Purpose
The RT2875B is a high efficiency, current-mode synchronous DC/DC step-down converter that can deliver up to 3A output current over a wide input voltage range from 4.5V to 36V. his document explains the function and use of the RT2875B 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 RT2875B is a high efficiency, current-mode synchronous DC/DC step-down converter that can deliver up to 3A output current over a wide input voltage range from 4.5V to 36V. The device integrates 95mΩ high-side and 70mΩ low-side MOSFETs to achieve high conversion efficiency. The current-mode control architecture supports fast transient response and simple external compensation. A cycle-by-cycle current limit function provides protection. against shorted output and an external soft-start eliminates input current surge during start-up. The RT2875B provides complete protection functions such as input under-voltage lockout, output under-voltage protection, over-current protection and thermal shutdown.The RT2875B is available in the thermal enhanced TSSOP-14 (Exposed Pad) package.
The RT2875B provides Hiccup Mode Under Voltage Protection (UVP). When the VFB voltage drops below 0.3V, the UVP function will be triggered to shut down switching operation. If the UVP condition remains for a period, the RT2875B will retry automatically. When the UVP condition is removed, the converter will resume operation. The UVP is disabled during soft-start period.
Product Features
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3A Output Current
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Internal N-MOSFETs
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Current Mode Control
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Adjustable Switching Frequency : 300kHz to 2.1MHz
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Adjustable Current Limit : 1.5A to 6A
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Synchronous to External Clock : 300kHz to 2.1MHz
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Adjustable Output Voltage from 0.6V to 24V
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High Efficiency Up to 95%
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Stable with Low ESR Ceramic Output Capacitors
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Cycle-by-Cycle Current Limit
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Input Under-Voltage Lockout
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Output Under-Voltage Protection
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Thermal Shutdown
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RoHS Compliant and Halogen Free
Application
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Point of Load Regulator in Distributed Power Systems
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Digital Set Top Boxes
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Broadband Communications
Key Performance Summary Table
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Key Features
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Evaluation Board Number : PCB013_V1
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Default 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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3.3V
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Default Marking & Package Type
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RT2875BQGCP, TSSOP-14 (Exposed Pad)
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Operation Frequency
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300kHz to 2.1MHz (Adjusted by RT Value, Defult RT = 100k,
Frequency = 500kHz)
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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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SW
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Switch Node
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Connect to external L-C filter.
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PGND
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Power Ground
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Ground. The exposed pad must be soldered to a large PCB and connected to PGND for maximum power dissipation.
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RT/SYNC
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Oscillator Resistor
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Oscillator Resistor and External Frequency Synchronization Input. Must connect a resistor from this pin to GND to set the switching frequency. If SYNC clock is requested, connect an external clock to change the switching frequency.
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AGND
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Analog Ground
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Ground.
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RLIM
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Current Limit Setting
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Connect a resistor from this pin to GND to set the current limit value.
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FB
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Feedback Voltage Input
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The pin is used to set the output voltage of the converter to regulate to the desired via a resistive divider. Feedback reference = 0.6V.
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COMP
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Compensation Node.
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COMP is used to compensate the regulation control loop. Connect a series RC network from COMP to GND. In some cases, an additional capacitor from COMP to GND is required.
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SS
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Soft-Start Time Setting.
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Connect a capacitor from SS to GND to set the soft-start period.
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EN
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Enable
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Enable Control Input. High = Enable.
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PGOOD
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Power Good Output.
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Power Good Indicator Output.
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VIN
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Input Voltage
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Input voltage range = 4.5V to 36V.
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BOOT
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Boot
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Bootstrap Supply for High-Side Gate Driver. Connect a 0.1µF ceramic capacitor between the BOOT and SW pins.
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Power-up & Measurement Procedure
1. Connect input power (4.5V< 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 = 0.6V (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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4.5
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36
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V
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Oscillator Frequency
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fOSC
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300
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2100
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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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EN Input Voltage
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VEN
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1.4
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1.5
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1.6
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V
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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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Q'ty
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Part Number
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Description
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Package
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Manufacture
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U2
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1
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RT2875BQGCP
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Step-down Converter
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PTSSOP-14
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RICHTEK
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Css, C3,
C6, C11
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4
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C1608X7R1H104KT000N
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0.1μF/50V/X7R
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C-0603
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TDK
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C1, C2
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2
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GRM32ER71H106KA12
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10μF/50V/X7R
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C-1210
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Murata
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C4, C5
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2
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GRM32ER61C226KE20L
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22μF/16V/X5R
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C-1210
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MURATA
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C7
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1
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0603B333K500
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3.3nF/50V/X7R
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C-0603
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WALSIN
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R5, C8, C9, C12, C13
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5
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NC
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C/R-0603
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|
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D1
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0
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NC
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|
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D2
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0
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NC
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|
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L1
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1
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NR8040T4R7N
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4.7μH
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8040
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TAIYO YUDEN
|
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744 771 404 7
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4.7μH/7A
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WE
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RT, R4, RPG
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3
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100K
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R-0603
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RLIM
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1
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33k
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R-0603
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R1
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1
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115K
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R-0603
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R2
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1
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25.5K
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R-0603
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R3
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1
|
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10K
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R-0603
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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
