PCA9901_1

© NXP B.V. 2009. All rights reserved.

Product data sheet

Rev. 01 — 3 December 2009

11 of 27

NXP Semiconductors

PCA9901

One wire single LED driver

8.8.4 ‘Synchronization’ signal not generated after RUN ONCE command

The PCA9901 is waiting for the ‘Synchronization’ signal (rising edge of CTRL) after a

RUN ONCE command has been issued.

1. Once the RUN ONCE command has been decoded (end of the 1.5 ms window), a

time-out counter starts counting as long as CTRL stays LOW.

2. The time-out counter counts until it reaches the maximum allowed ON value. The

maximum allowed ON time is greater than or equal to 255 ms.

Remark: If CTRL goes HIGH before reaching the maximum counter value, the

time-out counter is reset and the PCA9901 runs the sequence once.

3. If the maximum time-out value is reached, the RUN ONCE command is automatically

aborted and the PCA9901 goes to Shutdown mode.

8.9 Current source generation

The LED output contains a constant current driver that will source a current that is

determined by an external resistor connected between ISET pin and GND. The current

can be set using the following formula:

I

O

1.23

400

×

()

R

ext

------------------------------

=

(1)

be programmed.

Remark: LED current accuracy is proportional to the accuracy and temperature

When no external resistor is connected between the ISET pin and GND, the LED output is

able to source 5 mA through a fully internal current source. It is automatically shut down

when an external resistor is connected to ISET.

Remark: The LED current accuracy is proportional to the tolerance and temperature

coefficient of the resistor.

Remark: To save power, the current source generator is only enabled when the LED

needs to be turned on.

8.10 Short-circuit and thermal protection

A short-circuit and thermal protection circuitry disables the LED output driver and the

current generator when a short occurs or when a high temperature condition has been

detected.

The circuitry is active during normal mode operation (Programing, RUN ONCE, RUN or

Bypass modes). When a fault condition is detected, the reference current circuitry (ISET)

and the LED output stage (LEDOUT) are automatically shut down. This will cause

LEDOUT to be OFF as long as the fault condition is present. The other analog blocks

(oscillator, voltage reference) are kept enabled as long as the PCA9901 is in normal mode

operation.

The PCA9901 goes automatically to Power-down mode when it exits the programming,

RUN ONCE, RUN or Bypass modes.