RT7257B 3A, 18V, 1.2MHz Synchronous Step-Down Converter General Description Features 1.5% High Accuracy Reference Voltage The RT7257B is a high efficiency, monolithic synchronous 4.5V to 18V Input Voltage Range step-down DC/DC converter that can deliver up to 3A 3A Output Current output current from a 4.5V to 18V input supply. The Integrated N-MOSFET Switches RT7257B s current mode architecture and external Current Mode Control compensation allow the transient response to be Fixed Frequency Operation : 1.2MHz optimized over a wide input voltage range and loads. Output Adjustable from 0.8V to 12V Cycle-by-cycle current limit provides protection against Up to 95% Efficiency shorted outputs, and soft-start eliminates input current Programmable Soft-Start surge during start-up. The RT7257B also provides under Stable with Low ESR Ceramic Output Capacitors voltage protection and thermal shutdown protection. The Cycle-by-Cycle Over Current Protection low current (<3A) shutdown mode provides output Input Under Voltage Lockout disconnection, enabling easy power management in Output Under Voltage Protection battery-powered systems. The RT7257B is available in Thermal Shutdown Protection an SOP-8 (Exposed Pad) package. RoHS Compliant and Halogen Free Ordering Information Applications RT7257B Wireless AP/Router Package Type Set-Top-Box SP : SOP-8 (Exposed Pad-Option 1) Industrial and Commercial Low Power Systems Lead Plating System Z : ECO (Ecological Element with LCD Monitors and TVs Halogen Free and Pb free) Green Electronics/Appliances H : UVP Hiccup Point of Load Regulation of High-Performance DSPs L : UVP Latch-Off Note : Pin Configurations Richtek products are : (TOP VIEW) RoHS compliant and compatible with the current require- ments of IPC/JEDEC J-STD-020. 8 BOOT SS Suitable for use in SnPb or Pb-free soldering processes. 2 7 VIN EN GND 3 6 SW COMP 9 GND 4 5 FB Marking Information RT7257BxZSP : Product Number SOP-8 (Exposed Pad) RT7257Bx x : H or L ZSPYMDNN YMDNN : Date Code Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. DS7257B-03 September 2012 www.richtek.com 1RT7257B Typical Application Circuit V 2 1 IN VIN BOOT 4.5V to 18V C C BOOT IN L RT7257B 10F x 2 0.1F 3.6H V 3 OUT SW 3.3V R 100k EN R1 7 EN 75k C OUT 8 5 SS FB 22F x 2 C C C R SS C R2 2.2nF 0.1F 22k 24k 4, 9 (Exposed Pad) 6 COMP GND C P Open Table 1. Suggested Components Selection V (V) R1 (k) R2 (k) R (k) C (nF) L (H) C (F) OUT C C OUT 8 27 3 51 2.2 10 22 x 2 5 62 11.8 33 2.2 6.8 22 x 2 3.3 75 24 22 2.2 3.6 22 x 2 2.5 25.5 12 16 2.2 3.6 22 x 2 1.5 10.5 12 10 2.2 2 22 x 2 1.2 12 24 8.2 2.2 2 22 x 2 1 3 12 6.8 2.2 2 22 x 2 Functional Pin Description Pin No. Pin Name Pin Function Bootstrap for High Side Gate Driver. Connect a 0.1F or greater ceramic 1 BOOT capacitor from BOOT to SW pins. Input Supply Voltage, 4.5V to 18V. Must bypass with a suitable large ceramic 2 VIN capacitor. 3 SW Switch Node. Connect this pin to an external L-C filter. 4, Ground. The exposed pad must be soldered to a large PCB and connected to GND 9 (Exposed Pad) GND for maximum power dissipation. Feedback Input. It is used to regulate the output of the converter to a set value 5 FB 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 Pin. A logic high enables the converter a logic low forces the 7 EN RT7257B 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 8 SS from SS to GND to set the soft-start period. A 0.1F capacitor sets the soft-start period to 13.5ms. Copyright 2012 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. www.richtek.com DS7257B-03 September 2012 2