Datasheet

PCA6408A All information provided in this document is subject to legal disclaimers. © NXP B.V. 2012. All rights reserved.
Product data sheet Rev. 1 — 27 September 2012 13 of 40
NXP Semiconductors
PCA6408A
Low-voltage, 8-bit I
2
C-bus and SMBus I/O expander
9. Application design-in information
9.1 Minimizing I
DD
when I/Os control LEDs
When the I/Os are used to control LEDs, normally they are connected to V
DD(P)
through a
resistor as shown in Figure 12
. The LED acts as a diode, so when the LED is off, the I/O
V
I
is about 1.2 V less than V
DD(P)
. The I
DD
parameter in Table 15 “Static characteristics”
shows how I
DD(P)
increases as V
I
becomes lower than V
DD(P)
. Designs that must minimize
current consumption, such as battery power applications, should consider maintaining the
I/O pins greater than or equal to V
DD(P)
when the LED is off.
Figure 13
shows a high-value resistor in parallel with the LED. Figure 14 shows V
DD(P)
less than the LED supply voltage by at least 1.2 V. Both of these methods maintain the I/O
V
I
at or above V
DD(P)
and prevent additional supply current consumption when the LED
is off.
Device address configured as 0100 000x for this example.
P0 and P2 through P4 are configured as inputs.
P1 and P5 through P7 are configured as outputs.
(1) Resistors are required for inputs (on P port) that may float. If a driver to an input will never let the
input float, a resistor is not needed. Outputs (in the P port) do not need pull-up resistors.
Fig 12. Typical application
PCA6408A
P0
P1
SCL
SDA
V
DD
SCL
SDA
P2
P3
V
SS
MASTER
CONTROLLER
V
SS
V
DD(I2C-bus)
= 1.8 V
controlled
switch
A
B
enable
INT
V
DD(P)
INT
10 kΩ
SUBSYSTEM 1
(e.g., alarm system)
ALARM
(1)
P4
P5
P6
P7
10 kΩ10 kΩ
RESETRESET
V
DD(P)
002aaf829
V
DD(I2C-bus)
V
DD(I2C-bus)
ADDR
KEYPAD
10 kΩ
10 kΩ (× 3)