BA45F5xxx Smoke Detector Integrated Analog Front End Application Notes
AN0540EN V1.00 1 / 5 March 9, 2020
BA45F5xxx Smoke Detector Integrated
Analog Front End Application Notes
D/N: AN0540EN
Introduction
The BA45F5xxx series of MCUs include an integrated analog front end (AFE) for use by smoke
detectors, allowing designers to develop related products with a minimum of external components.
This application note introduces the principles and application description of this kind of AFE.
Functional Description
The following figure shows a block diagram of an independent type smoke detection alarm product.
When smoke enters the product, the smoke detection circuit will generate a tiny current signal. The
AFE is responsible for this current signal, converting it to a voltage signal and amplifying it, and
then determining whether an alarm threshold has been reached using an A/D. After this, the product
will generate an alarm output signal. This application note explains the AFE part in the figure below.
Smoke Detector
Circuit
AFE
A/D sampling and
alarm output
Note: 1. In the smoke detection products, the photoelectric smoke detector commonly
uses optical refraction and has a labyrinth cavity.
2. Optical refraction is composed of infrared receiving and transmitting transistors.
3. The labyrinth cavity is designed to minimise any noise detected by the infrared
receiver when no smoke is present.
4. When smoke enters the labyrinth cavity, the infrared radiation will be diffused,
which will then be detected by the receiving part.
BA45F5xxx Smoke Detector Integrated Analog Front End Application Notes
AN0540EN V1.00 2 / 5 March 9, 2020
Operating Principles
This section introduces an independent type smoke detector alarm application circuit along with a
description of AFE principles. When applied for use with different brands of labyrinth cavities or
infrared pairs, there may be differences in alarm thresholds and gain settings. These need to be
adjusted according to specific requirements.
Application Circuit
Fig. 1 Application Circuit
Fig. 1 shows the infrared pair which is driven by ISINK0/1 which provides a constant infrared
emission current. Inputs A0PI and A0NI process the infrared receiving signal.
The infrared receiver works in reverse bias.
The infrared receiver current is proportional to the detected smoke concentration.
The ISINK0/1 constant current output is adjustable.
There are two sets of ISINK for use by different smoke - white/black.
If using the BA45F5222, then a current limiting resistor needs to be used with the LED.