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Design
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RT7257J
3A, 18V, 340kHz Synchronous Step-Down Converter
General Description
The RT7257J 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 RT7257J'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 RT7257J 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 RT7257J is available in an SOP-8 (Exposed Pad) package.
Marking Information
RT7257JCHZSP : Product Number
RT7257JCH ZSPYMDNN
YMDNN : Date Code
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 : 340kHz Output Adjustable from 0.8V to 15V Stable with Low ESR Ceramic Output Capacitors Up to 95% Efficiency Programmable Soft-Start Cycle-by-Cycle Over Current Protection Input Under Voltage Lockout Output Under Voltage Protection Thermal Shutdown Protection RoHS Compliant and Halogen Free
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
Simplified Application Circuit
VIN Chip Enable
CIN
CSS
VIN BOOT RT7257J SW
EN SS FB
GND
COMP
CBOOT L
VOUT
CC RC
R1 COUT
R2
CP Open
Copyright ©2018 Richtek Technology Corporation. All rights reserved. DS7257J-04 March 2018
is a registered trademark of Richtek Technology Corporation.
www.richtek.com 1
RT7257J
Ordering Information
RT7257JCH
Package Type SP : SOP-8 (Exposed Pad-Option 2) Lead Plating System Z : ECO (Ecological Element with
Halogen Free and Pb free)
Note : Richtek products are :
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
Pin Configurations
(TOP VIEW)
BOOT VIN SW
GND
8
27
GND
36 9
45
SS EN COMP FB
SOP-8 (Exposed Pad)
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
BOOT
Bootstrap for High Side Gate Driver. Connect a 0.1μF or greater ceramic capacitor from BOOT to SW pins.
2
VIN
Power Input. The input Voltage range is from 4.5V to 18V. Connect two 10μF or larger ceramic capacitors from the VIN to GND.
3 SW Switch Node. Connect this pin to an external L-C filter.
4, 9 (Exposed Pad)
GND
Ground. The exposed pad must be soldered to a large PCB and connected to GND for maximum power dissipation.
5
FB
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
6
COMP
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 7 EN RT7257J 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 8 SS from SS to GND to set the soft-start period. A 0.1μF capacitor sets the
soft-start period to 13.5ms.
Copyright ©2018 Richtek Technology Corporation. All rights reserved. www.richtek.com 2
is a registered trademark of Richtek Technology Corporation.
DS7257J-04 March 2018
Function Block Diagram
RT7257J
VIN
Internal Regulator
Oscillator
Shutdown Comparator
VA VCC
1.2V +
Foldback Control
- 0.4V +
Lockout 5kΩ Comparator EN -
UV Comparator
1.8V +
VCC
6µA SS 0.8V ++EA
-
Current Sense Slope Comp Amplifier
+
-
RSENSE VA
S
+ R
Current Comparator
Q Q
110mΩ 90mΩ
BOOT SW GND
FB COMP
Copyright ©2018 Richtek Technology Corporation. All rights reserved. DS7257J-04 March 2018
is a registered trademark of Richtek Technology Corporation.
www.richtek.com 3
RT7257J
Absolute Maximum Ratings (Note 1)
Supply Input Voltage, VIN ----------------------------------------------------------------------------------------- −0.3V to 20V Switch Voltage, SW ------------------------------------------------------------------------------------------------ −0.3V to (VIN + 0.3V) VBOOT − VSW ---------------------------------------------------------------------------------------------------------- −0.3V to 6V Other Pins Voltage ------------------------------------------------------------------------------------------------- −0.3V to 20V Power Dissipation, PD @ TA = 25°C
SOP-8 (Exposed P.