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1.5A, 18V, 500kHz ACOT™ Synchronous Step-down Converter

EVB_RT7255AGV8

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The RT7255 is a synchronous step-down converter with Advanced Constant On-Time (ACOT™) mode control. The ACOT™ provides a very fast transient response with few external components. The low impedance internal MOSFET supports high efficiency operation with wide input voltage range from 4.3V to 18V. The proprietary circuit of the RT7255 supports all ceramic capacitors.

Purpose

The RT7255 is an Advanced Constant On-Time (ACOT™) mode control synchronous step-down converter with the input voltage range from 4.3V to 18V and provides 1.5A output current. This document explains the function and use of the RT7255 evaluation board (EVB), and provides information to enable operation, modification of the evaluation board and circuit to suit individual requirements.


Introduction

General Product Information

The RT7255 is a synchronous step-down converter with Advanced Constant On-Time (ACOT™) mode control. The ACOT™ provides a very fast transient response with few external components. The low impedance internal MOSFET supports high efficiency operation with wide input voltage range from 4.3V to 18V. The proprietary circuit of the RT7255 supports all ceramic capacitors.


Product Feature

  • 4.3V to 18V Input Voltage Range
  • Adjustable Soft-Start Time
  • PGOOD Function
  • 1.5A Output Current
  • Advanced Constant On-Time Control
  • Fast Transient Response
  • Support All Ceramic Capacitors
  • Up to 95% Efficiency
  • 500kHz Switching Frequency
  • Adjustable Output Voltage from 0.6V to 8V
  • Cycle-by-Cycle Current Limit
  • Input Under-Voltage Lockout
  • Hiccup Mode Under-Voltage Protection
  • Thermal Shutdown

 


Key Performance Summary Table

Key Features

Evaluation Board Number : PCB037_V1

Default Input Voltage

12V

Max Output Current

1.5A

Default Output Voltage

1.2V

Default Marking & Package Type

RT7255AGV8, SOT-23-8

Operation Frequency

Fixed 500kHz



Bench Test Setup Conditions

Headers Description and Placement

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Please carefully inspect the EVB IC and external components, comparing them to the following Bill of Materials, to ensure that all components are installed and undamaged. If any components are missing or damaged during transportation, please contact the distributor or send e-mail to evb_service@richtek.com



Test Points

The EVB is provided with the test points and pin names listed in the table below.

Test point/Pin name

Signal

Comment (expected waveforms or voltage levels on test points)

VIN

Input voltage

Power input. The input voltage range is from 4.3V to 18V. Must bypass with a suitable large ceramic capacitor at this pin.

EN

Enable test point

Enable control input. Connect EN to a logic-high voltage to enable the IC or to a logic-low voltage to disable. Do not leave this high impedance input unconnected.

GND

Ground

Power ground.

BOOT

Bootstrap supply for high-side gate driver. Connect a 1µF ceramic capacitor between the BOOT and SW pins.

FB

Feedback

Feedback voltage input. The pin is used to set the output voltage of the converter via a resistive divider. The converter regulates VFB to 0.6V.

SW

Switch node test point

Switch node.

PGOOD

Power good test point

Power good indicator output.

SS

Soft-start test point

Soft-start time setting. Connect a capacitor from SS to GND to set the soft-start period.



Power-up & Measurement Procedure

1. Apply a 12V nominal input power supply (4.3V < VIN < 18V) to the VIN and GND terminals.

2. Set the jumper at JP2 to connect terminals 2 and 3, connecting EN to enable operation.

3. Verify the output voltage (approximately 1.2V) between VOUT and GND.

4. Connect an external load up to 1.5A to the VOUT and GND terminals and verify the output voltage and current.


Output Voltage Setting

Set the output voltage with the resistive divider (R2, R3) between VOUT and GND with the midpoint connected to FB. The output is set by the following formula :

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Schematic, Bill of Materials & Board Layout

EVB Schematic Diagram

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Bill of Materials

Reference

Qty

Part Number

Description

Package

Manufacture

U1

1

RT7255AGV8

DC/DC Converter

SOT-23-8

RICHTEK

C3

1

GRM31CR71E106KA12L

10μF/25V/X5R

C-1206

MURATA

C8

1

C1608X7R1H104K080AA

1μF/25V/X5R

C-0603

MURATA

C4, C6, C10, C11, C12, C1, D2

7

N/A

C9

1

GRM32ER61C226KE20L

22μF/16V/X7R

C-1210

MURATA

R4, R5

2

10k

R-0603

C13

1

0603B103K500

10nF/50V/X7R

C-0603

WALSIN

R2

1

10k

R-0603

R3

1

10k

R-0603

L1

1

NR8040T3R6N

3.6μH/4.9A

8 x 8 mm

TAIYO YUDEN



PCB Layout

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Top View (1st layer)

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PCB Layout—Inner Side (2nd Layer)

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PCB Layout—Inner Side (3rd Layer)

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Bottom View (4th Layer)

Title Last Update Share Download
Evaluation Board User Guide 2015/10/27
Bill of Materials 2015/10/27
Gerber File 2015/10/27
Schematic 2015/10/27
RT7255

18V, 1.5A, 500kHz ACOTTM Step-down Converter in 8 Pin TSOT-23

RT7255

The RT7255 is a simple, easy-to-use, 1.5A synchronous step-down DC-DC converter with an input supply voltage range of 4.3V to 18V. The device build-in an accurate 0.6V reference voltage and integrates low RDS(ON) power MOSFETs to achieve high efficiency in a TSOT-23-8 package.

The RT7255 adopts Advanced Constant On-Time (ACOT®) control architecture to provide an ultrafast transient response with few external components and to operate in nearly constant switching frequency over the line, load, and output voltage range. The RT7255A operates in automatic PSM that maintains high efficiency during light load operation. The RT7255C operates in Forced PWM that helps meet tight voltage regulation accuracy requirements.

The RT7255 senses low-side FETs current for a robust over-current protection. It features cycle-by-cycle current limit protection and prevent the device from the catastrophic damage in output short circuit, over current or inductor saturation. An externally adjustable soft-start function prevents inrush current during start-up. The device also includes input under-voltage lockout, output under-voltage protection, and over-temperature protection (thermal shutdown) to provide safe and smooth operation in all operating conditions. The RT7255 is offered in a TSOT-23-8 package.

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