GPIOs and LED Drivers

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21

nominally 31ms after the start of a valid serial transfer,

the interface resets itself and sets up SDA as an input.

The MAX7306/MAX7307 then waits for another START

condition.

Applications Information

Hot Insertion

Serial interfaces SDA, SCL (and AD0 for the MAX7306)

remain high impedance with up to 5.5V asserted on

them when the MAX7306/MAX7307 are powered down

MAX7306/MAX7307’s reset threshold, all I/O ports

become high impedance. Ports P2 through P4 remain

high impedance to signals between 0V and the port

Port P1 goes high impedance to signals up to 5.5V. If a

signal outside this range is applied to a port, the port’s

signal. This behavior is safe if the current through each

protection diode is limited to 10mA.

If it is important that I/O ports remain high impedance

when all the supplies are powered down, including

or parasitic path for the MAX7306/MAX7307 input signals

achieve this is with a series small-signal Schottky

diode, such as the BAT54, between the port supply

I/O Level Translation

The open-drain output configuration of the ports allows

them to level translate the outputs to lower (but not

pullup resistor converts the high-impedance, logic-high

condition to a positive voltage level. Connect the resis-

inputs, a pullup resistor value of 220k

Ω is a good start-

ing point. Use a lower resistance to improve noise

immunity, in applications where power consumption is

less critical, or where a faster rise time is needed for a

given capacitive load.

Driving LED Loads

When driving LEDs, use a resistor in series with the

LED to limit the LED current to no more than 25mA.

Choose the resistor value according to the following

formula:

where:

the LED (

Ω)

LED (V)

For example, to operate a 2.2V red LED at 20mA from a

Ω.

Driving Load Currents Higher than 25mA

The MAX7306/MAX7307 can sink current from loads

drawing more than 25mA by sharing the load across

multiple ports configured as open-drain outputs. Use at

least one output per 25mA of load current; for example,

drive a 90mA white LED with four ports.

The register structure of the MAX7306/MAX7307 allows

only one port to be manipulated at a time. Do not con-

nect ports directly in parallel because multiple ports

cannot be switched high or low at the same time, which

is necessary to share a load safely. Multiple ports can

drive high-current LEDs because each port can use its

own external current-limiting resistor to set that port’s

current through the LED.

Power-Supply Considerations

0.1µF capacitor as close as possible to the device. The

between 1.40V and 5.5V and bypassed with a 0.1µF

supply and port supply are independent and can be

connected to different voltages or the same supply as

required.

either order or together.

For the MAX7307, when a push-pull port is acting as an