LT8609 Basic Description

Simply put, it is a synchronous rectifier buck regulator from Analog Devices that is small, highly efficient, and features high-speed switching.

Some more detailed features are summarized below:

LT8609A Features

- Wide input voltage range: 3.0V~42V

High efficiency: Over 93% efficiency(12V input, 5V/1A output)

- Maximum continuous output current: 3A

- Short Minimum On Time: 45ns

Adjustable and syncable frequency: 200kHz~2.2MHz

・Low EMI to achieve vinegarSpread Spectrum Modulation(*)

 

Here, I would like to add a little more information about (*).

LT8609A Pin Functions

First, let's look at what the pins on the LT8609A are. Below are the pin configuration and a standard recommended circuit diagram.

LT8609A Pinout

Standard Recommended Circuit

Details of each pin

1. BST (Bootstrap)

High-side switch (internal MOSFET) to drive Bootstrap Capacitor Terminal to connect. SW–BST In between 0.1µF It is essential to connect a capacitor of this magnitude.  SW Use the action, BST The charge stored in the capacitor is supplied to the high-side driver.

2. SW (Switch Node) 

internal MOSFET and the connection point of the inductor Switching Node. 0V → Vin To switch between these voltages at high speed, abrupt voltage changes (dV/dt) occurs. EMI This is the point where the inductor is most SW Place it close to the pin.

3.INTVcc

Supplies power to the control circuit and gate driver for 3.5V internal LDO Externally 1µF ~ 4.7µF A capacitor of some size is required.

4.RT（Run Time frequency setting）

Pin to connect a resistor to set the switching frequency.  The frequency range is 200kHz to 2.2MHz.

5.SYNC

Pin used for synchronous operation with external clock input or for setting the operating mode.

- External clock synchronization

When an external clock signal is input, the switching frequency is synchronized to the external clock.

・Operation mode

　　- When connected to GND, Burst Mode

　　-Pulse Skip Mode when connected to INTVcc or 3.2V-5.0V 

6.FB（Feedback）

Feedback input that determines the output voltage. The voltage at this pin is stable at 0.782V. The output voltage is set using a resistor divider based on 0.782V.

7.TR/SS（Tracking / Soft-Start）

Pin to set the soft-start time or tracking to another power supply during startup.

　
- By connecting a capacitor, the rise time can be set.

→Inrush current at startup is suppressed, preventing output overshoot.
- When an external reference power supply is applied, it rises up by following that power supply (tracking).

8.PG（Power Good）

​Open-drain status output indicates when the output voltage has reached regulation.

・ High → Output voltage is normal
　 ・ Low → UVLO / OCP / Thermal Shutdown Even when Low
 　　 A pull-up resistor must be connected (INTVcc or external 3.3V/5V etc.).

9.VIN

Input power terminal, operating range 3.0 to 42V.

The input capacitor (at least 4.7µF) should be placed as close as possible to minimize power line loops.

10.EN/UV

Enable + Under Voltage Lockout setting terminal.

・ Enable
1V Nearby (When rising 1.05V, When descending 1.00V) but ON/OFF Threshold

→ When it is turned ON, it starts switching, and when it is turned OFF, it shuts down.

　・UVLO

By dividing VIN by resistors, the minimum operating voltage can be set to prevent malfunction when VIN is low. If the EN/UV pin is unused and always on, connect it to the VIN pin.

11.GND（Exposed Pad）

It serves as the reference ground and also serves as an electrical ground and heat dissipation.

The above is an explanation of the functions and roles of each pin.

Summary

When I first read the datasheet, I only understood the IC 's features and didn't understand the specific pin functions. I thought I could operate the device even without understanding the TR/SS and EN/UV pins in particular. However, if you don't understand their functions and the recommended design, malfunctions will occur.

However, I think it would be quite difficult for beginners to understand the entire 30-page datasheet all at once. So, for those who, like me, are reading a datasheet for the first time, I have summarized the LT8609 in my own words. I hope it will be of some use.

In the next blog, I would like to discuss the implementation of the LT8609 and the measurement results using an oscilloscope. This will be a more practical blog than this one, so if you are using the LT8609 for the first time, I would recommend you take a look at the next blog. Until next time,