Table of contents

RoadTest: Richtek EVB_RT7275GQW Evaluation Board

Author: gpolder

Creation date: 21 Oct 2014

Evaluation Type: Evaluation Boards

Did you receive all parts the manufacturer stated would be included in the package?: True

What other parts do you consider comparable to this product?: Other brands buck converters, like TI.

What were the biggest problems encountered?: Modifying the board for 5V.

Detailed Review:

Introduction

A very good review is already done by Peter Oakes (http://www.element14.com/community/roadTestReviews/1884).

It is not my intention to duplicate his work. In this review I will mainly focus on two subjects:

1. Behavior of the board at varying input voltage. The reason for this is that I like to use the board with a solar cell with output varying from 4-18V. Main application is a steady 5V output USB charger under varying sun conditions.
2. Modification of the board to 5V output.

Output versus varying input

First test were with the unmodified board. Here is a picture of the setup. The resistor is 4.7 ohm, a little bit to large value to test the 1.05 output voltage.

Input voltage comes from a programmable power supply:

First I checked the unloaded output voltage versus the input, from 18 - 3V in steps of 1V.

As you can see around 5V the output is unstable and noisy, dropping to 0 for lower input values.

There are quite a number of testpoints available on the evaluation board, see the documentation (http://www.richtek.com/assets/evb_file/UG7275GQW-00.pdf) for details.

On of the signals is PG (Power good output test point) PG voltage is 5.1V when soft-start is complete and the output voltage is in regulation. Below is a graph of the PG signal versus the input voltage (still without load). This PG signal can be used to switch off the power in case there is no proper input signal.

Conversion to 5 volt output

The evaluation board is designed for 1.05 volt output. Other output values can be set by replacing R23 and R28 according to the following formula:

Unfortunately this is not an easy task, since both resistors are 0603 type smd components. Furthermore according to page 14 of the RT7275 IC datasheet (RT7275/76 - Richtek Technology Corporation) a larger value for L1 is advised.

For an evaluation kit I would expect user selectable output power, with a jumper or solder bridge.

For my tests I removed r23 and replaced it with a potentiometer, as can be seen in the next pictures.

Operating at 5 volt

Startup

Next graph shows a power up with a load 0f 4.7 ohm at 5V output. Within 2.5 ms the full output signal is available.

Output as function of input voltage

Next graph shows the output voltage as function of the input voltage with 5W load (5v, 4.7ohm).

Again we see a very stable output as long as the input voltage is larger than 6 volt.

Efficiency

I quickly looked at the efficiency, Peter Oakes did this also, but in my case its at 5v output level.

The numbers I got here are very good.

This mainly concludes my review. I'm planning to use this evaluation kit for a solar USB charger. I will keep you informed about the progress, either here or somewhere else on element14.

Please let me know when you have any questions.