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How to read a datasheet: the CAT4104 LED driver

Lesson 1.3 · Theory · free

A datasheet is the manufacturer's document about a part: what it does, its limits and how to use it. We use it for everything: the symbol, the schematic, the footprint and later for debugging.

In this lesson · 5 steps
  1. Page 1: the summary
  2. Limits: what the chip can take
  3. Pin descriptions: for the symbol
  4. Basic operation: the LED current
  5. Package dimensions: for the footprint

1Page 1: the summary

  1. Description and features: four channels, up to 175 mA each, current set by one resistor
  2. Typical application circuit and the two packages: SOIC-8 and TDFN-8. We use the TDFN-8
Why?
The first page is the manufacturer saying "look how easy this chip is to use". It gives a quick picture: what the chip does, the main features, a typical circuit and the pins.

2Limits: what the chip can take

  1. Recommended operating conditions: VIN from 3.0 to 5.5 V. Our 5 V USB fits
Why?
Absolute maximum ratings are the limits you must never cross. The recommended operating conditions are where the chip works as described.

3Pin descriptions: for the symbol

  1. Table 5: LED1–LED4 (pins 1–4), GND (5 and the TAB under the chip), EN/PWM (6), VIN (7), RSET (8) for the TDFN-8
Why?
Every pin must be right in the symbol, or the board will not work. This table gives each pin's number, name and function.

4Basic operation: the LED current

  1. Table 6: RSET resistor settings. 2.10 kΩ gives 60 mA per LED, the value we use
Why?
One resistor from RSET to ground sets the current of all four LEDs. The datasheet gives a table of resistor values.

5Package dimensions: for the footprint

  1. TDFN-8, 2 × 3 mm, 0.5 mm pitch: the recommended soldering footprint
Why?
At the end of the datasheet are the exact package dimensions and a recommended footprint. We use them in lesson 2.2.
Check & troubleshoot

What you should see

  • You found the supply range: VIN from 3.0 to 5.5 V, so 5 V USB is fine
  • You found the pin table: LED1–LED4, GND, EN/PWM, VIN, RSET
  • You found the RSET table and the TDFN-8 package drawing at the end

Good to know

  • The LED driver gives each LED a constant current and drops the extra voltage, so the LEDs don't burn. It also has thermal and short-circuit protection
  • In a real project, choosing the part takes a lot of work: comparing options and checking requirements. In this course the driver is already chosen, so we focus on how to use it
  • Get the datasheet from the manufacturer: search CAT4104 on onsemi.com, or open it from the distributor's product page

Next: Chapter 2: we draw the CAT4104 symbol from the pin table.