pca9685 NXP Semiconductors, pca9685 Datasheet - Page 35

• Manufacturer Part Number: pca9685
• Description: 16-channel, 12-bit Pwm Fm+ I 2c-bus Led Controller
• Manufacturer: NXP Semiconductors

NXP Semiconductors
PCA9685_1
Product data sheet
Question 1: What kind of edge rate control is there on the outputs?
Question 2: Is ground bounce possible?
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?
Question 4: I can’t turn the LEDs on or off, but their registers are set properly. Why?
Question 5: I’m using LEDs with integrated Zener diodes and the IC is getting very hot.
Why?
•
•
•
•
•
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 ISS 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
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.
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.
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.
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.
Rev. 01 — 24 July 2008
16-channel, 12-bit PWM Fm+ I
DD
= 3.6 V; C
L
= 50 pF; R
2
C-bus LED controller
PCA9685
© NXP B.V. 2008. All rights reserved.
PU
= 500 ).
35 of 50