LED Flashlight

Goal

Design and build a color accurate LED flashlight for high end applications.

Example Block Diagram

At right is a simple LED driver with a simple digital control system. It's documented here.
It appears to be a current controlled topology. It's constant frequency and turns off when the sense voltage (driven by the output current) exceeds $V_{ref}$.

The eventual goal is to replace much of the digital side with an FPGA, but it's illustrative here to get a general idea of a simplified control loop.

It uses a GaN Mosfet, EPC8010 datasheet and runs about \$2 in 100 quantity. This part allows very high frequency switching.

For efficiency reasons D1 should probably be replaced by another Mosfet making it a synchronous switcher.

Background Material

Here is a mind boggling paper on control techniques using state space .

Also here is a set of videos from TI

Seminar videos from Analog Devices on switchers. (Maxim is now part of Analog Devices).

• Module 1 Intro.

• Module 2 Control.

• Module 3 Synchronous.

• Module 4 Filter Conponents.

• Module 7 Theory.

• Module 11 Protection.

• Module 12 Protection ICs

Additions

Bootstrapping

An additional supply (perhaps just a capacitor) to supply the gate drive with a high drive voltage.

Synchronous Mode

This uses a Mosfet to replace D1 (the catch diode) resulting in much less power loss since the $\small RDS_{on}$ is much smaller than even a schottky diode.

Protection

• Voltage
  • Input de-bounce
  • Input under/over volatge (UVLO/OVLO)
  • Reverse input voltage protection
• Current
  • Inrush current limiting
  • Overcurrent short circuit protection
  • Reverse current protection
• Thermal
  • Over temperature protection
  • Thermal shutdown

Example of a 'protection IC'. This gives a naive implementation that can be used as a guide.

Video for understanding the specifications of protection ICs.

Herman Gate Driver Design

This is Herman's P-Channel high side gate driver, part number IRS10752LTRPBF datasheet. We are using a low side driver so this is to see a gate driver in situ. We will need a similar compensation network before the gate.

This is the gate driver power supply. It's a bit tricky with two 12 volt regulators, part numbers L79L12ACUTR datasheet, and L78L12ACUTR datasheet; positive and negative regulators.

OK! Let's Start Designing

High Side Current Sense

  After a discussion with Herman, putting a small resistor (100mOhm) in the Source to ground leg is Ok.
It will only raise the gate voltage requirement a small amount.

The circuit is from a TI App Note. High side sensing has noise advantages also I believe (I'll check with Herman) that we can't use low side because the gate drive would have to go through the sense resistor which would limit the current! Something we are desparately trying to increase.

Here's a shi-shi-foo-foo (albeit excellent) op amp, OPA2991IDGKR datasheet

Low Side Gate Driver

Fundamentals of Gate Drivers .. 48 pages!

Low Side Gate Driver with Totem Pole. We don't want a totem pole. There are issues. See Doc.

Choosing a Gate Driver

Part	Manuf	Datasheet	Source	Price (100)	Spice	Comments
FAN3111E	Fairchild	(datasheet)	Mouser	\$ 0.67	model.zip)	Looks good
IRS44273L	Infineon	(datasheet)	src	price	Y	Need more info
ZXGD3009W6	Diodes Inc	(datasheet)	src	price	Spice	Need more data
NCP51100					N
TPS2819QDBVRQ1					N
TPS2816					N
IRS44273L	Infineon	(datasheet)	src	price	Y	more info

LTSpice

I will send the .asc Sim file to you via email.

The image here is for reference.

Below is what you have to do to get it running.

• To add the FDMS8622 FET model to LTSpice (spice here) follow these instructions.

• To add the ZXGD3009W6 Gate Driver model (spice here) to LTSpice follow these instructions

To add the FDMS8622 FET model to LTSpice (spice here) follow these instructions.

To add the ZXGD3009W6 Gate Driver model (spice here) to LTSpice follow these instructions

Sim Results 10us/10ns