PCA9685 4-Channel 8W Open Collector FET Driver 4-Channel GPIO w/ I2C Interface Datasheet

Product data sheet Rev. 4 — 16 April 2015 36 of 52

NXP Semiconductors

PCA9685

16-channel, 12-bit PWM Fm+ I

C-bus LED controller

Question 1: What kind of edge rate control is there on the outputs?

• The typical edge rates depend on the output configuration, supply voltage, and the

applied load. The outputs can be configured as either open-drain NMOS or totem pole

outputs. If the customer is using the part to directly drive LEDs, they should be using it

in an open-drain NMOS, if they are concerned about the maximum I

and ground

bounce. The edge rate control was designed primarily to slow down the turn-on of the

output device; it turns off rather quickly (~1.5 ns). In simulation, the typical turn-on

time for the open-drain NMOS was ~14 ns (V

= 3.6 V; C

=50pF; R

=500).

Question 2: Is ground bounce possible?

• Ground bounce is a possibility, especially if all 16 outputs are changed at full current

(25 mA each). There is a fair amount of decoupling capacitance on chip (~50 pF),

which is intended to suppress some of the ground bounce. The customer will need to

determine if additional decoupling capacitance externally placed as close as

physically possible to the device is required.

Question 3: Can I really sink 400 mA through the single ground pin on the package and

will this cause any ground bounce problem due to the PWM of the LEDs?

• Yes, you can sink 400 mA through a single ground pin on the package. Although the

package only has one ground pin, there are two ground pads on the die itself

connected to this one pin. Although some ground bounce is likely, it will not disrupt the

operation of the part and would be reduced by the external decoupling capacitance.

Question 4: I can’t turn the LEDs on or off, but their registers are set properly. Why?

• Check the MODE1 register SLEEP (bit 4) setting. The bit needs to be 0 in order to

enable the clocking. If both clock sources (internal osc and EXTCLK) are turned OFF

(bit 4 = 1), the LEDs cannot be dimmed or blinked.

Question 5: I’m using LEDs with integrated Zener diodes and the IC is getting very hot.

Why?

• The IC outputs can be set to either open-drain or push-pull and default to push-pull

outputs. In this application with the Zener diodes, they need to be set to open-drain

since in the push-pull architecture there is a low resistance path to GND through the

Zener and this is causing the IC to overheat.