4A, 18V, 650kHz, ACOT™ Synchronous Step-Down Converter

EVB_RT2855ALGQW

ACTIVE

The RT2855A is a high-performance 650kHz 4A step-down regulator with internal power switches and synchronous rectifiers. It features quick transient response using its Advanced Constant On-Time (ACOT™) control architecture that provides stable operation with small ceramic output capacitors and without complicated external compensation, among other benefits. The input voltage range is from 4.5V to 18V and the output is adjustable from 0.765V to 7V.

Purpose

The RT2855A is an Advanced Constant On-Time (ACOT™) control architecture step-down converter with the input voltage range from 4.5V to 18V and provides 4A output current. This document explains the function and use of the RT2855A evaluation board (EVB), and provides information to enable operation, modification of the evaluation board and circuit to suit individual requirements.

Introduction

General Product Information

Product Feature

Fast Transient Response
Discontinuous operating mode at Light Load
4A Output Current
Advanced Constant On-Time (ACOT™) Control
Optimized for Ceramic Output Capacitors
4.5V to 18V Input Voltage Range
Internal 70mΩ Switch and 30mΩ Synchronous Rectifier
0.765V to 7V Adjustable Output Voltage
Externally-adjustable, Pre-biased Compatible Soft-Start
Cycle-by-Cycle Current Limit
Optional Output Discharge Function
Output Over- and Under-voltage Shut-down in Latched
Input Under Voltage Lockout
Thermal Shutdown

Key Performance Summary Table

Key Features	Evaluation Board Number : PCB023_V1
Default Input Voltage	12V
Max Output Current	4A
Default Output Voltage	1.05V
Default Marking & Package Type	RT2855ALGQW, WQFN-16 3x3

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.

Test point/ Pin name	Signal	Comment (expected waveforms or voltage levels on test points)
VIN	Input voltage	Power input. Support 4.5V to18V Input Voltage. Must bypass with a suitable large ceramic capacitor at this pin.
EN	Enable test point	A logic-high enables the converter; a logic-low forces the IC into shutdown mode reducing the supply current to less than 10µA.
GND	Ground	Ground.
VCC	Internal regulator output	VCC supplies power for the internal linear regulator that powers the IC. Connect VIN to the input voltage and bypass to ground with a 0.1µF ceramic capacitor.
BOOT	Bootstrap supply test point	Bootstrap supply for high-side gate driver. Connect a 0.1µF capacitor between BOOT and SW to power the internal gate driver.
SW	Switch node test point	SW is the source of the internal N-channel MOSFET switchand the drain of the internal N-channel MOSFET synchronous rectifier. Connect SW to the inductor with a wide short PCB trace and minimize its area to reduce EMI.
VREG5	Internal regulator	Connect a 1µF capacitor to GND to stabilize output voltage.
SS	Soft-start control	Connect an external capacitor between this pin and GND to set the soft-start time.
PG	Open drain power good output	PGOOD connects to PGND whenever V_FB is less than 90% of its regulation threshold (typical).

Power-up & Measurement Procedure

1. Apply a 12V nominal input power supply (4.5V < V_IN < 18V) to the VIN and GND terminals.

2. Connect JP1 to connect VIN to VCC. Set the jumper at JP2 to connect terminals 2 and 3, connecting EN to VCC to enable operation.

3. Verify the output voltage (approximately 1.05V) between VOUT and GND.

4. Connect an external load up to 4A 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 :

Schematic, Bill of Materials & Board Layout

EVB Schematic Diagram

Bill of Materials

Reference	Qty	Part Number	Description	Package	Manufacture
U4	1	RT2855ALGQW	DC/DC Converter	WQFN-16 3x3	RICHTEK
C1	1		NC
C2, C5, C6, C8, C12	5	C1608X7R1H104K080AA	0.1μF/±10%/50V/X7R	C-0603	TDK
C3, C4	2	UMK325BJ106MM	10µF/50V/X7R	C-1210	TAIYO YUDEN
C7	1	0603B332K500	3.3nF/50V/X7R	C-0603	WALSIN
C9	1	C3225X5R1E226MT	22µF/25V/X5R	C-1210	TDK
C10	1	C3225X5R1E226MT	22µF/25V/X5R	C-1210	TDK
C13	1		NA
C14	1		NA
L1	1	NR8040T1R4N	1.4µH/7A	8 x 8 x 4.2 mm	TAIYO YUDEN
R1	1		0	R-0603
R2	1		8.25k	R-0603
R3	1		22.1k	R-0603
R4, R5	2		100k	R-0603
R6, R7	2		NA

PCB Layout

Top View (1^st layer)

PCB Layout—Inner Side (2^nd Layer)

PCB Layout—Inner Side (3^rd Layer)

Bottom View (4th Layer)

Title	Last Update	Share
Evaluation Board User Guide	2015/10/27	Email RSS Facebook Google+ twitter Linkedin
Bill of Materials	2015/10/27	Email RSS Facebook Google+ twitter Linkedin
Gerber File	2015/10/27	Email RSS Facebook Google+ twitter Linkedin
Schematic	2015/10/27	Email RSS Facebook Google+ twitter Linkedin

RT2855A/RT2855B

The RT2855A/B are high-performance 650kHz 4A step-down regulators with internal power switches and synchronous rectifiers. They feature quick transient response using their Advanced Constant On-Time (ACOT^®) control architecture that provides stable operation with small ceramic output capacitors and without complicated external compensation, among other benefits. The input voltage range is from 4.5V to 18V and the output is adjustable from 0.765V to 7V.

The proprietary ACOT^® control improves upon other fast-response constant on-time architectures, achieving nearly constant switching frequency over line, load, and output voltage ranges. Since there is no internal clock, response to transients is nearly instantaneous and inductor current can ramp quickly to maintain output regulation without large bulk output capacitance. The RT2855A/B are stable with and optimized for ceramic output capacitors.

With internal 70mΩ switches and 30mΩ synchronous rectifiers, the RT2855A/B display excellent efficiency and good behavior across a range of applications, especially for low output voltages and low duty cycles. Cycle-by-cycle current limit, input under-voltage lock-out, externally-adjustable soft-start, output under- and over-voltage protection, and thermal shutdown provide safe and smooth operation in all operating conditions.

The RT2855A and RT2855B are each available in
WQFN-16L 3x3 package, with exposed thermal pads. The RT2855B switches continuously even at light loads to avoid low-frequency interference while the RT2855A features a power-saving discontinuous operating mode at light loads.

Part Number	Free Sample	Buy From Authorized Distributor
EVB_RT2855ALGQW	Not available	Buy from Distributor

Please confirm with our distributor or sales representative for actual stock status.