Datasheet

PCA9557 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
Product data sheet Rev. 7 — 10 December 2013 13 of 30
NXP Semiconductors
PCA9557
8-bit I
2
C-bus and SMBus I/O port with reset
9. Application design-in information
9.1 Minimizing I
DD
when the I/Os are used to control LEDs
When the I/Os are used to control LEDs, they are normally connected to V
DD
through a
resistor as shown in Figure 18
. Since the LED acts as a diode, when the LED is off the
I/O V
I
is about 1.2 V less than V
DD
. The supply current, I
DD
, increases as V
I
becomes
lower than V
DD
.
Designs needing to minimize current consumption, such as battery power applications,
should consider maintaining the I/O pins greater than or equal to V
DD
when the LED is off.
Figure 19
shows a high value resistor in parallel with the LED. Figure 20 shows V
DD
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
and prevents additional supply current consumption when the LED is off.
Device address configured as 0011 100x for this example.
IO0, IO2, IO3 configured as outputs.
IO1, IO4, IO5 configured as inputs.
IO6, IO7 are not used.
Fig 18. Typical application
PCA9557
IO0
IO1
SCL
SDA
V
DD
002aad285
SCL
SDA
IO2
IO3
V
DD
V
SS
MASTER
CONTROLLER
V
SS
V
DD
(5 V)
620 Ω
SUBSYSTEM 1
(e.g., temp. sensor)
INT
SUBSYSTEM 2
(e.g., counter)
RESET
controlled switch
(e.g., CBT device)
A
B
enable
RESETRESET
2 kΩ 2 kΩ
SUBSYSTEM 3
(e.g., alarm system)
ALARM
IO4
IO5
V
DD
A2
A1
A0
IO6
IO7
100 kΩ
(× 5)
1.8 kΩ1.8 kΩ