PCA9557 datasheet(13/26 Pages) NXP

13 / 26 page

PCA9557

Product data sheet

Rev. 06 — 11 June 2008

13 of 26

NXP Semiconductors

PCA9557

8-bit I2C-bus and SMBus I/O port with reset

Application design-in information

9.1 Minimizing IDD when the I/Os are used to control LEDs

When the I/Os are used to control LEDs, they are normally connected to VDD through a

resistor as shown in Figure 18. Since the LED acts as a diode, when the LED is off the

I/O VI is about 1.2 V less than VDD. The supply current, IDD, increases as VI becomes

lower than VDD.

Designs needing to minimize current consumption, such as battery power applications,

should consider maintaining the I/O pins greater than or equal to VDD when the LED is off.

Figure 19 shows a high value resistor in parallel with the LED. Figure 20 shows VDD less

than the LED supply voltage by at least 1.2 V. Both of these methods maintain the I/O VI

at or above VDD 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

VDD

002aad285

SCL

SDA

IO2

IO3

VDD

VSS

MASTER

CONTROLLER

VSS

VDD (5 V)

620

Ω

SUBSYSTEM 1

(e.g., temp. sensor)

INT

SUBSYSTEM 2

(e.g., counter)

RESET

controlled switch

(e.g., CBT device)

enable

RESET

2 k

Ω

2 k

Ω

SUBSYSTEM 3

(e.g., alarm system)

ALARM

IO4

IO5

VDD

IO6

IO7

100 k

Ω

(

× 5)

1.8 k

Ω

1.8 k

Ω

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