Purpose
The RT2808A is a current mode asynchronous step-down converter that achieves excellent load and line regulation. This document explains the function and use of the RT2808A evaluation board (EVB) and provides information to enable operation and modification of the evaluation board and circuit to suit individual requirements.
Introduction
General Product Description
The RT2808A is a current mode asynchronous step-down converter that achieves excellent load and line regulation.Over a wide input voltage range from 5.5V to 36V and supports output current up to 3A. The Current mode operation provides fast transient response and eases loop stabilization. An adjustable soft-start reduces the stress on the input source at start up. In shut down mode, the regulator draws only 25μA of supply current. The RT2808A requires a minimum number of readily available external components, providing a compact solution. The RT2808A provides protection functions inducing input under voltage lockout, cycle-by-cycle current limit, short circuit protection and thermal shutdown protection.The RT2808A is available in the SOP-8 (Exposed Pad) package.
Product Features
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3A Output Current
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Wide Operating Input Range 5.5V to 36V
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Adjustable Output Voltage from 1.222V to 26V
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High Efficiency up to 90%
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Internal Compensation Minimizes External Parts Count
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Internal Soft-Start
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110mΩ Internal Power MOSFET Switch
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25μA Shutdown Mode
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Fixed 500kHz Frequency
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Thermal Shutdown
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Cycle-by-Cycle Current Limit
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Available In an SOP-8 (Exposed Pad) Package
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RoHS Compliant and Halogen Free
Application
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Distributive Power Systems
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Battery Charger
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DSL Modems
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Pre-regulator for Linear Regulators
Key Performance Summary Table
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Key Features
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Evaluation Board Number : PCB011_V1
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Default VIN Input Voltage
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12V
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Max Output Current
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3A
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Default Output Voltage
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2.5V
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Default Marking & Package Type
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RT2808AGSP, SOP-8 (Exposed Pad)
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Bench Test Setup Conditions
Headers Description and Placement
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.
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Test point/Pin name
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Signal
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Comment (expected waveforms or voltage levels on test points)
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Vin
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Input voltage
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Input voltage range = 5.5V to 36V
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Vout
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Output voltage
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Output voltage = 2.4V
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SW
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Switch Node
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EN
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Chip Enable Operation
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The EN pin is externally pulled to High by adding a 100kΩ resistor (R4) from the VIN pin in this EVB setting. For another enable operation, the converter is turned on when the EN pin is higher than 1.4V and turned off when the EN pin is lower than 0.4V.
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GND
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Ground
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Ground. The exposed pad must be soldered to a large PCB and connected to GND for maximum power dissipation.
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Power-up & Measurement Procedure
1. Connect input power (5.5V < VIN < 36V) and input ground to Vin and GND pins respectively.
2. Connect positive end and negative end of load to Vout and GND of output pins respectively.
3. The output voltage of (VOUT) can be set by R1 and R2.
VOUT = VREF * (1 + R1 / R2)
where VREF = 1.222V (typ.)
Specification
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Parameter
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Symbol
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Min
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Typ.
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Max
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Units
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Input Voltage Range
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VIN
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5.5
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36
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V
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Output Voltage
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VOUT
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2.4
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V
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Oscillator Frequency
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fOSC
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400
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500
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600
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kHz
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Output Current
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IOUT
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3
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A
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Current Limit
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ILIM
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4
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5.25
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6.5
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A
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Schematic, Bill of Materials & Board Layout
EVB Schematic Diagram
Bill of Materials
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Reference
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Qty
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P/N
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Description
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Package
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Manufacture
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U1
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1
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RT2808AGSP
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Step-down converter
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SOP-8
(Exposed Pad)
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RICHTEK
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C1, C2
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2
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GRM32ER71H475K
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4.7μF/50V/X7R
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C-1210
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MURATA
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C3, C6, C11
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3
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C1608X7R1H104K080AA
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0.1μF/±10%/50V/X7R
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C-0603
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TDK
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C4, C5
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2
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GRM32ER61C226KE20
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22μF/±10%/16V/X5R
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C-1210
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MURATA
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C12
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1
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82pF
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C-0603
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L1
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1
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NR8040T6R8N
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6.8µH
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TAIYO YUDEN
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D1
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1
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B550C
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50V/5A
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DIODES
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R1, R2, R4
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3
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100k
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R-0402
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C8, R5
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2
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NC
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PCB Layout
Top View (1st layer)
PCB Layout—Inner Side (2nd Layer)
PCB Layout—Inner Side (3rd Layer)
Bottom View (4th Layer)
Step-Down Converter Efficiency Test
