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3A, 1MHz, 6V CMCOT Synchronous Step-Down Converter

EVB_RT5797AHGQWA

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The RT5797A is a high efficiency synchronous step-down DC-DC converter. Its input voltage range is from 2.7V to 6V and provides an adjustable regulated output voltage from 0.6V to 3.4V while delivering up to 3A of output current. The internal synchronous low on-resistance power switches increase efficiency and eliminate the need for an external Schottky diode. The RT5797A is available in the WDFN-8SL 2x2 packages.

Purpose

The RT5797A is a high efficiency synchronous step-down DC-DC converter. Its input voltage range is from 2.7V to 6V and provides an adjustable regulated output voltage from 0.6V to 3.4V while delivering up to 3A of output current. This document explains the function and use of the RT5797A 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 RT5797A is a high efficiency synchronous step-down DC-DC converter. Its input voltage range is from 2.7V to 6V and provides an adjustable regulated output voltage from 0.6V to 3.4V while delivering up to 3A of output current. The internal synchronous low on-resistance power switches increase efficiency and eliminate the need for an external Schottky diode. The RT5797A is available in the WDFN-8SL 2x2 packages.


Product Feature

  • Efficiency Up to 95%
  • RDS(ON) 100mΩ HS / 70mΩ LS
  • VIN Range 2.7V to 6V
  • VREF 0.6V with ±1% Accuracy at 25℃
  • CMCOTTM Control Loop Design for Best Transient Response, Robust Loop Stability with Low-ESR (MLCC) COUT
  • Fixed Soft-Start 1.2ms
  • Power Saving in Light Load

Key Performance Summary Table

Key Features

Evaluation Board Number : PCB103_V1

Default Input Voltage

5V

Max Output Current

3A

Default Output Voltage

1.2V

Default Marking & Package Type

RT5797AHGQWA, WDFN-8SL 2x2

Operation Frequency

1MHz in CCM mode



Bench Test Setup Conditions

Headers Description and Placement

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Carefully inspect all the components used in the EVB according to the following Bill of Materials table, and then make sure all the components are undamaged and correctly installed. If there is any missing or damaged component, which may occur during transportation, please contact our distributors or e-mail us at 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)

FB

Feedback Voltage Input

An external resistor divider from the output to SGND, tapped to the FB pin, sets the output voltage. keep away from high current loops and swithing voltages.

PGOOD

Power Good Indicator

The output of this pin is an open-drain with external pull-up resistor. PG is pulled up when the FB voltage is within 90%, otherwise it is LOW.

VIN

Supply Voltage Input

The RT5797A operates from a 2.7V to 6V input.

PGND, GND GND1, GND2

Power Ground

Power ground. The exposed pad must be soldered to a large PCB and connected to PGND for maximum thermal dissipation.

NC

No Internal Connection

No internal connection.

LX

Switch Node

Switch node.

EN

Enable Control Input

Enable control input. A logic-high enables the converter; a logic-low forces the device into shutdown mode.

SGND

Signal Ground

Signal ground of the FB network. SGND should be connected to PGND close to the IC PGND pins.

VO

Output voltage

Output voltage.



Power-up & Measurement Procedure

1. Apply a 5V nominal input power supply (2.7V < VIN < 6V) to the VIN and GND terminals.

2. Set the jumper at JP1 to connect terminals 2 and 3, connecting EN to VIN through resistor R4 (100kΩ). The Enable pin can connected to VIN directly as well to enable operation.

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

4. Connect an external load up to 3A to the VOUT and GND terminals and verify the output voltage and current.


Output Voltage Setting

Set the output voltage with the resistive divider (R1, R2) between VOUT and GND with the midpoint connected to FB. The output is set by the following formula :

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The placement of the resistive divider should be within 5mm of the FB pin. The resistance of R2 is suggested between 10kΩ and 150kΩ to minimize power consumption, and noise pick-up at the FB pin. The resistance of R1 can then be obtained as below :

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For better output voltage accuracy, divider resistors (R1 and R2) should have tolerance of ±1% tolerance or better.



Schematic, Bill of Materials & Board Layout

EVB Schematic Diagram

RT5797AHGQW demo board : VIN 5V, VOUT 1.2V / 3A

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Note:

1. Do not hot-plug a live 5V supply to the board; if hot-plugging is required, add ~100µF electrolytic capacitor at the input.

A small feedforward capacitor (C1) can be introduced into the feedback network to speed up the transient response of high output voltage circuits. Adding C1 can also improve the light load PSM switching behavior. The feedforward capacitor is added across the upper FB divider.

To optimize transient response, C1 value is chosen so that the gain and phase boost of the feedback network increases the bandwidth of the converter, while still maintaining an acceptable phase margin. Generally, larger C1 values provide higher bandwidth, but may result in an unacceptable phase margin or instability.



Measurement Results

Output ripple measurement at VIN = 5V 10mA load

Cyan : V-SW; Blue : VOUT

Output ripple measurement at VIN = 5V 3A load

Cyan : V-SW; Blue : VOUT

TEK00076

TEK00075

Output ripple at 10mA load : 19.8mVpp, PSM mode

Output ripple at 3A load : 13.4mVpp

PWM frequency is 1.062kHz.

Dynamic load 0.01A to 2.2A load step

Blue : VOUT; Green : load current step

Dynamic load 1A to 2.6A load step

Blue : VOUT; Green : load current step

TEK00068

TEK00069

Overshoot : 21mV, Undershoot : 33mV in light load mode

Overshoot : 22mV, Undershoot : 24mV

OCP measurement : Dynamic load close to OCP

Blue: VOUT; Purple: Pgood; Green: Iout

OCP measurement : Full output short circuit

Blue: VOUT; Purple: Pgood; Green: Iout

TEK00070

TEK00071

Vout drops at 4A peak load.

Frequency reduction, hiccup mode at 4.24A peak load current

Start-up measurement from Enable :
EN pin 7 low – high

Cyan : V-Enable; Blue:VOUT; Purple : Pgood; Green : IOUT

Start-up measurement from VIN :

Cyan : VIN; Blue : VOUT; Purple : Pgood; Green : IOUT

TEK00073

TEK00074

Start-up time 1.24msec. Soft-start 1.1msec

VIN > 2.24V initiates start-up.



Efficiency Measurements

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Bill of Materials

Reference

Qty

Part Number

Description

Package

Manufacturer

U1

1

RT5797AHGQWA

DC-DC Converter

WDFN-8SL 2x2

Richtek

C1

1

NC

C-0603

C2, C3, C4

3

GRM31CR61C226KE15L

22µF/16V/X5R/1206

C-1206

muRata

C5

1

NC

C-1206

C6

1

GRM188R71H104KA93D

100nF/50V/X7R/0603

C-0603

muRata

L1

1

7447714015

1.5µH

10x10x5mm

WURTH ELEKTRONIK

R1, R2, R3, R4

4

WR06X1003FTL

100k/0603

R-0603

WALSIN



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 2018/08/21
Bill of Materials 2018/08/21
Gerber File 2018/08/21
Schematic 2018/08/21
RT5797A

3A, 1MHz, 6V CMCOT Synchronous Step-Down Converter

RT5797A

The RT5797A is a high efficiency synchronous step-down DC/DC converter. Its input voltage range is from 2.7V to 6V and provides an adjustable regulated output voltage from 0.6V to 3.4V while delivering up to 3A of output current.

The internal synchronous low on-resistance power switches increase efficiency and eliminate the need for an external Schottky diode. The Current Mode Constant-On-time (CMCOT) operation with internal compensation allows the transient response to be optimized over a wide range of loads and output capacitors. The RT5797A is available in the WDFN-8L 2x2 and WDFN-8SL 2x2 packages.

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