MAX7327 Maxim, MAX7327 Datasheet - Page 15

• Manufacturer Part Number: MAX7327
• Description: The MAX7327, a 2-wire serial-interfaced peripheral, features 12 push-pull outputs and four configurable open-drain I/O ports with selectable internal pullups and transition detection
• Manufacturer: Maxim

diodes to V+ and GND continue to appear as a diode
clamp from each output to GND (Figure 10).
Each of the four I/O ports P2–P5 has a protection diode
to GND (Figure 11). When a port output is driven to a
voltage lower than GND, the protection diode clamps
the output to a diode drop below GND.
Each of the four I/O ports P2–P5 also has a 40kΩ (typ)
pullup resistor that can be enabled or disabled. When a
port input is driven to a voltage higher than V+, the
body diode of the pullup enable switch conducts and
the 40kΩ pullup resistor is enabled. When the
MAX7327 is powered down (V+ = 0V), each I/O port
appears as a 40kΩ resistor in series with a diode con-
nected to 0V. Input ports are protected to +6V under
any of these circumstances (Figure 11).
When driving LEDs from one of the 12 push-pull out-
puts, a resistor must be fitted in series with the LED to
limit the LED current to no more than 20mA. Connect
the LED cathode to the MAX7327 port, and the LED
anode to V+ through the series current-limiting resistor,
R
the resistor value according to the following formula:
where:
R
LED (Ω).
V
V
Figure 10. MAX7327 Push-Pull Output Port Structure
SUPPLY
LED
LED
LED
. Set the port output low to light the LED. Choose
is the forward voltage of the LED (V).
is the resistance of the resistor in series with the
is the supply voltage used to drive the LED (V).
R
LED
OUTPUT
MAX7327
= (V
______________________________________________________________________________________
SUPPLY
- V
I
V+
LED
2
Driving LED Loads
C Port Expander with 12 Push-Pull
V+
- V
OL
) / I
O6–O15
O0–O2
LED
Outputs and 4 Open-Drain I/Os
V
sinking I
I
For example, to operate a 2.2V red LED at 10mA from a
+5V supply:
The MAX7327 can be used to drive loads such as
relays that draw more than 20mA by paralleling out-
puts. Use at least one output per 20mA of load current;
for example, a 5V 330mW relay draws 66mA, and
therefore, requires four paralleled outputs. Any combi-
nation of outputs can be used as part of a load-sharing
design because any combination of ports can be set or
cleared at the same time by writing to the MAX7327. Do
not exceed a total sink current of 100mA for the device.
The MAX7327 must be protected from the negative
voltage transient generated when switching off induc-
tive loads (such as relays), by connecting a reverse-
biased diode across the inductive load. Choose the
peak current for the diode to be greater than the induc-
tive load’s operating current.
The MAX7327 operates with a supply voltage of +1.71V
to +5.5V. Bypass the supply to GND with a ceramic
capacitor of at least 0.047µF as close as possible to the
device. For the TQFN version, additionally connect the
exposed pad to GND.
Figure 11. MAX7327 Open-Drain I/O Port Structure
LED
OL
Driving Load Currents Higher than 20mA
PULLUP
OUTPUT
is the desired operating current of the LED (A).
is the output low voltage of the MAX7327 when
ENABLE
INPUT
LED
R
LED
(V).
= (5 - 2.2 - 0.1) / 0.01 = 270Ω
V+
Power-Supply Considerations
V+
40kΩ
MAX7327
P2–P5
15