3.6µH
R1
75k
FB 5
COMP
6
C
C
2.2nF
R
C
22k
R2
24k
V
OUT
3.3V
C
OUT
22µF x 2
C
SS
0.1µF
4, 9 (Exposed Pad)
C
P
Open
Table 1. Suggested Components Selection
V
OUT
(V) R1 (kΩ)
8
5
3.3
2.5
1.5
1.2
1
27
62
75
25.5
10.5
12
3
R2 (kΩ)
3
11.8
24
12
12
24
12
R
C
(kΩ)
51
33
22
16
10
8.2
6.8
C
C
(nF)
2.2
2.2
2.2
2.2
2.2
2.2
2.2
L (μH)
10
6.8
3.6
3.6
2
2
2
C
OUT
(μF)
22 x 2
22 x 2
22 x 2
22 x 2
22 x 2
22 x 2
22 x 2
Functional Pin Description
Pin No.
1
2
3
4,
9 (Exposed Pad)
5
6
Pin Name
BOOT
VIN
SW
GND
FB
COMP
Pin Function
Bootstrap for High Side Gate Driver. Connect a 0.1μF or greater ceramic
capacitor from BOOT to SW pins.
Input Supply Voltage, 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 Pin. A logic high enables the converter; a logic low forces the
RT7257B into shutdown mode reducing the supply current to less than 3μA.
Attach this pin to VIN with a 100kΩ 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 0.1μF capacitor sets the
soft-start period to 13.5ms.
7
EN
8
SS
Copyright
©
2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
2
DS7257B-05
February 2018
RT7257B
Function Block Diagram
VIN
Internal
Regulator
Shutdown
Comparator V
A
V
CC
1.2V
+
-
Lockout
Comparator
-
+
2.5V
V
CC
6µA
SS
0.8V
+
+EA
-
0.4V
Oscillator
Foldback
Control
+
-
UV
Comparator
Current Sense
Amplifier
Slope Comp
+
-
R
SENSE
V
A
BOOT
S
+
-
Current
Comparator
R
Q
Q
110m
Ω
EN
5kΩ
SW
90m
Ω
GND
FB
COMP
Copyright
©
2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7257B-05
February 2018
www.richtek.com
3
RT7257B
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
Recommended Operating Conditions
(Note 4)
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
4.7
1.2
270
78
100
Max
3
1.2
0.812
--
--
--
10
--
--
1.4
--
--
--
Unit
μA
mA
V
μA/V
mΩ
mΩ
μA
A
A/V
MHz
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
ΔI
C
= ±10μA
--
--
--
1
--
--
--
Copyright
©
2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS7257B-05
February 2018
RT7257B
Parameter
EN Input Threshold
Voltage
Logic-High
Logic-Low
Symbol
V
IH
V
IL
V
UVLO
ΔV
UVLO
I
SS
t
SS
T
SD
V
SS
= 0V
C
SS
= 0.1μF
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
μA
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
©
2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7257B-05
February 2018
www.richtek.com
5
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