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Richtek

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 ACOT® Buck轉換器

RT7255

RT7255是簡單易用的1.5A同步Buck轉換器,輸入電壓範圍為4.3V至18V,內建0.6V高精度參考電壓,集成低RDS(ON)功率MOSFET開關,轉換效率高,使用TSOT-23-8封裝。
RT7255使用改進型固定導通時間 (ACOT®) 控制架構,具有超快速瞬態回應能力,週邊元件少,在全輸入電壓、負載和輸出電壓下的工作頻率保持基本恒定。RT7255A在輕載條件下自動進入PSM工作模式以維持高效率工作,RT7255C則工作在強制PWM模式下以滿足高輸出電壓精度的要求。
RT7255通過檢測下橋開關電流實現過流保護功能,具備逐週期電流限制保護能力,可在輸出短路、過流或電感飽和等嚴峻狀態下確保器件安全,另有外部可調的軟啟動功能避免出現啟動器件的電流衝擊,還有輸入欠壓鎖定、輸出欠壓保護、過熱保護 (熱關機) 等確保其在各種條件下都能安全工作。

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