Q

Q

CLR

PRE

D

CLK

GND

CAN Controller

CAN Controller

Power

EN

RX

nSTB

RX

I/O

R1

C1

R2

TX

TX

11

SN74HCS72

www.ti.com

SCLS801 – FEBRUARY 2020

Product Folder Links: SN74HCS72

Submit Documentation Feedback

Copyright © 2020, Texas Instruments Incorporated

9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

9.1 Application Information

The SN74HCS72 is an ideal device for taking a CAN wake-up request and converting it to a power supply enable

due to its low power consumption and noise rejecting inputs, which eliminate false triggers. CAN communication

can occur when the vehicle ignition is off. Therefore, many circuits are designed to work in a standby or low

power mode. Because a CAN wake-up request causes the RX pin to pulse LOW, the SN74HCS72 will trigger off

the falling edge enabling the power for the CAN controller. Then the CAN controller powers on for the incoming

communication. When communications are finished, the controller sends a reset pulse to the SN74HCS72 and

CAN transceiver putting the circuit back into a standby mode.

9.2 Typical Application

Figure 12. Power Enable Using CAN Wake-up Request

9.2.1 Design Requirements

The SN74HCS72 device allows flexibility by having complementary outputs for active-high or active-low enables.

The supply should be selected such that the device is always powered along with the CAN transceiver. The

same supply for both devices is recommended.

With the SN74HCS72, a power on reset circuit only requires a resistor (R) and capacitor (C) to create a delay.

The R and C values create a delay that is approximately 2.2×RC. In this application, it is desired to have the

output (Q) in the HIGH state at startup, so R1 and C1 are connected directly to the CLR pin, as shown in

Figure 12. A second resistor is needed to limit the current into the CAN controller when it sets the circuit back

into standby mode. It is required for the R1 resistor to be at least ten times larger than R2 to avoid a divider

circuit (R2

≤ 10R1).